How to Calculate Solar Battery Backup Time

Buying a solar battery or a solar inverter system at large is not like buying Television in an electronics store. You need to do a little bit of calculations in other to get the desire output and value for your money. This article is written to make simple how to calculate solar battery backup time.

First Step: Calculate the electric load

You have to have an inventory of all electric gadgets you intend to run on the inverter, and then write out all there power outputs in watts. For example, 70watt TV, 35watt Bulb, 90watt fan, 30watt laptop. all totals 225watts.

Second Step: Determine the Inverter Capacity

You calculate the inverter capacity in VA by dividing the your power load in watts by the inverter efficiency. The amount of power lost during conversion of DC to AC is estimated to be around 25 per cent. Hence we have VA = watts/0.8. i.e, 225/0.8=281VA. This means your power requirement is 281VA. It is advisable to buy an inverter with a slightly higher capacity than this, say 400VA. (These numbers are not accurate as gadgets have different capacities). Sometime because of instability of gadgets startup power, you can give enough space by buying inverter that is three times the capacity of the power output.

Third step: Calculate Solar battery power Backup

To calculate the battery backup time, multiply the battery size in AH by the number of batteries needed by battery voltage (usually 12V) by battery efficiency rate (usually 0.80) then divide your answer by your power load in Watts. i.e, AH*N*12*0.8/watts.

For instance, if you have a 100AH battery and your load is 281watts, your battery backup time is 100*12*0.8/281) =3.4 hours. If you buy a 200AH battery, your battery backup time will be 200*12*0.8/281, giving you 6.8 hours.


In summary, Before you buy your solar battery backup, it is important to determine your electric load. This will help you to determine the best kind of inverter and battery to buy. If you live in an area with frequent power interruptions, you may use the hours you are likely to stay in blackout every day and your power load to determine the most suitable inverter and battery to keep you powered up. The good thing about inverter battery is if you do your calculations right, you can manage and control your power usage.