# Questions about inverting DC to AC

Hello,

I have a question, How can I calculate the working time for the following scenario:

I have 12V 7.2AH LEAD ACID SLA DEEP CYCLE BATTERY
Then I used DC 12V to AC 220V Power Inverter USB 1000W to give me AC

I connected one 100w lamp .. For how long it will work ? how can I calculate it

another question .. How can I calculate the watt of the inverter (to know which one I should buy) .. is it 220v x amp ? or 12v x amp ?

thanks

mfb
Mentor
I connected one 100w lamp .. For how long it will work ? how can I calculate it
You will need the efficiency of your converter to find the power needed as input.
Assuming your battery is designed to deliver ~100-150W (check this!), it can store 12V*7.2AH = 86,4Wh, so it will run for something like 30-50 minutes.

another question .. How can I calculate the watt of the inverter (to know which one I should buy) .. is it 220v x amp ? or 12v x amp ?
The power rating of the transformer is probably for the output, so you should use the 220V-side. If the losses in the converter are small, the other side should give a similar result (the current will be much larger there, to account for the lower voltage).

For power inverters, the rating is always for the load. If you have 1000W of light bulbs (a resistive load), you need at least a 1000W inverter (can be larger but not smaller).

Something else to consider besides the efficiency of the inverter is your battery. The faster you discharge a battery, the lower the Ah rating becomes. Usually the Ah rating is taken for a 20 hour discharge period. If you discharge over a longer time it actually goes up slightly, but if you were to discharge your battery in say 5 hours, the Ampere-hours you realize can decrease considerably (plus you can damage the battery if the drain is too high).

In your example of a 7.2 Ah battery, the 20 hour rating would result in a 360 milliampere drain (for 20 hours), but with a 100W load, the current drain would be over 8.3 Amperes so the 7.2 Ah battery rating would be diminished substantially. Lower ambient temperatures will also decrease the Ah rating.

Lastly, the Ah rating of a given battery usually falls with the age of the battery and how it has been treated during that lifetime (rate of discharge, depth of discharge, number of discharge cycles, etc.)

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