Will a 120-volt, 12.5 amp tankless electric water heater fill a large, therapeutic bath tub with hot water?

I’ve done a lot of research into tankless water heaters over the years, and though there are models out there that will work for you, the one you’re looking at won’t. It simply can’t provide hot water fast enough.

Tankless Water Heaters Offer Two Advantages:

1. they’re physically smaller than tank-style heaters
2. they are more energy efficient while keeping a tank full of water hot between uses

The price you pay for these benefits is the very high rate of energy consumed by tankless heaters when they are actually heating water. For instance, a quick online search led me to an electric unit that heats water to 105ºF at a flow rate of 3.7 gallons per minute. This is about as fast as one bathtub faucet flows and just barely hot enough for filling a tub like yours. Despite the relatively small output, this unit consumes more power than is provided by 14 conventional 120-volt wall outlets, each on their own circuit. To actually install a heater like this you’d need a 240-volt, 120-amp circuit, and that’s more than many household electrical systems can handle, given all the other demands. There’s also the very real prospect that householders may someday be required to pay extra for peak electrical demands above a certain level. Industrial users have faced this situation for years. This means that even though tankless electric units might not use much total electricity, they could trigger financial penalties because of the very high, though short-lived strain they put on the electrical system during use.

The situation with tankless water heaters is entirely different with gas-fired models because high rates of energy use are more easily met with this technology. If you’ve got access to natural gas or propane, tankless technology will work just fine for you.