FWIW, I do plan on some sort of propane-powered back-up system - at mimimum a generator and maybe also a propane furnace - but the goal is to use as little propane as possible. I feel like this should be doable (though, yes, probably expensive) but I'm trying to find the math to verify it.

There was a very useful post from 7 years ago: https://www.houzz.com/discussions/2417141/carrier-greenspeed-25vna-massachusetts-cold-weather-data

Per the data there, a Carrier 25VNA heat pump (which I believe has a HSPF of 13) used 1435kWh/month for a cold Massachusetts January. I'm in California where our January is probably closer to their March. For March they used 1039kWh. That averages to 34.6 kWk/day.

This was for a smaller house, but also a much older house. I think this was also a forced air system which is (supposedly) 30% less efficient than radiant. Also, I'm looking at the Chiltrix CX34 Air-To-Water Heat Pump, which is supposed to be even more efficient than conventional heat pumps.

I just feel like there should be some formula out there that I could use to actually calculate the kWh usage.

First you need to clarify your desire for radiant floor heating. It doesn't work that well in the situation you are describing for the following reasons.

- Air based heat pump that heat water are not common. The vast majority of heat pumps are designed to heat air. When things are not common, they are expensive to buy and maintain. And that can be a resale issue.

- A tight house in a modest climate will require the smallest of temperature difference at the floor. That is the floor will be 74 degrees (as an example) vs 68 degree air temp. You won't notice it. Which means you spent $50k on something unnoticeable

- A/C is expected in a new house in most of the US. Then you have duplicate systems.

I live in central NC with Jan nights of 30. I have heat pumps and solar. Mine are super conventional - read cheap - Trane Seer 15. I have an Ecobee stat and solaredge monitoring so I can give you run times and energy use for any scenario that I have experienced in the last year (since I installed the Ecobee).

3800 sqft, what the energy gurus would call a "pretty-good" house. 2x6 with packed glass and zip walls with 1/2 inch foam. R-98 in attic. Best available conventional 2 pane windows - but lots of them. Compromises like conventional wood door with single latch. Some foam in tough areas. Architecture far from ideal with 14 ft ceiling great room and 10 ft down/9 up otherwise.

A 40 degree night requires close to zero heat. We don't run the heat upstairs unless it is 25 and we have older guests (ie want it 70).

We have 4 tons but only use the downstairs 2 tons. It uses 1.8 kw when running. The last cold day was end of November with average temp around 42 - so figure a 52 high and 27 low or around there. It ran 7 hours - so 12 kwh.

In Jan, it ran 280 hrs, Feb 250, March 86. So if you figure your Jan is my March then about 140 kwh. The real issue with solar is December usually. The 20 shortest days of the year are in December so your generation is weak and your solar gain to heat the house is weak. Then there are the xmas lights if you do that. For me, last Jan and Dec had the same generation and consumption but typically December is worse.

I have 50 panels or so. Last year generated 16.7 Mwh and used 12.7 Mwh. Those 4 used from utility were basically 1 per month for the winter and the rest of the year about zeroed out. 2 EVs. I think if March was our coldest and we had no EVs (or charged them in town), then we could survive without much propane (I don't have propane - just in your case)

. I just checked and we used right around 1 Mwh for March and Dec generation was .7. Mwh. Cars are tough because they also use more power in the winter.

I think off grid solar used to be living with specialty refrigeration and wood heating but now it can mean conventional house with a lot of panels. You don't need geothermal radiant floor heating (although that is the most efficient), you just need lots of panels. The panels are the cheaper option. We have massive TVs and massive refrigeration just like everyone else. Subzero 30 inch columns, 2 fridge drawers and a undercab wine cooler. Not that we are off-grid but without EVs and with some insulated window treatments, we could be.

Don't forget to factor in the inrush current when your compressor first starts.

My compressor requires 5x the current to start than while running normally.

@David Cary Thanks so much for your feedback. Your house sounds *a lot* like what we are planning. 3400sqft., 13ft ceiling in greatroom/9 ft in rest, 2x6 walls, lots of good quality windows, Subzero, etc, etc. so this is a very good use-case example. Do you mind sharing more about your panel system? Brand, system size. Do you have backup batteries for overnight use or do you default to grid for that? 50 panels is a lot of physical space. Are they all roof mounted? How much square footage is that taking?

We've been debating a lot about the radiant vs. forced air question. Contractor is recommending both, which sounds like over-engineering. I'm not terribly concerned with cooling. We will have a lot of passive cooling via window placements and ceiling fans. We don't have AC now and generally only regret it about one week a year. Even on most hot days, nights are cool. Though I suppose I need to plan for things getting worse. And it will be a lot cheaper to add ducting (or radiant) during construction than try to retrofit later. Every new high-end house around here seems to be doing radiant. Don't know if it's just "trendy" or beneficial.

For me the big benefit over forced-air is energy (electricity) savings. Since we are off-grid, I'm trying to find ways to save energy costs without sacrificing comfort. This is why I'm trying to calculate the mid-winter scenerio. This is when my panels are producing the least, but my heating needs are the highest, especially at night when panels are producting nothing.

If the goal is to use a minimum amount of propane, then why have a propane furnace? Electric resistance heating could be the back when the heat pump is not able to supply enough heat.

The compressor start up is a big current surge. There are start up kit devices that help reduce that surge. You also have to account for other high current draw items like domestic hot water, cooking range, oven, dishwasher and washing machine.

Do you live in an area where the solar panels could be covered in snow for several days? Even if you had no snow, a rainy day in the winter would significantly reduce the amount of power the solar panels can supply. Something to consider when determining the size of the back up battery.

Will the battery only be charged by the solar panels? In the situation where the battery is depleted the solar panels may not be able to charge the battery and supply enough power to the house at the same time. I suppose the battery could be programmed to do a slow charge. The system design of solar panels, battery, and generator can become complex.

@mike_home The backup propane generator and (maybe) furnace are for the winter scenario that you described. No snow here, but a week of overcast days would strain the batteries, so this is in the case where I have sparce electricity. I either burn propane to run a generator to make electricity to run the heat pump OR burn propane to run a furnace.

That’s another efficiency tradeoff calculation that I’m finding it hard to get numbers for. That is, I need propane BTU in vs. heat BTU out along both paths. I can’t find any useful numbers in any product spec sheets. E.g. like the startup surge. I agree it’s important. So where do I find that data? I’m looking at manufacturer websites and their “technical” spec sheets are a joke. It’s all just marketing.

"So where do I find that data? I’m looking at manufacturer websites and their “technical” spec sheets are a joke. It’s all just marketing."

You have to look at the product data/installation instructions, not the brochures for electrical information. For example, I just purchased the Carrier 50VR package heat pump. Its electrical information is shown starting on page 44 of the Product Data document. Of course, you have to figure out the meanings of the terms and how to use the data provided.

"LRA - Locked Rotor Amps: The current you can expect under starting conditions when you apply full voltage. The current builds up in milli-seconds when the compressor contactor closes at start up and decays to the running current over 100-200 milli-seconds (the time it takes for the motor to start spinning at a significant fraction of running speed). Note: The running current is affected by starting in Stage 1 or Stage 2, but the initial LRA current surge into a non-spinning motor is not affected by what stage is enabled.

"RLA - Rated Load Amps: The maximum current a compressor should draw under any operating

conditions. Often mistakenly called running load amps which leads people to believe, incorrectly, that the compressor should always pull these amps."

-- http://www.miamiheatpumps.com/Appendix.pdf

David Cary:

- 50 solar panels is a lot - I just took a quick walk outside and my neighbor has 16. How much did your system cost?

- What temperature do you set your thermostat at during heating months?

- What is your backup heat source - electric resistance? (Not practical for an off grid house)

I find your usage numbers a bit light but that's fine. Thanks for responding.

Calculating the amount of energy using SEER. HSPF, or COP is not going to work in my opinion. I suggest you use the EER measurement which is BTU/Watt at 95 F degrees in cooling mode. Is is an actual measurement under difficult conditions. This will be a good approximation of how much electricity the heat pump will use when running under very cold conditions. You can add 10% to approximate a worst case number.

The heat pump has to be sized for cooling. A 3400 sq. foot house with very high insulation values in your climate conditions may only need 3 tons of cooling. The heating and cooling load calculations will determine the required amount.

Elmer - it was $35k; 18k net after utility credit and fed tax credit.

68 during day, typically 65 at night. Stat is smart enough to not do electric resistance by my criteria. We didn't use backup heat last winter. I do keep it higher at night if it is particularly cold out.

It is actually 44 panels - had to count from this pic. Not perfectly located at all. That is west facing.

In my area, the pool pump has to run below freezing - not something I would do if off-grid.

No batteries here. We have no TOU pricing, $14 a month base charge which has net-metering. So battery would just for proof of concept and electric outages. We are at 3 years next month and have just 2 hours without power once.

Small point Mike - you don't have to size for cooling. In an off grid situation, I would probably size for heating if it was higher but lots of variables to consider. As you probably know, high insulation helps reduce heating loads more than cooling. Ours is very close to balanced. I imagine in the drier West, oversizing cooling probably isn't too big of a deal - which you might do if you size by heating load.

The panels are Q cells 295 Mono - so about 38 feet x 22 ft if I did my math right and spacing is minimal

David you could size the heat pump for heating, but then you may be oversized for cooling and have poor humidity control. For dryer climates with multi zones or stage it may not be a problem.

Your roof is ideal for solar panel installation. You must of put a lot of thought and planning into it.

Thanks for the info David Cary.

Just a point of comparison -

I live in Northern NY where the average Dec temperature range is 19-32 F. My roof-mounted solar panels were installed in December of 2019, so I now have two full years of data. I have 54 304 watt panels which produced 499 kWh in December of 2020 and 705 kWh to date in December of 2021 (today is December 28).

On Dec 19 we had a snowfall which has not yet melted, although most of the snow slid off the roof last week. My heat source is an electric boiler (resistance) feeding in-floor radiant piping in the basement and in the main floor and it is plenty to keep the home comfortable. Although I am grid-tied, usually my electric bill is zero, although I don't have the exact data conveniently available.

I think if I had an air-to-water heat pump system, I would be easily able to supply all my heating needs from the solar panels.

@David Cary David, you mentioned that your Trane SEER 15 2-ton heat pump uses just 1.8kW when running. How did you figure out that number? I can't see any wattage info in any spec sheets for any heat pump. When you say you only use the 2-ton downstairs does that mean the area you are heating is actually just 1900 sqft (1/2 of 3800)? Do you have the Manual J data for your house?

I have a whole house energy monitor and the heat pump is very easy to see in the graph as it runs for a long time - and is one of the bigger spikes. I do agree this info is very hard to find.

I heat downstairs and the upstairs stays comfortable. Our downstairs is bigger than up so I think up is more like 1600 sqft. There is an open stairwell that transmits air and heat fairly well.

I am sure I have a manual J somewhere. It was not perfect but it was at least done by our energy rater and not the HVAC company. Is there something you were curious about specifically?

@David Cary Hi David. 12kWh for a cold winter day is quite good, so I'm just trying to "reverse engineer" the number. Even with a top-of-the-line HSPF 13 heat pump the math still gives me almost 3x that, so I was curious if your square footage or Manual J heating load numbers were significantly different from mine. It sounds like the square footage might be part of the difference since I’m calculating for 2900 sq.ft. My Manual J heating load is 32,000 BTU/h for a 31 degree "design day".

I found it and was reminded about some of the give and take. It started at 47k/hr and ended at 42k/hr. At the time, I thought that was too high based on some of their numbers but I gave up after getting down to 42k.

They had an estimated ACH at 4 and it was 1.9 in the end. By rough math, that "mistake" is 4k. So even just fixing that mistake has us at 38k. I suspect 35k is probably closer. Then we have a large southern window exposure (with overhangs) which doesn't effect Manual J but does lower heating bills.

Design temp of 23. Mind you a temp that is breached for 20-30 hours per year if I had to guess.

I would not be doing HPSF of 13 if your design temp is 31. That sounds like north Florida....

Similar to D Cary's experience but with equipment that's AC only and not heat pumps, the electricity use I've experienced is that a 2 ton unit running for one hour uses +/- 2 KW so 2 KWh and a 4 ton unit +/- 4 KW if running for one hour so 4KWh.

Re: Elmer J Fudd

“Similar to D Cary's experience but with equipment that's AC only and not heat pumps, the electricity use I've experienced is that a 2 ton unit running for one hour uses +/- 2 KW so 2 KWh and a 4 ton unit +/- 4 KW if running for one hour so 4KWh.”

Does anyone understand what he just said?

IMPO

SR

Sure. The question was, what is the electricity consumption of a heat pump.

I shared that my AC units (which I guess you don't know, are heat pumps that run in just one direction) consume about 1 KWh per ton. 4KWs per hour with a 4 ton, 2 KWs per hour with a 2 ton. D Cary said "but only use the downstairs 2 tons. It uses 1.8 kw when running." My experience is comparable.

If you have no experience with this type of equipment, it's okay to sit out.

Do I need a ground source heat pump? Maybe you'll recommend I can use it to heat under-driveway heating tubes to defrost snow and ice? No matter that there's no weather like that where I live, you'll still suggest it.

Re: Elmer J Fudd

“the electricity use I've experienced is that a 2 ton unit running for one hour uses +/- 2 KW so 2 KWh and a 4 ton unit +/- 4 KW if running for one hour so 4KWh.”

You still have no idea how to read or interpret electrical specifications. You should stick to shoes as your stock and trade (Elmer is a shoe salesman, not an HVAC anything) as you offer no useful new content.

To the OP, how has this helped you and why is this suddenly about Elmer (again) ?

IMPO

SR

The heat pumps don't generally list electricity use in the specifications. In fact, it is really hard to get the information. And that is fairly frustrating to people trying to plan things like solar or generator or batteries.

Or even someone trying to decide between different models of heat pumps with different SEER or HSPF specifications.

I have no idea why my 2 ton heat pump with a HSPF of around 9 only uses 1.8 kw when running when the caclulations would say that it should use more like 2.7 kw (24,000/9). Perhaps, my nominal output is only 16,000 BTUs in heat pump mode. (of note, my unit almost keeps up at design temp in a estimated 35-40k loss house).

The manufacturers have not made this information easy to figure out and so it takes some backwards calculations to get there. The general public has very little idea about HSPF and I personally wouldn't trust the HVAC salespeople to be the ones to educate them.

Home owner's are left wondering how to sort these things out.

Would Elmer's observation or mine be more relevant if we were HVAC installers or salespeople? There is a small percentage of people employed in customer-facing HVAC that have engineering/physics degrees, truly understand the thermodynamics involved and spend time educating consumers.

I have talked to plenty of HVAC people and they don't know how to answer my question. The "math" they use gives the wrong answer (it's only designed to size the HVAC system not the electrical supply system), which is why I posted here to get real-world results from people who 1) have heat pumps and 2) have a way to measure how much actual energy (kWh) they use in a day to run the heat pumps. That's the number (or at least one of the numbers) that will help specify a solar battery system. I now have three data points. I'd like to get more, but I'll take what I can get.

To OP margaret17:

I understand that you’re in California and that you want to build a 3400sq ft house. That your nighttime temperatures are as low as 30˚F with an avg. 40˚F nighttime temperature.

You’re looking at Carrier Greenspeed, in-floor radiant and a ‘Source’ that will require the lowest or least amount of ‘Lift’ (difference between source temperature and indoor temperature) possible to achieve the greatest efficiency.

“How many kWh will I use over a 24h period in the middle of winter to keep my house warm? I can't find kW info in spec sheets for most heat pumps, and even when I do, I still don't know if that means the pump will be running 24/7 or something else. There must be some math out there to calculate this.”

“Also, I'm looking at the Chiltrix CX34 Air-To-Water Heat Pump, which is supposed to be even more efficient than conventional heat pumps.

I just feel like there should be some formula out there that I could use to actually calculate the kWh usage.”

“We've been debating a lot about the radiant vs. forced air question. Contractor is recommending both, which sounds like over-engineering. I'm not terribly concerned with cooling. We will have a lot of passive cooling via window placements and ceiling fans. We don't have AC now and generally only regret it about one week a year. Even on most hot days, nights are cool. Though I suppose I need to plan for things getting worse. And it will be a lot cheaper to add ducting (or radiant) during construction than try to retrofit later. Every new high-end house around here seems to be doing radiant. Don't know if it's just "trendy" or beneficial.

For me the big benefit over forced-air is energy (electricity) savings. Since we are off-grid, I'm trying to find ways to save energy costs without sacrificing comfort. This is why I'm trying to calculate the mid-winter scenerio. This is when my panels are producing the least, but my heating needs are the highest, especially at night when panels are producting nothing.”

“That’s another efficiency tradeoff calculation that I’m finding it hard to get numbers for. That is, I need propane BTU in vs. heat BTU out along both paths. I can’t find any useful numbers in any product spec sheets. E.g. like the startup surge. I agree it’s important. So where do I find that data? I’m looking at manufacturer websites and their “technical” spec sheets are a joke.”

I hope I’ve summarized most of your situation and questions; I get what you’re asking! You already sound like you know more than many posters.

Before answering, I’d first like to understand your motivation for being ‘Off Grid’? What are your electric, gas and propane rates and where do you think they’ll be in 5-years time?

IMPO

SR

margaret, the load calculations needed to size the HVAC equipment for new construction will be able to tell you what heat loss figures your home will have. "Heating" means replacing heat lost with heat. Or raising an indoor temperature by adding more heat than is lost. Compare heat lost with heat needed to be added to determine how long the run times will be. Longer is better from a comfort standpoint but of course that can mean more power consumed.

I'd be surprised even with the mild temperatures you describe if an affordable, completely off-grid system can be counted on to produce enough power to charge or recharge enough batteries and provide for electricity consumed with a heat pump running. You probably would be well served to have a backup power or heating source available, maybe both. A large permanent generator using propane and perhaps in addition a propane boiler.

The OP wants kWh usage. That's 1000 watts (1 kW or multiples thereof) consumed over a 1-hr period. The kW (wattage/1000) draw should be reasonably known via the specifications of the compressor & fan and the indoor blower. Then kWh can be determined for a maximum-usage day by assuming the unit will run 100% of the time (24 hrs), either with no defrost periods (involving strip elements) or with x-number of defrosts for x-mins each. That's not possible to calculate? There may be some variables involved but the OP probably isn't expecting six-sigma accuracy.

Margaret - can you turn on your messages? I can't say that I know how but I did receive one from someone who happened to live a few miles from me (many years ago). Might be helpful as specifics are probably not applicable to the general population.

My browser crashed with a long winded post (maybe saved you some time....)

But I had done some calcs on battery size/generator time etc. In my climate and my use, I need about a 20 kwh battery and a small generator. The smallest I see is 10 kw which is plenty for me even on a record cold/cloudy day with 22 hours of heat pump run time.

The cost is somewhere like $20k in batteries and $5k generator. Given that, I am probably waiting for a V2G solution unless our utility goes PG&E on us. I think in CA, you can often justify based on costs.

If I had water based fossil fuel heat, it doesn't change the cost - still need the generator and the battery size is needed anyway (for a/c). Or the generator events per year (which is 2 or 3). Around 1/22/22 we had a 200 kwh deficit over 4 days. It would have been perhaps 100 kwh deficit with fossil fuel heat. And maybe 85 with water based circulation. Either way, a generator event. The average temp for the day was 26 degrees with 3 defrost cycles (total of 2-3 kwh) and 22 hours of heat pump run time. 2 kwh were generated.

Of note, we have relatively sunnier winter days than much of CA. My winter generation is relatively better than typical CA posters.

How many kWh will I use over a 24h period in the middle of winter to keep my house warm? I can't find kW info in spec sheets for most heat pumps, and even when I do, I still don't know if that means the pump will be running 24/7 or something else. There must be some math out there to calculate this.

Hi Margaret17,

Here is how I would approach the calculation. If the heat pump is already installed the power consumed can be calculated by measuring the current draw while it is running and then multiplying by the voltage (usually 240 V). You can buy a clamp ammeter for about $40 and measure the current at the electric whip connection feeding the heat pump. This will give you the number of watts. To calculate KWh you need to multiply the number of hours each day the heat pump will run. This will vary greatly by how cold is on any given day.

If you want to calculate how much current if a heat pump you are considering. Then I would use the SEER and EER ratings of the heat pump. Even though these ratings are for cooling, they can give an estimate of how much power will be used during heating. Find the model and size you are considering and look up the actual amount of BTUs it will produce. Divide the BTU number by the SEER and EER values. The calculation with the SEER is an average power over a typical season. The calculation with the EER is the worst power draw. This is the power used when the outdoor temperature is 95 degrees. I would use the worst case number for planning.

In both cases you have to estimate the number of hours you will use the heat pump on a day. I would also worst case this number to something in the range of 12 - 16 hours per day. That could be way off, but at least you have some numbers to start your analysis.

Thanks all. The “motivation” for being off-grid is that there is no grid connection at the property. PG&E won’t even tell me how much it would cost unless I submit an application for service. However, others suggest it could be $300-500K. Given the reputation of PG&E, no thanks!

I am definitely planning to have backup propane systems. I don’t expect the heat pump to cover 100% of my heating needs, but I do want to minimize my propane burn. The goal is to size my batteries to cover, say, 95% of typical heating needs. Batteries are still relatively expensive so it doesn’t make sense to oversize them to cover emergency situations that might happen a few times a year. Propane isn’t cheap either, but there is a point where it’s more cost effective to burn propane than to add another battery bank.

I can’t measure the heat pump usage since the house isn’t built yet. However, I can easily come up with a number that massively oversizes the batteries by multiplying either the heat pump kW rating or the Manual J BTUH by 24. For example, the heat pump I’m considering takes ~2.5kW so if I assume it’s running 24/7 that’s 60kWh/day. That’s a LOT of batteries. Similarly, the Manual J heat (loss) number can’t just be multiplied by 24. Let’s assume 35,000BTUH heating from Manual J. That’s 840,000 BTU/day. Using an average heat pump with HSPF of 9 (HSPF = BTUs / (kWh x 1,000)) gives 93 kWh/day. Even more batteries!

Based on real-world data, both of these numbers are wrong. It makes no sense for me to install a 100 kWh battery bank. (Or 300 kWh, or 500 kWh depending on how many days I want to last when my panels aren’t producing much energy.)

It *is* possible to come up with a better approximation using Manual J, but it takes more work. What the Manual J numbers gives you (unless I’m mistaken) is the input heat needed to maintain a certain indoor temp given a certain outdoor temp. To size the heat pump you put in the lowest expected outdoor temperature (let’s say 30) and the highest desired indoor temperature (let’s say 70). However, that number will be very different if the outdoor temperature is actually 50 (for example during the day vs. at night), or if the desired indoor temperature is actually 60 (for example at night when you don’t want it as warm). So the BTUH actually changes, sometimes dramatically, throughout the day, hence you can’t just multiply the Manual J worst-case number by 24.

I did the “correct” math for my house for a typical winter day and get a more reasonable number of around 30 kWh/day. However, that is still very high compared to what David Cary is getting which is why I wanted to know more about his setup.

Just a thought, Can you just install 20 kwh of batteries as a start but have the option to double that later if your assumptions are incorrect? After a year of real world data on generator events and cost of those events, you can make a better decision.

Obviously, you should have help with design from an energy consultant but I can understand wanting to know things for yourself.

In your position, you probably should oversize your solar and optimize for winter - more vertical placement on a south roof. You should also have a little bit east and a little bit west to extend the length of time getting generation.

At some point, using batteries to get heat is not particularly cost effective compared to superinsulation - if you haven't started building yet.

If you like it cool at night, remember that heat pumps don't do a morning warmup that effectively. On a cool morning, I get 1 degree of temp increase per hour so I don't usually go below 65 degrees inside. Morning sun helps quite a bit.

Batteries are not only expensive, they are finite. Their lifespan is a combination of time and cycling. That should be factored into the financial decision.

30 kwh per day on a cold day sounds about right. It is 40 kwh for me but my cold is a bit more than your cold. Of course on that kind of day, you would just heat with propane unless it was sunny. This winter, there were 9 days that I had 10+ hours of runtime - 10% of winter days. Those days would be average 50% propane and you are about 95% of heat is heat pump. I would get there with 18 kwh in a day. Somewhere about 20 kwh in batteries.

Are you planning to size the solar panels so they will charge fully charge the batteries on a daily basis? If so then you have to think about rainy days when there is little sun, or when the panels are covered with snow for several days after a major storm. You are going to have to account for those scenarios.

You also have to design the batteries to be able to handle the start up current of the heat pump and any other large appliance it the house. An electric hot water heater is another large user of electric power.

On my energy monitor, there is no significant start up current of the heat pump. Not anything that would challenge a big battery.

OP is in CA. Most of CA doesn't rain much and snow is not likely given a design temp of 31. Possible but not likely a big issue. Certainly not days of coverage.

But long rainy periods or snow are covered by a generator.

The reality is that with a generator and a decent battery, off gird is not hard. It doesn't make sense for most of us but it isn't that hard.

I would suspect, she is sizing to cover at least 75% of days with a battery to get another 15% of days. The details matter if you are shooting for generator for 10 days a year or 30 days a year. Probably 10. Which is probably a 10-15 kw solar and 20 kwh battery.

A heat pump hot water heater is a really cheap battery.

I see why you’re off grid. Given that a heat pump is part of your plan, what I would recommend beyond the obvious of a tight, energy efficient envelop to bring down the size, energy requirements and cost of the equipment as much as possible and given that you live in California, I would recommend an air-to-water heat pump system. Mainly because 1-integrated system can provide space heating, air-conditioning and DHW. The other reasons are that a liquid hydronic system offers the greatest active electrical efficiency, hydronics also offers the possibility of thermal storage that air-to-air cannot offer and that a solar thermal liquid system can be tied into the HP system and requires minimal electrical energy to operate.

Design for the lowest fluid temperature possible to supply all your heating needs and the least amount of ‘Lift’ in heating mode. I suggest designing for a water temperature of 110˚F for heating mode using a combination of in-floor radiant and fan coil units deigned for the correct Btu output at 110˚F and chilled water for air-conditioning.

Thermal storage might mean that you can heat and cool your home at night with minimal electrical usage, minimal generator usage and minimal natural gas usage, as you will be using less active heating and cooling while your PV panels are producing nothing at night. Mostly low wattage circulators and small blower fans for air-conditioning, possibly condensate pumps required with less compressor run time.

Regarding equipment run times, you would have to tabulate the Bin Data or ‘Mean Coincident’ ( averaged or ‘trimmed’ data) Bin Date for your location to determine the total daily hours of run times for the size of selected HVAC equipment, then determine how many hours of operation your selected equipment will run at each stage, along with blowers, pumps, defrost elements (also at each stage), etc. This is done after Manual J calculation and equipment selection. This is still at best a highly educated guess. The real results might not be known for a couple of years after commissioning, based on real weather, of the equipment selected and adherence to installation best practices and the addition of solar thermal if you so choose. When done well, the systems should perform as well as or better than the design estimates. Some tweaking will likely be required and should be included in the installation budget.

As for the estimate of power to run equipment that you have asked about, look at the link provided from a manual. Page 82 for example, provides the electrical specs for the compressor in both watts and amps for both heating and cooling at various outdoor temperatures for water produced at either 105˚F or 120˚F as well as COP and cooling specs.

Full electrical specs are found on page 85 that indicate a ATW-55 would require a total of 104A (compressor), 27.6A (outdoor unit) and 4A (circulators) for at total of about 135A at 230V or about 31kW at 230V Locked Rotor Amps (LRA) just for starting. You would need a generator of an even larger capacity to have the reserve capacity to complete the start. Speak with an electrician and/or the manufacturer.

Nordic Air-To-Liquid Heat Pump

IMPO

SR

Are such products known in California and commonly used? Are there contractors near the proposed building site with experience in installing and servicing such equipment? Are you recommending this without asking if the home also needs air conditioning?

Don't you ever ask any questions to understand needs and parameters before making what seems like the same one or two recommendations no matter what or where?

The 2 challenges I see

- cost of install and maintanence. KISS really helps. Solar PV, air-air HP and even batteries are mature technology that are nearly idiot proof from a design standpoint.

- cooling at night is rarely a problem. Dehumidification is a problem that is much harder to deal with in a system you are describing

But sure - water can store heat and moving water is way more efficient than moving air. So given the right local expertise, it can be helpful. But what is the cost increase? I would expect something over $20k (probably more like $50k) - and that money is probably more helpful in batteries/panels/envelope upgrades.

The start up current surge for a heat pump lasts a few milliseconds. I doubt an energy monitor has the frequency response to capture and display such a short event. A generator would also have problems starting up the heat pump. We had an interesting discussion about this topic a few months ago. It is something that needs to be designed correctly up front.

The control system which manages the generator, battery, and solar panels would an interesting engineering problem to solve. I am not aware if anyone sells such a system.

I think the plan right now is to start with something like 40 kW battery system, make sure it’s expandable, and add more later if needed. We’ll be taking a similar approach to the solar panels. Basically, put in the largest system our roof can support. We are hoping 20kW, but we might not have enough room. If whatever we roof-mount ends up deficient (i.e. we are burning too much propane), we would add a ground-mounted system. Yes, I want to size the PV system to fully charge the batteries each day. We don’t get any snow, so no issues there. Sadly, these days, we don’t get much rain either. :-( Backup generator will also be around 20kW since we’re planning to use the same one as for construction power. I do plan on maximizing insulation and envelope tightness. Also adding heat recovery ventilation. In our current badly insulated house (single-pane windows, 2x4 frame which may or may not have any insulation), we don’t run the heater at night very often.

The Chiltrix system I am looking at is an air-to-water chiller. I plan on using it for radiant floor heating and domestic water. It also includes a buffer tank option. I agree that new tech is always risky. They claim they have around 100 installed near me so at least I wouldn’t be the first. I’m also considering ground-source heat pumps since this is new construction and I have the space for it.

I’ll have to look into the startup current issue. Something like 31kW would be a problem.

@mike_home Not sure if this is what you meant, but SimpliPhi includes a power management system with “Automatic Generator control for additional standby power”.

Brave choice. In your shoes I'd probably stick to something better known and established. I'd forgotten about minisplits, which modulate and are efficient, a good choice in an off grid situation. A ducted install with many minizones would probably be very efficient.

I find it interesting that the OP themselves mentioned air-to-water heat pumps and the proliferation of radiant that they are seeing, that is why I highlighted it. Most on this site feel that modern hydronics are foreign, difficult and not much is know of these systems as they are too new, finicky and that there’s no one to service them.

They are all wrong. You have all probably been in office towers, high rise and other buildings built years ago that are both hydronically heated and air-conditioned - in every part of North America. This is a mature efficient and reliable technology with readily available technical expertise. It’s just that it’s now beginning to cross over to the residential markets due to its proven reliability, efficiency and with the huge governmental push to decarbonization and electrification of virtually all things HVAC.

The next generation of HVAC technicians are eager to learn about hydronics coupled with heat pumps. I see this all the time at technical seminars and training, many young techs are as excited about forced air as they are about oil heating, even though forced air is an important sub specially of HVAC. Its nature will evolve as it becomes further integrated with hydronic heating and cooling.

I have not suggested geothermal due to the OP being in California and air-to-water HP efficiencies are quite high year round at those outdoor temperatures. There is little doubt that geothermal is technically almost always the best choice, which is why I usually suggest it - despite the fact that I invariably get savaged by the same posters on this site, in this case the OP mentioned it first.

What I like about geothermal is that it is always the most efficient source for active heating and cooling. It is about the only form of solar energy readily available for harvesting 24/7 - whether the sun shines or not. A form of solar energy that provides storage like a battery that is local, is resilient. Unlike electrical storage batteries, the ground will likely not require maintenance - ever! It also doesn’t transfer as much of our wealth abroad as do some other renewable technologies.

As the OP mentioned, “I’m also considering ground-source heat pumps since this is new construction and I have the space for it.” That being the case, you might consider a horizontal 'Slinky' that would take up less space. Also, if you will have domestic water supplied by a well, depending on local regulations, you might simply have a well drilled that can supply both domestic water and geothermal (Open Source) water supplies from the same well and just have one additional well drilled at the appropriate distance down stream to discharge the water used for geothermal back into the same aquifer with the only modification to the water being a modest change in temperature of just a few degrees higher or lower, depending on whether the HP is in heating or cooling mode. You can discuss this with a driller, in particular the electrical requirements for pumping that will effect your PV systems. A horizontal Slinky would be more efficient to pump as it’s a ‘balance’ system.

Geothermal Water-To-Water HP’s coupled with hydronics also offers the possibility of thermal storage that would mean lower electrical energy required when the sun is not shining.

Integrated Solar Thermal might also be investigated to be ruled in or out of your plan.

IMPO

SR

A soft start kit could be added to reduce the high current surge of the heat pump's compressor.

I would be interested to know how control system turns on the generator when the battery is about to die.

"You have all probably been in office towers, high rise and other buildings built years ago that are both hydronically heated and air-conditioned - in every part of North America."

This is a home, not a large public building. Most have their HVAC equipment on the roof, are you suggesting that be done for this house?

Also, that same large number of large buildings have forced air heating and cooling. The water system simply carries the heated or chilled water to heat exchangers through which interior air passes to be heated or cooled. The same process as with conventional furnace and AC systems or split heat pumps in homes.

Is your saying that these building are hydronically heated and cooled an intentional misuse of the term? As far as I know, hydronic systems in residential structures refers to floor embedded tubes and or external heat radiators of various sorts. These are not conducive to air conditioning. Not a stand alone term used for this but rather what I thought was called hydro-air systems or chilled water systems. That's what large buildings have.

"How many kWh will I use over a 24h period in the middle of winter to keep my house warm?" Kwh consumption will stay fairly constant any day of the year when the heat pump is running but thermal transfer of energy from the outside will vary as the outside temperature varies.

To determine the BTU output a performance graph and balance point is required from the actual unit used and must be known. The balance point is when the performance (COP) is at 1 and would equal the power consumption of the unit times 3.41 to give the BTU's per hour, a unit running at a COP of 2 would be (3.41 x 2) x kwh. But all of this means nothing without knowing every variable of the house you're building so there isn't an answer to your question.

I believe you're looking in the wrong direction and focusing on what you need to heat the house but should focus on what you need to not heat the house, and critical to living off grid. Applying many passive building techniques not to heat the house is the first step, and will last the life of the house and not just the the life of the extra batteries you will need to satisfy peak demand. Shifting the peak demand would be the first priority along with slowing heat loss and increasing the time constant of the house.

Shifting peak demand not only involves a well insulated and air tight home but also involves energy storage in the floors and walls as thermal mass. During low demand the heating sources charge the mass, like a trickle charger on a battery waiting to be used when demand rises. In passive solar much of the charging source is radiant heat through south facing windows from the sun, and wouldn't be a prerequisite to take advantage of thermal mass to shift demand but I highly recommend using solar for the undeniable benefits it give.

Time constant is a fancy way to calculate how slow a home will lose heat when also using thermal mass in a build as energy storage. It's complicated but a well built certified passive home depends on thermal mass and some can go many days before needing to recharge the mass. There is a formula but will make your head spin.

Passive doesn't necessarily mean all glass on the south side and nothing on the other sides. In fact I wouldn't recommend more than 25% glass on the south exposure do to overheating, and still would use glaze on the other sides for light, views and opening for fresh air. Common sense is all one needs, and if you want a wall of windows on the North you will lose an uphill battle. An average window will lose more heat than the entire wall it's in so the amount should be regulated.

I could go on and on as a favorite subject of mine since the early 70's but this is getting long. One thing about Fsq4cw, he is correct on everything he stated and isn't something to shy away from just because some display negativity because of there own ignorance on the subject. The entire Baylor campus is heated and cooled using hydronics from a central point through miles of pipes for 4 decades. St. Paul Mn, energy district does the same using thermal solar, thermal storage and biomass serving 200 buildings in the business district including the capital just to name a few.

There are specific terms and their use and misuse matter.

What I think you're describing for St Paul is what's called a district heating system. A large one that's quite old is in New York city and provides steam energy. Hot water systems are quite common in parts of Europe. Over there, for heating and not for cooling. Radiators not forced air distribution. The steam or hot water often being a by-product of electricity generation.

I think a system that heats a campus would probably be described as having a centralized heating and cooling plant. Are these not called chilled water systems and not hydronic? Buildings are conditioned by forced air systems that use heat exchangers and blower fans. Doesn't the term "hydronic" in residential use imply radiant heating?

I once had a challenge explaining to a not so knowledgeable person light was a form of radiation. I'm afraid if I told you Elmer that radiant is also a form of radiation I would be up to the same challenge.

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It should be stated even though obvious, hydronic heat makes a ton of sense in a large building. That doesn't necessarily carry over on a scale of 3600 sqft. It really isn't a great argument.

Domestic hot water is really a small amount of electricity in a heat pump setting from a relatively cheap device. So, while that is an advantage of water based systems, it is a minor one.

But, we can all disagree about stuff and still hopefully give salient advice. I am biased to simple systems that are manufactured in the millions per year. I am also biased toward the envelope vs. Complex or expensive systems. Doesn't make it the perfect answer.

margaret17, this thread is priceless. I'm in a similar situation, but with a larger house. We're building 6,000sqft and now considering methods for off-grid after we got absolute sticker shock from PGE install price.

My home requires dropping a new pole, then taking the high-voltage lines underground pipe about 1000'; two underground vaults are required. Total price came in at $250,000+ (pole, trenching, conduit, wire and transformers). In addition to all this, we also paid $14k (sunk cost now) for an outside designer to perform an "Applicant Design" to get our global approvals and joint-trench for internet/phone. Good news, this was the first quote and we can bid it out to various contractors. Bad news, even if this quote is 50% higher than the next one, it's still extremely expensive.

And of course, with us having PGE...it would be smart to also have some form of large solar arrays, battery storage (due to PGE NEM2), and also a solid generator when they shut us off during wildfire season for days on end. So tack on another $60k-100k for all that. :-)

A question for anyone in California still following this thread. Anyone know if you CAN do off-grid legally now for new construction? I know there was some reference to the old energy code requiring "interconnection", but interconnection was not really defined anywhere. Now I see the latest energy code calls out batteries and generators, so it's looking more promising.

The solar guy I'm talking to has done multiple off-grid homes in California. Per my (unprofessional) interpretation of the code, there is nothing saying you can't do it. Now, whether or not your local jurisdiction will allow it is another story. I'm fully expecting a fight with my local jurisdiction about this, but I'm used to fighting them. I think my PG&E connect cost would be similar. I don't think a jurisdiction can force you to pay a quarter million dollars for something you don't even want just so a private company can make some money off of you AND get some free electricity. PG&E is a racket.

I think a jurisdiction can absolutely require you to pay $250k - if that is what it costs to make something code. Code is about future owners (among other things) and them not being surprised after purchasing something. Imagine a future buyer not realizing what "off-grid" means and there probably not being a requirement to disclose that anyway. They arrive with their electric SUV thay they expect to charge with 200 kwh every night since they drive 130 miles to work each day. Or 30 years from now, the batteries and solar have lost some capacity - again you sell the house - and the next person can't a/c all day with the windows open without running out of juice.

I 100% don't agree with a requirement for a grid connection but I can see why some knuckle dragger on a permitting board would not see it that way. Grid interconnect is redundancy and safety. Now they probably can't force you to sell to PG&E but who cares about that anyway.

When you fight with your jurisdiction, you should definitely know the arguments.