We just upgraded out house service from 125 amps to 200 amps, and the house is all electric, no gas at all. The same day that happened my 41 year old water heater developed a leak and one of the elements fried. I don't know if they are related, but at that age I think it probably is unrelated.

I'd like to replace it with a tankless water heater. The electrician who upgraded the main panel recommended a sub panel, since I also do hobby woodwork in the garage and that would give expansion for woodworking equipment. That sounds really attractive. My question is about electrical loads.

I live in the desert in southern California. This is about the coldest time of year and my inlet water temperature is about 65 degrees. I also calculated the flow rate of both showers in my house running at the same time and depending on the setting of the shower head it is between 4 and 5.5 gallons per minute. Granted that is worse case scenario. In the summer the water heater would be doing very little (but the AC makes up for that).

I'm not very familiar with electric tankless water heaters, but for that flow rate it looks like I'd need at least a 20kw heater, probably a 24kw.

The few 20kw heaters I've looked at generally require 2 double pole 50A breakers, and the 24kw ones require 3 double pole 40A breakers.

If I did a sub panel for that much power, the way I'm reading the situation is that I would need at least a 125A sub panel for just the water heater, with no extra space for future tool expansion. I know it isn't drawing that current often, but you've got to plan for worst case. I don't want to be running a table saw or welder, then a kid jumps into the shower and starts tripping breakers. What do you all think?

It is starting to seem like just replacing the tank water heater with another tank water heater is a very simple way to go.

  • 5
    Electric tankless is not good on only 200 amps. If your whole house is electric, then having an extra 120 amps for the heater is probably a pipe dream.
    – crip659
    Commented Feb 8 at 22:26
  • 2
    Stick with the standard tank.
    – JACK
    Commented Feb 8 at 23:06
  • Why tankless? Curious.
    – jay613
    Commented Feb 8 at 23:11
  • 5
    Heat loss from an insulated tank is very low. The cost of a service upgrade to run an electric tankless heater is almost certain to exceed any nominal power savings. Commented Feb 9 at 9:03
  • 1
    Both the "endless" concept and the "only use energy" concept are counterproductive. A good tanky water heater will use less energy and will obviously use much less peak power, and if selected appropriately will reliably provide as much hot water as you need even during a power outage, even if you have two showers running along with the laundry machine. If you frequently run out, get a bigger tank, or if you have one critical application, get a small point of use instant heater.
    – jay613
    Commented Feb 9 at 12:39

1 Answer 1


Let's see. 5.5 GPM at 8.35 pounds per gallon, so 46 pounds per minute. 2755 pounds per hour.

To raise 1 pound of water 1 degree F takes 1 scientific BTU ( not to be confused with the commercial BTU unit, which is a rate of BTU/hr).

You have 65F and want 120 so 55F rise. So at 2755 pound/hour rate, you need 151,000 BTU/hr rate.

What's that in watts? A watt-hour is 3.41 BTU. A watt is 3.41 BTU/hr. I'm getting 44443 watts or 185 amps.

I think you're gonna need a bigger boat.

Since you want air conditioning most of the time anyway, heat pump water heater sounds more feasible. Plugs into a common 120V outlet. Uses 1/4 the energy of a normal tanked water heater, but chills the utility room. So it helps to be in a place where you'd like the utility room cooler.

  • You can even put a duct on it to cool paces other than the utility room, when desired.
    – Ecnerwal
    Commented Feb 9 at 1:16
  • I've never heard of that type of heater. Definitely something to look into. With summer temps a little higher than 110, that sounds really nice.
    – JBurt
    Commented Feb 9 at 2:22
  • 2
    @JBurt Technology Connections has your back youtube.com/watch?v=zheQKmAT_a0 Commented Feb 9 at 18:26

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