Backup hours from battery Ah and load watts, or the Ah you need for a set number of hours
Runs in your browser. Nothing you type is sent anywhere. Updated on 24 Oct 2026.Backup hours equal battery Ah times volts times the number of batteries, times the usable depth of discharge, times inverter efficiency, divided by the load in watts. A 12 V 150 Ah lead-acid battery at 50% discharge and 85% efficiency runs 300 W for about 2 hours 33 minutes.
Under Work out, choose Backup time or Battery size.
Pick the Battery type, then enter the voltage and number of batteries.
Enter the Load on the inverter in watts, and the capacity or the backup wanted.
Check the Inverter efficiency and Depth of discharge.
Read the backup time or the capacity each battery needs, then press Share result.
This calculator answers two questions about a home inverter. How long will my batteries run this load? And what battery size do I need for a set number of hours? It uses the energy stored in the batteries and nothing else, so every step can be checked by hand.
The formula. Each battery stores its Ah rating times its voltage in watt-hours: a 12 V 150 Ah battery holds 1,800 Wh. Only part of that should be used. Lead-acid batteries, tubular or flat plate, last much longer when drained to about half, so the default depth of discharge is 50%. Lithium (LiFePO4) batteries tolerate deeper use, so the lithium default is 80%. The inverter then loses some energy as heat; 85% efficiency is typical for a home sine-wave inverter. Divide what is left by the load in watts to get the hours.
Series or parallel makes no difference to the energy. Two 12 V 150 Ah batteries hold 3,600 Wh whether the inverter wires them as 24 V 150 Ah or 12 V 300 Ah. Enter the rating of one battery and the number of batteries.
Sizing works the same formula backwards. Multiply the load by the hours, divide by efficiency and depth of discharge, then by the bank voltage, to get the Ah each battery needs. Choose the next size up that is sold.
To find your load, add the wattage on each appliance's label: a ceiling fan is usually 50 to 75 W, an LED bulb 9 W, a television 60 to 120 W.
The calculator does not model the Peukert effect (lead-acid batteries deliver fewer Ah at high current than their rating), battery age, heat, or the inverter's own standby draw. Real backup is usually a little shorter than the figure shown, especially above 500 W on one battery.
About 2 hours 33 minutes for a 12 V lead-acid battery. The battery stores 1,800 Wh; using half of it gives 900 Wh, and an 85% efficient inverter delivers 765 Wh. Dividing 765 by 300 W gives 2.55 hours. A lithium battery of the same rating, used to 80%, lasts about 4 hours.
Depth of discharge is the share of a battery's stored energy you actually use before recharging. Lead-acid batteries wear out quickly if drained fully, so about 50% is a common limit for a long life. Lithium iron phosphate batteries handle 80% or more. A deeper discharge gives more backup today and fewer years of battery life.
No. Two 12 V 150 Ah batteries store 3,600 Wh either way: in series they make 24 V at 150 Ah, in parallel 12 V at 300 Ah. The inverter's design decides the wiring. Enter one battery's rating and the number of batteries, and the energy comes out the same.
Add up the power rating printed on each appliance that runs during a cut. A typical ceiling fan uses 50 to 75 W, an LED bulb 9 W, a tube light 20 W, a television 60 to 120 W and a Wi-Fi router about 10 W. Leave out anything that will stay off.
Lead-acid batteries deliver fewer amp-hours at high current than their rated figure, known as the Peukert effect. Older batteries hold less, cold weather reduces capacity, and the inverter draws some power even with no load. The calculator leaves these out, so allow a margin, particularly for loads above 500 W on a single battery.
Most home sine-wave inverters convert 80 to 90% of battery energy into mains power at moderate loads, so 85% is the default. The figure is often printed in the inverter's manual or datasheet. Efficiency usually drops at very light loads, where the inverter's own consumption is a larger share of the total.
Formula only: no rates or limits are read from an official source.
Still stuck? Ask about Inverter Battery Backup Calculator and anybody who knows can answer. No account needed.
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