I would not use electric tankless for anything. Period.

If you're worried about an eventual leak from a tank, put it in a pan that drains to the outside. Problem solved.

Propane will probably cost you the same a electricity to use.

If you're worried about water running at the 2nd home, turn it off when you leave.

For the 2nd home, I would recommend tankless propane over tankless electric.

Hi weedmeister,

May I ask why you wouldn't use an electric tankless? Is it the cost of upgrading the electrical panel (which isn't applicable in our case)? If the cost of electricity is equivalent to propane, I'd rather use electricity so I don't have to have a huge tank of flammable liquid sitting on my semi-rural property in a fire-sensitive zone.

I also like the idea of tankless for the space savings in my primary house and the laziness factor in my second home (no need to turn it on/off each time we visit). In my searching, I also read we should NOT be using the vacation setting because of Legionnaires' disease. Oops, we've been setting it on vacation for the past 5 years and only turning it to HOT when we're there.

I don't want to take 150 amps of my 200 amp service just for a water heater.

Huge tanks around here are buried. Unless you mean something like 2000 gallons. I have an acquaintance who has one of these, but he also has a 40kw generator. The tank is behind the barn.

If you're really sure you want tankless (and you kinda seem to be), I'm not going to argue with you. I'll just say a couple of things and then I'll bow out.

Disclaimer: I'm not an expert on these matters. They just interest me and I hope to be more energy independent in the future, so I read up on them. I always welcome corrections from knowledgeable folk.

First point: IMO electric resistive heating of ANY kind basically just doesn't play nice with PV, off grid or on.

The only time it really makes sense is when you're off grid and your system produces more energy during the day than you can store in your battery.

This can also apply to some extent if you're intertied and your utility pays you wholesale or less when you sell your power, essentially penalizing you for conserving.

Either way, dumping your excess PV energy into water heaters is one way to store it for later use. However, it should be obvious that to store that energy, you have to use traditional storage water heaters, not tankless.

Second point: Peak demand matters. It matters to your electric utility, and it matters to your bill.

You probably know more about CA rates than I do, but it appears to me that right now California is more into time of use (TOU) metering than peak demand metering. However, more and more utilities are adopting peak demand metering for residential customers (they've had it for business for ages). So even if you're not dealing with it now, it may well be in your future.

Demand based metering watches your power (not just energy) usage. The higher it is, the more you pay for all your electricity.

In one demand scheme, you pay a surcharge based on your peak power usage (in kilowatts), In another, the price they charge you for every kilowatt hour you use depends on your peak usage -- the higher your peak, even if it's only once in the month, the more you pay per kWh that month.

Let me give you an example of the second scheme. Let's say your house's big energy hog right now is an electric range.. Most of the time, you just use one or two surface units at a time. So your peak demand is relatively low, and you pay (say) 14 cents per kWh most months.

But when you cook Thanksgiving dinner, you have all the surface units going, plus the oven, and a microwave. On that one day, you have a much higher peak demand. Your utility takes note. And that month, you pay 20 cents per kWh for EVERY kWh you use all month.

So even though that month's usage is only a little higher than usual, the final bill for the month is MUCH higher.

Now imagine what could happen to your bill if you add an appliance that increases your peak demand every day of every month. Guess what: when it comes to peak demand, tankless electric water heaters are MONSTERS.

Think about it.
With its mild climate, California is a great place for heat pump storage water heaters. That's what I'd get if I were you. Not tankless, and definitely not electric tankless.

And that's all I have to say about that.

When I lived alone I turned my water heater on twice a week for 30 minutes each time. My electric bill was very low compared to when I turned the water heater on full time (after my wife moved in). Living alone I took a shower every day and did not use the dishwasher. Laundry was done at her house on weekends. Now that the households are combined we use the hot much more regularly and leave it on.

DavidR: that was very helpful! I'm going to look into heat pump storage water heaters, as I have no idea what those are. I was told our HVAC system at the primary house is a heat pump, as it's all electric as well and the outdoor fan unit turns on whether it's AC or heat. I'm not sure how old that unit is, but perhaps we can get an integrated heat pump HVAC and water heating system?

I also wanted to chime in that we're already on Time-of-Use plans for both houses because of our solar PV systems. Fortunately, we rarely get into the "second tier" of electricity use because of solar generation. We also currently have two large electric water heater tanks at our primary house because there is no propane/gas connection to the house. Replacing 2 large (75-100 gallons each?) electric tank water heaters with an on-demand tankless or a heat pump storage water heater may even result in net electricity savings.

Thanks again!

I have posted often about electric tankless: I would not ever recommend them except for limited Point of Use (POU).

In addition to requiring massive electrical service and not really providing much in the way of savings, they don't provide the quantity of hot water you need. For example: A 36kw heater will provide about 5 gallons per minute (2 showers only) when the incoming water temp is 58. If the water was any cooler than that (winter) you wouldn't get even 2 showers at the same time.

You do not want to use electric resistance with PV solar. That makes no sense.

To avoid problems with a standard tank rusting out use a Marathon fiberglass standard electric water heater.

If you want energy efficiency and need it to be electric, use a heat pump (hybrid) water heater.

DavidR makes a good point about Electric Tankless and demand based billing. If you now have or would likely have demand based billing you do not want an electric tankless.