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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.

Edit #1: I've supplement material: https://www.apc.com/shop/id/en/tools/ups_selector/home/load/recommendations?power=50&powerUnit=w&operatingVoltage=230&voltageScheme=singlePhase&runtime=120&powerMargin=0&pageNumber=1&sortOption=RUNTIME_ASC

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.

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    \$\begingroup\$ peukert effect looks to give 65% capacity when using 1C discharge rate on a battery \$\endgroup\$
    – uglyoldbob
    Jul 2, 2020 at 18:57
  • \$\begingroup\$ @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. \$\endgroup\$
    – tonny
    Jul 2, 2020 at 20:10

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A 66 Watt-hour battery generally can't supply 66 Watts for 1 hour. Probably it can do 660mW for 100 hours, and 6.6W for almost 10 hours, but after drawing 66 Watts for 39 minutes, it can't supply that much power anymore.

This is because batteries are physical things that rely on chemical reactions to re-generate the charge that they supply. The chemical reactions happen at a finite rate, and as you discharge the battery, the rate slows down.

This isn't really an efficiency thing, the other 25% or so of the battery charge is still in there, but it can't be extracted at a fast enough rate to power a 66W load.

At higher rates the same problem becomes even worse, which is why most UPSes only last a couple minutes at rated load, unless they are provided with very generous batteries relative to the power handling capacity of their electronics.

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  • \$\begingroup\$ I've updated my question, with another example. In that example, we use about 1/10 C. But still, The efficiency is that bad, according to the link. \$\endgroup\$
    – tonny
    Jul 2, 2020 at 20:12
  • \$\begingroup\$ I've recently have better APC UPS that shows the VA and Watt usage. 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. \$\endgroup\$
    – tonny
    May 8, 2022 at 3:50

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