[mike31154]

There are pros & cons to UPS as backup for something like a powerhead. As ReefPilot mentioned already, the key to run time of your powerhead is the ampere hour (a/h) capacity of the battery in the UPS, which typically is not that great, since the primary design is to keep a computer running long enough for you to shut it down without losing data. The math isn't that difficult, it's Ohm's Law in conjunction with the a/h rating of the battery in the UPS. The variable that's a bit obscure in all this is the Hour (h) and that's why you need to now the a/h rating of the battery in the unit you intend to use. Most of them do provide charts listing typical loads & how long you can expect them to last. In your case an ideal set up would be a 100 watt UPS with 40 a/h battery. You don't need a high power (1,000 watt) UPS to run a 5 watt load, but you do need a big honkin' a/h battery so that you can run those puny 5 watts for a very long time. Sadly, most UPS units are not built that way. Note that the 'a' in a/h is amps or I(current) as used in Ohm's Law.

P(watts) = E(voltage) x I(current in amperes). You already know the powerhead uses 5 watts, so you need to calculate the current (I) it draws at it's nominal operating voltage (E). Transpose the formula & plug in the numbers I=P/E. Once you have that number use it in conjunction with the a/h rating of the UPS battery(or any deep discharge battery) & you will have an approximate run time for your 5 watt powerhead. Most reasonable sized UPS units should handle a 5 watt load for a few hours at least. Keep in mind that as any battery ages, it's original a/h rating will decline, even if it's not used. In some cases battery life is actually shortened if it's not given a deep discharge from time to time.

Personally I'm not a fan of these residential consumer grade UPS units. I've never owned one but my Dad did & it failed after a couple of years. I dismantled it to see what was up & found the battery bulging at the sides. It could have burst or started a fire IMO. Some of these units have next to no ventilation & that's why they use these sealed batteries. I think they are time bombs. I see that some of them now offer replacement batteries so the unit is not trash once the battery fails, as it ultimately will. That's great, but I doubt that the average consumer keeps an eye on these things & essentially run them 'til they quit. They are plugged in 24/7 & even though the chargers are pretty smart these days, constant charge cycles eventually cause the electrolyte to evaporate & since you can't top them up, they overheat & fail.

The alternative is of course to use a good old deep discharge wet lead acid battery that you can keep an eye on & maintain. Might be what's in those Can Tire boost units. The issue now becomes proper ventilation of the hydrogen gas produced during charging. Probably not that big an issue if there's only one battery & it's a reasonable size.