Calculate BUT!?

Question

Hi there,

I want to know how to calculate the following:

- The battery Ah I need for UPS 8KVa

- Number of batteries I'll need to operate the above UPS for 40 minutes

I hope you could give me a general equation I could use each time I want to install a new UPS.

Answer 1

But you need the rating in watts, real power, to calculate the battery size. This is usually a lot lower than the VA rating, and is the only one that counts. The VA rating is not very useful.

Say the UPS is rated at 3 kW or 3000 watts, typical

Battery size depends on the power it has to deliver (3kW) and the time (in hours). multiply them together and you get 3*40/60 = 2 kW-hrs

A 12 volt battery with 20 amp-hr rating has about 200 watt-hr rating, which you get by multiplying voltage 12x20=240 and derating 20% for losses.

So if you have 20 amp-hr (200 watt-hr) batteries, and you need 2000 watt-hr, dividing you get 2000/20 = 100 batteries. Clearly not a good result.

If you switched to 100 amp-hr batteries, about the largest easily available, you need 2000/100 or 20 batteries.  You can see why UPS hold times are usually in minutes. And they tend to rate battery life at half power or lower.

In general, you get the wattage rating of the UPS, W in watts.

Get hold up time needed in hours, T

Battery capacity in Watt-hrs, B

(12v battery, B = 10*amp-hr rating)

then battery capacity needed = W*T in watt hours.

Number of batteries = W*T/B