Yeah, although electric heat is 100% efficient it takes a lot of energy to heat water quickly. You’re better off going with gas for hot water on demand. I imagine the unit price is more, but the install price will probably be lower.

400V in residential sounds exciting. That’s one way to cut down on wire cost I guess.

Yup. That’s why people use smaller heating elements and slowly heat up lots of water in an insulated tank instead. Just the difference in the cost of wire you need to run is massive.

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I’m an electrician. I haven’t been impressed by most of the tankless systems I’ve hooked up. I would definitely go with gas unless you are willing to run some serious power. Most hot water tanks pull about 3000-4500W or 12.5-18.75A at 240V. A good tankless system is upwards of 18000W or 75A at 240V and some can be around 36000W which works out to 150A. This will easily be the single largest electrical load in your house, unless you also have an electric furnace or something. Depending the size of your electrical service and what else you have in your house you may need to upgrade the electrical service going to your house as well. Most Residential Electrical services are usually 100-200A in my area.