# How to calculate the power of an uninterruptible power supply (UPS)

I was looking at the datasheets of some UPSs and I noticed some oddities:

https://www.durantesrl.it/assets/files/serie-compact.pdf

For example, the COMPACT 1000 model claims to have a capacity of 600W, but contains only 2 12Volt batteries with 7Ah ..

The doubt arose when I tried to calculate the wattage of the battery pack:

2x12x7 = 168 Watt

Did I do something wrong, or is the declared capacity not true?

## 2 Answers

You are confusing watts and watt-hours. Watt is a measure of power, while watt-hours is a measure of energy.

This means you can pull 600 W, but not for an hour. You can pull 168 W for one hour.

The watt rating gives you an idea of how much current you can draw through the device at any one time.

The amp-hour or watt-hour rating will tell you how long before the battery is flat.

• OK, perfect! :) thank you so much for the explanation! :) Commented Mar 10, 2022 at 17:52
• Re, "You can pull 168 W for one hour." Maybe. The actual number of Watt-hours you can get from a full charge can depend on the number of Watts your application demands. If some battery is able to continuously supply 16.8W for ten hours, then I would not be surprised if it could only supply 168W for somewhat less than one hour. How much less? I don't know. I don't have a lot of experience with that. Commented Mar 10, 2022 at 18:18
• @SolomonSlow That's a good point. Those numbers tend to be...optimistic. Commented Mar 10, 2022 at 19:04

Amp hours is not watts. Watts is Joules per second.

2x 12Vx7Ah = 168Wh

So this means the unit could supply 168W for 1 hour

Or it could supply 600W for ~1/3 hour (600W*0.28h is 168Wh)

• OK, perfect! :) thank you so much for the explanation! :) Commented Mar 10, 2022 at 17:53