I have a line-powered (standard AC) device that sinks 50W in steady-state and I need to power it for 10 hours in case of service interruption.

I'm assuming that I can use, say, a 500W UPS (50W * 10hr = 500Whr = 500W * 1hr) to power it for 10 hours.

But where can I find that kind of UPS? Looking at, for example, CyberPower, I see the model I currently own will supply 600W for only 1 minute (if I'm reading the spec correctly here, under product features "Runtime (half/full)." (Actually, I don't know how to read that since it says full = 1 minute, half = 6 minutes, so maybe it isn't linear?)

Anyway, where can I find a battery backed UPS that will supply 50W for 10 hours?

Bonus questions: The UPS specs generally also use VA as units as well as W. Is this due to RMS factors for sine wave power or what? And does it affect my calculations?

  • I recommend rewording your question to directly ask for a product recommendation (and also add more specific requirements). Asking about where you can find something is off-topic.
    – Adam
    Mar 14, 2016 at 6:30
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    Thanks for your edits. But asking "where to find" something is an idiom for asking for information, it isn't a literal request to name a store or website. I'm also not specifically interested in a product recommendation - though I'll take any - as I'm more interested in getting an answer to "what kind of battery backed UPS products are there that supply power for an extended period of time, i.e., on the order of hours not minutes." Maybe I have to know how to read a specification differently, maybe I need to look at specific suppliers, maybe UPSs don't "scale" linearly.
    – davidbak
    Mar 14, 2016 at 6:36
  • I'm sure you mean it as an idiom, but not everyone will see it as such. I recommend replacing the current question with the one you just provided in your comment because it's a lot more specific and clear.
    – Adam
    Mar 14, 2016 at 6:57
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    Based on a quick search, your best bet is probably to build your own from a small RV inverter/charge controller and a marine deep-cycle battery or two. Computer UPSs are optimized for high load/low runtime, and get quite inefficient at low power draw.
    – Mark
    Mar 14, 2016 at 7:14
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    @davidbak can confirm what Mark is saying. I too have a need for a 50W PSU. My fish tank pump pulls 48W and a PC PSU from CyberPower (I forget if its the 850 or 600 model) runs exactly 108 minutes before it completely craps out.
    – user588
    Mar 14, 2016 at 17:59

1 Answer 1


Computer UPSs are optimized to provide high power levels for a short period of time, the idea being to bridge a brief flicker in the mains power, or let you shut down cleanly during an extended outage. At low power draws, they are quite inefficient, wasting much of the battery's power as heat.

Your best bet is to build your own UPS from an inverter-charger and a marine/RV deep-cycle battery. Something like this 600-watt inverter-charger ($400) plus a 60AH-80AH battery should work; given your low power draw and assuming you don't switch to battery power very often, you might even be able to get away with using an automotive battery.

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