You need more of the specs on the power to determine the requirements.

A lot of today's electronics do not like modified or worse sine wave electrical producers.
All UPS's are rated for a load and time period. Increase the load shorten the time.

I would call tech support for your new heater and find out the specs on the power needed to keep the unit functioning

 

I'm not sure what a "standard" ups is. There are all kinds of battery capacities available for a basic 120v, 1400va ups.

The 90w is probably only when it's running. If you're not using water, it's usage is likely a few watts. You'll need to pick a UPS before we can say how long it'll work for.

IE. Something like this will last maybe an hour of heating water. Much longer on standby.

APC Back-UPS Pro, 1500VA, Tower, 120V, 10 NEMA 5-15R outlets, AVR, LCD - BX1500M | APC USA

BX1500M - APC Back-UPS Pro, 1500VA, Tower, 120V, 10 NEMA 5-15R outlets, AVR, LCD | APC USA

www.apc.com

 

I'm not sure what a "standard" ups is. There are all kinds of battery capacities available for a basic 120v, 1400va ups.

The 90w is probably only when it's running. If you're not using water, it's usage is likely a few watts. You'll need to pick a UPS before we can say how long it'll work for.

IE. Something like this will last maybe an hour of heating water. Much longer on standby.

APC Back-UPS Pro, 1500VA, Tower, 120V, 10 NEMA 5-15R outlets, AVR, LCD - BX1500M | APC USA

BX1500M - APC Back-UPS Pro, 1500VA, Tower, 120V, 10 NEMA 5-15R outlets, AVR, LCD | APC USA

www.apc.com

Fair point - I was thinking something like this:

Amazon.com

I could go a little bigger if that's not going to get me enough run time like this:

Amazon.com

Ideally I would like to have something that could run the hot water for approximately 2 hours over a 24 hour period. That's more than enough than I'll ever need probably. At minimum an hour of water usage over 8 hour period.

 

The idea of a UPS is to give you ample time to back up your work and turn off your computer.....that's it.
Obviously the larger one you get, the longer it will last.
Maybe you should have stayed with a regular gas water heater to avoid this problem.

 

I'm sure I'm not the only one using a UPS for something other than a computer. They're intended for other purposes besides "back up your work and turn off your computer". I just need some time between power going out and firing up my generator to keep my wife happy and since it's a natural gas water heater it doesn't require much energy. I should get about 45 minutes on a 550w 1000va UPS that I just ordered which is plenty, and that 45 minutes is assuming the hot water will be running the whole time. It's not a bad idea to put a line interactive UPS with AVR on a $1500 appliance regardless.

 

just an update - the amazon basics ups I was previously using would cause the water heater to throw a fault code when the power went out. I have since upgraded to an APC UPS and now it works just fine. The gas tankless water heater has minimal electrical draw.

 

The thing with 'Amazon Basic' or 'Amazon Essential' anything is they are sourced from all over the place and any given product might change suppliers depending on where they can get it cheapest. Chances for product or tech support are probably pretty slim. At least with a name brand like APC or CyberPower you can get some level of support.