DavidR, I had discussed with my Hawaiian contact regarding the increase cost of solar panels and the recent administration discouragement of PE, so he suggested the PowerWall route. Our local provider is still offering a 30% rebate but I am interested in energy independence also.

I don't really know much about the Powerwall. I seem to recall an initial impression that it cost a fair bit for the storage capacity it provided, but I might be remembering wrong.

There are other battery options if you go the 100% off grid route, and you'd need to examine the pros and cons of each for your situation.

I will say that I don't think electric resistive heating is a good choice when you're making all your own electricity. I'd be looking instead at some kind of fuel (natural gas, propane [$$}, wood, or even oil if you must).

If you still want to heat electrically, you'll need something with a COP (coefficient of performance) well above unity, which is where you are now. A ground source heat pump ("geothermal") is a traditional electric heat source for cold regions, but today I'd look into mini-splits. The Mitsubishi Hyper Heat units can keep heating well below zero F. You can use the cheaper +5F capable splits if you supplement with wood or other fuel heat.

Much depends on the size of your house, how well sealed and insulated it is, and what your budget can stand.

Again, though, I'm somewhat out of my depth here; I've never actually done any of this, just read up on it. I think you'll get better answers on the RE forum.

Powerwall provides an extremely small amount of reserve power for a house. Cost would be obscene (and space requirements would be very high)to put in enough units to provide power overnight for house with electric heat. While I hope batteries improve enough in a few years, the costs of storage greatly exceed the cost of a generator. I have looked at using a backup battery (enphase A/C Battery--similar to powerwall but compatible with Enphase microinverter system) while hooked up to the grid, these systems still need a lot of work. A grid tied system gets its timing (60Hz frequency wave) from the power company and must follow it exactly. When you go off grid, you must generate your own timing signal--that is why there are different inverters for grid tied/not grid tied systems. The A/C battery must also decide when to charge and when to discharge(discharge to replace line power or only during an outage)

Ideally, I would like to charge a battery with solar, then feed surplus back to the grid while power is on. Switch to draw off battery AND charge battery with solar panels when the grid is down. Theoretically this is doable, but off the shelf components do not seem to be available

If you're in NY, you might look to the Amish. I was watching a show the other day about barns in Pennsylvania and they spent a lot of time discussing how they are ideally suited for solar (the roofs face the sun in order to provide warmth in the winter months for the cows). The Amish have accepted the use of solar to run the dairy machinery as well as to recharge the batteries they have to use in their buggies for lights.

I used to work for a company that made something we called a 'solar appliance'. It was about the size of a squat refrigerator and contained several telecom class AGM batteries (50v). It was simple to install (3 connections: Line, Load, PV) and provided something like 150 Ah during an outage. It was grid-tied, normally, and came with a meter reading power in and out. Battery storage could be doubled if needed.

Thing was, most of the components were off the shelf. The batteries were telco, but marine would also work if you put them outside or somewhere ventilated. The invertor/charger was made by Outback but we were looking at others (but they cost more).

"Ideally, I would like to charge a battery with solar, then feed surplus
back to the grid while power is on. Switch to draw off battery AND
charge battery with solar panels when the grid is down. Theoretically
this is doable, but off the shelf components do not seem to be available."

Actually, they are available, and have been for several years now.

I know of at least two major manufacturers. Outback makes intertied inverters with battery backup., and SMA (Sunny Boy) offers an accessory called Sunny Island that you add to one of their grid-tie inverters for backup. I think Schneider Electric also offers one.

I even found a cheapo inverter from Taiwan on Ebay which seems to do all that, at a fraction of the big-name inverter cost. I didn't look at it very closely, though, so I'm not 100% sure. I also have no idea what its reliability would be.

As you'd expect, the cost for an intertied system with battery backup is going to be higher. But if you want the capability, it's available.

Yes, the fact is that for the average homeowner a backup genset is less expensive than purchase and maintenance of batteries and their associated demands. That all needs to be recalculated when natural gas is not available. liquid fueled gensets are more costly in terms of $/btu and upkeep demands.

Batteries, ah batteries. It is easy to forget that the most widely used batteries are about 85% efficient when new and they go downhill from there. Are the Tesla batteries magic? That is what we really need, magic batteries that are inexpensive and nonpolluting to make, last forever, and 100% efficient. Wikipedia has a pretty good entry on the Powerwalls including a section on ROI. That is what it boils down to. You just have to run the numbers. 1) Liquid fuel genset. 2) conventional batteries 3) Tesla might be closer to magic than lead-acid, but they are probably not magic enough for the average household in the USA.

People that are off grid in cloudy areas like upstate NY usually, maybe always have batteries and a genset. The genset acts as supplement in times of low sunlight. I think that the necessary part might be to condition the batteries. I could be wrong, but I think that it is difficult to provide the conditions required to equalize the batteries (put them into a moderate overcharge state for some period) with the PV cells only. At least now at least one major genset supplier is marketing DC gensets for off grid residential use. It seems like a good approach to lowering fuel consumption, wear on the equipment and noise generation.

Gensets running on Diesel or gasoline are unreliable and high maintenance. Most rural areas still have propane available. Besides being a cost effective way to heat, cook and run a dryer, propane is a relatively clean generator fuel and does not cause problems with deposits in the engine.

Regarding Lithium ion batteries, they are expensive but quite reliable and long lasting provided that they are properly controlled. They need circuits to be sure they are not overcharged or discharged. Control circuits must be sure the battery is not discharged too quickly as this also causes permanent damage. They need to be kept warm while charging. In colder climates, charging off a 110V circuit is extremely inefficient since much of the electricity must be used to keep the battery warm! 220V chargers face less of an issue not only because they charge faster, but because the heat released by charging generates much of the heat needed to keep the battery warm enough to charge. As Tesla has shown by selling electric cars in Norway, all these problems are manageable with good design.

I believe Lithium ion or similar technologies have a lot of promise, but the price is not where it needs to be yet. If you are looking at the Powerwall, focus on max amp draw requirements. Besides, total watthours of capacity, the batteries are very limited in draw rate. You are likely to need to pull from several at once to run demanding electrical loads.

I should have considered propane. Yes, the clean burn aspect is huge on the maintenance side. Exercise needs to be watched closely with diesel to avoid slugging up the prime mover.. If one already has a heating fuel in place, it makes some sense to use that for both. I don't know how relative cost breaks down at this time for fuel oil vs. propane.

Sorry this post is long. If you get too bored, just skip to the next-to-last paragraph.

So far nobody has yet found the perfect battery. All of them are balances of cycle life, cost, safety, temperature range tolerance, specific energy (Wh/kg), energy density (Wh/l), specific power (W/kg), charge efficiency, and probably some factors I've forgotten about.. No one battery type has them all nailed down, and probably none ever will.

The battery that might make its way into a home power system today would most likely be lead or lithium ion. A few others have been used but are now mostly on the sidelines. These include nickel cadmium, nickel iron, and nickel metal hydride (which is really just improved nickel iron).

Lead is the traditional battery for home power systems, but it has significant downsides. Specific energy and energy density are low. Flooded types need watering and periodic testing. Cycle life is good for heavy, expensive tubular plate types, not so great for conventional flat plate batteries.

Lead used to be the uncontested leader in cost per watt-hour delivered, but in recent years lead metal prices have increased and it's no longer as competitive as it once was.

Nickel iron is legendary for extremely long cycle life. I've read that some of the original NiFe cells built by Thomas Edison are still in service! Their specific energy and energy density are better than lead, but not dramatically so. Efficiency is a weak point because they gas too freely (too much energy going into electrolysis), and this also requires careful ventilation. The few NiFe manufacturers charge nosebleed prices, which limits their use.

Nickel cadmium has long cycle life and decent specific energy and energy density. Their big problem is that cadmium is nasty, toxic stuff. Because of this they face severe restrictions in the EU.

Nickel metal hydride batteries have sort of been lost in the lithium shuffle in recent years. They have long cycle life and quite good specific energy and energy density. The problem has been that they're encumbered by restrictive terms issued by the patent holder, but I think the relevant patents may expire later this year. The other big negative with NiMH, as with other nickel chemistries, is that nickel is expensive. If given a chance, I think NiMH could still become competitive, but don't hold your breath.

Recently lithium ion batteries have grabbed the spotlight almost to the exclusion of any other type. Note though that they aren't all alike. There are several different chemistries with different balances of those concerns I mentioned above. Beyond that, each chemistry is optimized differently by different manufacturers with different design goals.

In general I'll agree that lithium ion can be a bit touchy about temperature, certainly more so than lead.

Some lithium ion cells shouldn't be charged below freezing (0C / 32F). Others can accept a charge down to -10C, and a few are OK as low as -30C. Again, it depends on what the battery manufacturer is optimizing for.

In most home power systems this isn't a problem, especially if the battery is located in conditioned space. The battery's own internal resistance will keep it warm with any normal current use. Insulation and controlled ventilation will keep the temperature in the acceptable range.

OTOH very warm temperatures can reduce lithium batteries' cycle life. The more fully charged they are, the more they suffer.

So you limit the charging rate at low temperatures, charging slower and slower as the temperature falls, and eventually not at all. On the upper temperature end, you don't charge the battery completely if you expect, or already have, very hot temperatures.

Because lithium batteries are fussier than other types about over-charging and over-discharging, they almost always are sold with battery management systems (BMSes).

Good lithium ion BMSes know what their batteries are capable of. They monitor the cells' state of charge (or at least their voltages, which is not quite the same thing) and usually also temperature. They control the charging and discharging functions to suit, for a compromise between capacity and cycle life.

To cut to the chase, based on Tesla's track record (no pun intended) with their cars, I expect that they'll be more than competent when it comes to their Powerwalls' battery management.

As for whether a Powerwall or two can store enough energy for your household needs, that depends on many factors, including how you heat and your overall energy efficiency. Most off-grid folks have to make some lifestyle adjustments when they're no longer drawing their energy from an apparently bottomless source (the grid). If they don't, it probably means their pockets are pretty much bottomless. :-\

That is not at all too long, DavidR. There should be lots of interest from anyone that wants to understand how difficult it is to store electrical energy at a reasonable cost. I fear that the general public throws out the idea of batteries as the solve-all without the smallest concept of what a battery is. There was a recent political debate in a city near me that really irked me in that respect. There are other ways to store wind and solar, but need to work on a large scale (pumping water uphill, for example.). Regenerative fuel cells and fuel cells in general have been the next best thing for decades now :-(

I learned a thing or two. I did not know that the common lithium secondary cells have such a high internal resistance. Good to know that NiMh may become more free with a patent expiration as well.

From a personal standpoint, I don't look at NiCad loss as such a big problem, but I am still angry that the EPA wrecked my SLR camera when they took away my exceptionally voltage-stable mercury cells. I briefly thought of trying to make them myself. I've seen purpose-made examples in very old lab equipment.

Lithium cells don't have especially high internal resistance. They have some, like any battery; how much depends (like everything else) on what the designers' goal was. It's pretty typical for specific energy and specific power to move in opposite directions, for example. They design for the application.

NiMH has pretty well replaced NiCd for small cells. Disposal problems are much less, and it has higher specific energy to boot.

OTOH you can do some things with NiCd that you can't with NiMH. For example, you can charge a NiCd cell at C/10 nearly forever with few consequences. With NiMH you'll lose capacity. Unfortunately many designers of NiMH rechargeable consumer gadgets still design their chargers as if they were charging NiCd cells.

I don't entirely agree with you on the impracticality of batteries for utility scale storage. I agree that mechanical storage will probably be with us for a very long time, but I do see a potential for lithium batteries to take over some utility scale storage.

The reason is electric vehicles. They may be a niche item for a long time here in the US, but several EU nations are already planning to phase out fuel powered vehicles (or at least require them to be hybrids) in the next few decades. That's a LOT more batteries, almost certainly all lithium type. What will we do with all the spent cells?

It turns out that lithium cells that have lost too much capacity to still use in electric vehicles (say < 70% of design capacity) are fine for stationary applications. So I think we'll see these partly depleted cells used increasingly for utility scale storage. I could be wrong, but IMO that's one reason that Tesla is getting into the business.

That would be a good thing, because lithium's charge efficiency is within hailing distance of 100%. You are never going to even approach that with any kind of pumped storage.

Fuel cells can't do it either. From what I've read, a hydrogen fuel cell's theoretical maximum efficiency is 80-85%, with real world numbers probably no better than 60%.. Electrolysis is around 40% efficient, for a round trip efficiency of around 25-30%, and that doesn't even count the energy you'll use storing and transporting the hydrogen. High temperature electrolysis is more efficient, but then you use energy for the heat input, driving efficiency down again.

As with EVs, it's quite possible that the US will continue to lag behind the rest of the world in renewable energy. We seem to be dragging our feet now, with the recent protectionist efforts to limit PV imports and prop up coal and gas. But again, it's a big world out there; it's passed us by and soon will lap us. Renewables are closing the affordability gap elsewhere, if not here.

I think that long term the US tariffs and especially the utilities' lobbying to abolish or restrict net metering will backfire. If I were building a house today, I probably wouldn't even bother connecting it to the grid. I expect that before long others will start to think that way too.

The Japanese I believe are moving on with sodium batteries. I saw a documentary where Hitachi was putting these into office buildings and such in a city as a test. Their problem with home use would be their size, something like a chest freezer, and the fact that they run at 350* with liquid sodium. However, they are running up the manufacturing and reducing the price of the cell, the issues being that the cell is a porous ceramic and is brittle.

I understand there are other sodium chemistries that work at room temperature and have the potential in a few years to be cheaper than lithium.

DOE is looking a sodium cells as power storage for large wind generators. These batteries on of the size of a rail car.

I remember when NiMH first starting to compete with NiCd. I looked and thought one has twice the energy than the other, but half the cycles so that's a wash. Yes, NiCd produces power and charges in the same class as lead acid and they are at the top of the class for those types commonly-used by our peasant class as opposed to industrial. My biggest use was in camera flash and NiMH could not touch the NiCd. Neither can they for the RC crowd either.

I assumed that heat, slower charging, and slower power delivery equates to high internal resistance.

So sad that what might be a world leader in renewable energy tech and manufacturing might be letting it all slip away. I think that I've seen discussion about mobile batteries going to stationary as their capacity slips somewhere else, but i'd forgotten it. Thanks for refreshing.

If I had a battery of the week, this week it would be lithium thionyl chloride. I have some transmitters for a wireless (Zigbee/mesh) alarm/monitoring system that needed new batteries, but I had not looked into them before. I saw that they have an amazing shelf life due to insulating passivation on the anode that breaks down when power draw begins. I had noticed a sizable capacitor on the board. I don't know if that is supposed to make power withdrawals from the battery less frequent or smooth out the power available while it is breaking down.