It's prohibited for one. And for two, to actually run an electric tankless will require most of the power that you currently have in your panel. 125-200 amps are needed. If you live in a cold climate, electrical is not really doable at all because of the incoming winter water temperature. Gas tankless is much easier to retrofit, and even it may require new gas service for the home.

Hi, is the cost of operation an issue? So let's rule out electric tankless from the beginning. It doesn't achieve anything that you want, and adds a lot of cost to the electrical side of things. Your generator would not be able to provide power for electric tankless . Options are: -Standard Electric Resistance Tank -Hybrid Electric Tank (very low operating costs, but needs air from garage, basement, etc from which to pull heat). -Standard propane tank -Propane Tankless (less operating costs than above) A 200k BTU unit at those water temps will provide all the water you need. -Hybrid tankless w/tank (low operating costs compared to standard propane tank, but high volume) I would go with Electric hybrid and avoid propane. Propane has had some crazy cost volatility. Something to consider on your new build is grid tie Solar PV (electric) with battery backup. This might be in the ballpark cost wise if you were otherwise considering a generator and propane install. Grid-tie solar would provide some percentage of your electric power that your home needs and feeding any excess back into the power company. The battery backup would provide you with emergency power. You are in a good part of the country for Solar and since it's a new build, you can do it right

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.

Okay, so I supplied you with the numbers and the assumptions that got to those numbers. It's possible that some of the assumptions (like the cold water temp) is wrong... but what we designed for was 3 gpm (slightly more than a shower) and 50 degree temp rise. The heater to do that has a max draw of 100 amps. That's all math. You may only load a circuit to 80% of its rated capacity. That's code. Now, if you are able to run this heater on a 200 amp service depends on the rest of your power usage. You have 60 amps left to work with. It's borderline. Patricia Colwell Consulting is not correct. An electric tank heater and an electric tankless heater are both almost 100% efficient at heating water. An electric tank heater has standby losses of about 3% in normal usage. The issue is that any tankless heater must heat water very quickly. That requires a lot of energy over a short amount of time. For that reason, both gas and electric tankless require very large inputs. For example, my gas tankless heater is 200k btu/h which is twice as much gas input as my furnace. There is no magic here at all. You can do the math yourself - 1 BTU = the energy to raise 1 pound of water 1 degree F. Water is 8.34 pounds per gallon - so it takes 8.34 BTU to raise a gallon 1 degree F or 1251 BTU per minute to raise 3 gal of water 50 degrees in one minute or 75,060 BTU/hour. There's 3412 kw per BTU/hr so that's about 22 kw (the heater was a 24 kw model- so slightly higher). dadoes is correct - this heater isn't going to be running at 100 amps all the time. However, you do have to design it for the times that it will run at maximum capacity (when someone is taking a shower and the inlet water temp is 50 degrees)