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I have a Takagi T-K3-SP tankless gas water heater, which is installed outdoors. It still needs electricity though, so when I lose power at home, there's no hot water.

The manual states it uses 86W during operation, 6.2W during standby, and 111W during freeze protection (but my area doesn't get cold enough). I thought of just plugging the heater in to a UPS backup battery for my computer (rated 600VA) during power outage, but it's hardwired to AC outlet as shown in the photo.

What are my options? I looked at Hugo backup battery for tankless water heaters, but that seems to just provide a regular 3-prong AC outlet.

So I'm now thinking of putting a UPS battery system indoors. Behind the water heater is a crawl space. The AC power for water heater comes from a junction box inside this crawl space (see photo below). The white Romex cable is what goes outside and supplies power to the tankless heater. The yellow cable is the power supply. The metal conduit goes to the HVAC unit.

Instead of white Romex cable going into this junction box, can I create a power inlet like this next to the junction box, and terminate the white cable there? I will also create a new power outlet from the junction box. I will buy a pure since wave UPS like this, and plug it into the AC outlet. And then I will use an extension cord to connect the UPS to the inlet.

To summarize, electricity flows from the junction box --> new AC outlet I'm creating --> UPS backup battery --> white Romex cable --> water heater outside.

Please share your thoughts on this setup. Thank you!

(I got this idea from the DataComm TV wall mount kit I have, where I put an inlet next to an existing outlet by the floor)

enter image description here enter image description here Crawl Space Junction Box

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Technically, a UPS could be wired in, but there are a few caveats:

  • It depends on the electrical codes and gas codes in your area. The thought of making a change which might affect a gas-powered appliance would make me very cautious. Another worry would be an improperly isolated device feeding power back into the line, creating a shock hazard. Be sure to use an approved device, installed by a technician qualified in both gas and electric maintenance.
  • Though the UPS has ample power to operate the heater, the run-time is limited by battery size. A quick "off-the-cuff-" calculation: Assuming an 18 ampere-hour battery in the UPS, at 12 volts, gives a maximum of 216 watt-hours. Assuming a 20% duty-cycle, i.e. moderate use of water, the heater would use ~18 watts on average. With an 18 amp battery, you'd have about 12 hours of operation.
  • The UPS would need to be kept indoors, because a battery loses significant capacity in cold weather, and because the UPS is doubtless not designed for outdoor use.
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