Backup battery for tankless gas water heater for loss of power?

We have a Navien natural gas tankless water heater that is about 2 years old. At the time I was researching battery backup UPS systems and found out that many would not work properly for our Navien tankless water heater because they were not pure sine wave systems. We purchased the following PFC Sine wave system. UPS systems with active power factor correction (PFC) design are Pure Sine Wave. The best one for the money at the time for what we wanted was the following. It has worked flawlessly in testing and in a couple of actual power outages. You can buy the same model with less Volt Amps for a lower price, but just know you will have less run time.

CyberPower CP1500PFCLCD PFC Sinewave UPS System, 1500VA/1000W, 12 Outlets, AVR, Mini Tower,Black

My natural gas tankless water heater controller requires approximately 3A to run. The CyberPower CP1500PFCLCD PFC Sinewave UPS is rated at 9AH (Amp Hours).

9AH / 3A = 3 hours

So my tankless water heater will run for approximately 3 hours total cumulative duration during a power outage, or approximately 18 ten minute showers. Since it takes about 1 minute before the water turns warm/hot in the shower, then 18 nine minute showers or 9 nineteen minute showers as an equivalent example of maximum use before the UPS runs out of battery power.

Its possible that your make/model tankless water heater will work on a less expensive modified sine wave UPS battery backup. You should investigate that, but if you purchase a modified sine wave battery backup and it doesn't work for your tankless water heater, then that is likely the reason and you may likely need a pure sine wave system.

If you have a pure electric tankless water heater, you would need a significantly better UPS battery backup than what I listed. In fact, you might need a whole home generator to run a pure electric tankless water heater at the performance you desire as a UPS battery backup would likely be extraordinarily expensive for pure electric tankless systems, which could need 125A or more to run which is more than many homes 100A service panels. My 9AH UPS would only provide hot water for 4.3 minutes or less minus a minute for the water to warm up at the shower head or faucet.

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We have a Navien natural gas tankless water heater

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My plumber installed one for my friends, he told them what back up system to use.

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Tankless GAS!

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Definitely the way to go. Also recommend a whole house water filter installed at the same time to greatly reduce scaling and maintenance for the tankless water heater. You're supposed to flush a gas tankless water heater annually. My plumber said we can wait at least two years to do this with the water filter installed. After 1 year, I had it flushed with the descaler and the water coming out of the unit was clear. My plumber said without the water filter, the water would have a brown tint. I'm going to do the descaling maintenance myself going forward now that I understand how to do it.

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I am confused. What tank are you flushing with a tankless heater???

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I should have used the word "unit" instead of tank. There is no tank, but scaling will occur inside the tankless water heater. I'll fix my original post. Thanks.