Battery Backup For a Tankless Water Heater #RWTH

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Tankless Water Heaters Need Electricity, so should you have a battery backup?

A tankless water heater was in the plans for my new garage from day one. The little closet where the old style tank water heater and old furnace where hiding perfectly fit my new garage refrigerator. Surprisingly it was not a hard sell for my wife. The fact that it used less gas and saved water was a perfect #RWTH fit for me.

As the garage was getting finished our neighbor came over and asked about the two pipes sticking out of the side of the house. The pipes were the inlet and outlet for the new tankless water heater. When I told her, she laughed at me saying “Tankless water heaters stink, the first time the power is out when your wife needs to work that thing will be gone.”

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It didn’t take long to realize that since tankless water heaters had to be plugged it, no power meant no hot water. It was double jeopardy. No tank to hold hot water and no electricity to run the little computer meant zero chance of any hot water.

Being the smart guy that I am, I figured that a quick trip to the big box store for a battery backup would solve the problem. I picked up a 650VA battery backup for $99. Hundreds less than the 500VA battery backup the plumber tried to sell me. 650VA had to last longer than 500VA right.

The Noritz model that I have doesn’t specify the electrical load, so I had to add a little meter to figure out what was going on. After our first power failure, 30 minutes later I still had zero hot water. The hot water heater only draws about 25 watts when it isn’t making hot water and about 150 watts when the fan and pump are running when it makes hot water and runs the recirculation pump. 65w when it only makes hot water.

battery backup time for tankless water heater isn't much
APC UPS Runtime Chart

What I figured out is that the battery backups made for computer have a larger inverter to handle 300w plus computers and monitors. The inverter itself takes a lot of power even if the tankless is only drawing 65 watts or less. If you look at the run time charts, the curve is steep. Once you break 50 watts the time drops quickly.

The part of the chart that makes no sense is that far left side. It looks like if there is nothing plugged in it would last forever. With only the 25w computer I should get 90 minutes, and at least 15 if I run hot water. We got about 20 minutes without running hot water and 10 when we ran the hot water.

Finding a long lasting battery backup that wasn’t $500 didn’t happen. The Hugo was the only one we found that claimed to last up to 7 days under normal use. It is only a 550 KVA battery, so my guess is they use a very small inverter that is very efficient. I don’t have one yet, but come back, I may end up with one very soon.

If you are thinking of getting a battery backup for your tankless hot water heater, think about the runtime. Once the computer backup starts beeping, you only have a little while before the cold showers begin. Even the biggest APC backup at the local store was limited to 45 minutes.

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