# How to Calculate UPS Batteries DC to AC Efficiency?

For simplicity's sake, we will use this APC UPS for example: https://www.apc.com/shop/id/en/products/APC-Power-Saving-Back-UPS-Pro-550/P-BR550GI. It has 330W rating, loaded with 66 Watt-Hour (Volt-Amp-Hour in their site), and 91.7% efficiency when using 20% load (can be accessed from their Efficiency Graph).

Theoretically, the battery can supply 66 Watt for 1 hour. But their Runtime Graph showed that we can only get around 39 minutes with 66 Watt load. It is supposed to have 91.7% efficiency, but we just get about ~65% (39/60 minutes).

Can someone enlighten me where's those remaining efficiency gone to? We have about ~26.7% difference in efficiency here.

If I need 50 Watt and 2 hour runtime, I can get 5 hour 40 minutes from BR1500GI + External batteries with total Watt 187 + 372 = 559 Watt-Hour. BR1500GI rating is 865 Watt. With total power draw 50 Watt translate to 5.7% load, efficiency ~86%. Rough estimate how much hour we can get from the batteries is 559/50 = ~11.2 hour, but as we can see in the link, they calculate it can only supply power for 5 hour 40 minutes. That about ~51% (340/672) efficiency.

I've recently have better APC UPS that shows the VA and Watt usage, SMT750i. I tested it with LG 32" TV. The wattage is ~50, but the VA is ~150. So, looks like that's the problem. The Load efficiencies is for VA, not Watt.

• peukert effect looks to give 65% capacity when using 1C discharge rate on a battery Jul 2, 2020 at 18:57
• @uglyoldbob I've updated my question, with another example. In that example, we only use about 1/10 C. But still, The efficiency is that bad if we refer the estimated time from the link. Jul 2, 2020 at 20:10