# Calculating time for a fully charged UPS

I have a UPS of 1000 Volts connected with 2 batteries each of 150 Amp. How much time it will take to consume the whole UPS (after fully charged) when a device of 1Amp is getting electricity form that UPS.

Please also explain me the calculation.

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There are some missing data in your question.

• What voltage does the batteries have, I'm going to assume 12V since it is common.
• Battery capacity, you typed it as 150A but I guess it was 150Ah. Please note that a normal car battery on a car like a VW Golf has approx 60Ah so 150Ah is a quite big battery.
• Output power, you state that you have 1000V (Volts) out, but I guess you are talking about 1000W (Watts). And that should be maximum power.
• Output voltage should be either 230V (or 110V). I will assume 230V.

The first thing is that a device connected to the UPS and is using 1A, that is 1A at 230V or $230V * 1A = 230W.$

Then we go inside the UPS and have a look. We still has the 230W but since it is now a 12V system, we need to reverse the conversion into amps again and get something like $\frac{230W}{12V}=19A$.

And then we divide the battery capacity with this current and get something like, $\frac{(150*2)Ah}{19A}=15h$.

Please note that those 15 hours is the best value you can expect to see, but since there is losses in the conversion from 12V to 230V we probably loose some 10-20% of the energy, and that translate directly into a shorter time.

Let's say 80% of 15h would be approx 12h.

Then I must add that Fortunato does have a point. Batteries degrade over time and can't hold the same charge, so make sure you have some margin and check/service your UPS on yearly basis.

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Unless they are special deep discharge batteries (ie just cheap car/alarm lead-acid) you can typically only take 50% of the rated capacity without damaging them. – Martin Beckett Jul 11 '11 at 3:45

Battery capacity is normally rated in ampere-hours (Ah), not ampere. This means what it sounds like - if you have 2 * 150 Ah of batteries, you can (ideally) pull 1A for 300 hours like in your question (*), or 300 amps for 1 hour. This is an intuitive value of battery capacity but is not strictly speaking a measurement of the energy stored since this depends on the voltage which you pull the current at (and both of these vary during the discharge of a real battery).

You wrote that your batteries are rated at 150A, but you might have misread the 150 Ah figure, in which case your answer is above. You could also have quoted the maximum current capacity of the battery of 150A (typical of a car battery for example), which isn't related to the capacity at all and I agree with the other answer here that you cannot calculate it without further information.

To use the old water analogy - the voltage is, roughly speaking, the water pressure of the battery and the ampere is the rate of water in the stream coming from it. You would have no indication of the size of the reservoir so you cannot calculate for how long your device can run.

(*) This is of course assuming that you can connect your device at all, i.e. an UPS with an output of 1000 volts will not work very well if you connect it to a domestic 110V or 220V piece of equipment :)

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Don't think you can calculate this. I depends on the type, age and history of the batteries. All his factors make a big difference. If you rely need to know the TCT "total charge time" put them threw the cycle once and measure it. And remember this will change with time.

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## protected by Qmechanic♦Apr 9 '13 at 7:56

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