How to calculate Battery run-time?

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1. Jul 4, 2016

Artahir

I can't get my head around question about energy consumption, energy supply and run-time. I think I forgot (Im sure I forgot) something in my equation. I am trying to find a way to calculate for how long could UPS supply my PC I dunno why I can't find Amp-hours of UPC on it but let's assume it's 1500Ah and my PC consumption is 150W. I tried to do some digging first I found some explanations but none of them made sense to me I feel like it can't be so complicated. Or when I found easier equations it didn't make sense (the result). could anyone give me answer?

2. Jul 4, 2016

Svein

What voltage is the battery in the UPS? If it is 24V, 1500Ah you have a raw capacity of 24⋅1500Wh=36kWh. Divide by 150W and you get 240h.

This is just a coarse calculation, assume at least 10% less going from theory to practice.

3. Jul 4, 2016

Artahir

Exactly this is my calculation which I tried based on my logical assumption. but I used Voltage 230 AC for my desk PC and it doesn't make sense because if you add Voltage it means more hours and with 230 Iam around 2k hours which is stupid right?

4. Jul 4, 2016

Staff Emeritus
You need volt-amp-hours, not amp-hours. An APC 1500 (retail \$400, so you know where that fits in their product line) stores 220 V-A-hours. (792 kJ in SI) That has the energy needed to run a 150 W device for 88 minutes: 720,000 kJ / 150 W = 5280s = 88 minutes. This is an absolute upper limit: you don't have perfect efficiency, and you can't get the last bit of charge out of the battery. APC claims 50 minutes for that load.

5. Jul 4, 2016

Tom.G

Watch out for Watts versus Volt-Amps. Consumer Battery Backup devices are advertised in VAs not Watts. Usually the Power Factor is 0.6 but some of the off-brand ones use a Power factor as low as 0.4. So a 1500VA rating could be anywhere between 900W and 600W; that is the level that the overload shutdown is set for. Also, many modern computer power supplies have Power Factor Correction built-in to keep their power factor high, 0.8 to 0.95. That being the case, the actual runtime could be only 1/2 of the 'computed' runtime. And no-load losses of Battery Backup also seem to be significant but I haven't measured them.