Battery Ah needed to run a given load for a chosen number of hours
Runs in your browser. Nothing you type is sent anywhere. Updated on 24 Oct 2026.To size a battery, start from the load and the hours: 300 W for 4 hours is 1,200 Wh at the plug. Divide by the inverter efficiency and the share of the battery you can safely use, then by the bank voltage. On a 24 V bank of two lead-acid batteries that is about 118 Ah each. Pick the next size up that is sold.
The page opens on Battery size; pick the Battery type and the voltage of one battery.
Enter the Number of batteries and the Load on the inverter in watts.
Enter the Backup wanted in hours.
Read the Capacity of each battery in Ah and the energy the bank must store.
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.
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.
The rest of the article, and all 6 questions about Inverter Battery Backup Calculator, are on the Inverter Battery Backup Calculator page.
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.
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.
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.
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