HisabCalc › Discounts & tips

Inverter Battery Backup Calculator

Shows the usable energy in an inverter battery and how many hours it can run a given load, using efficiency and depth of discharge.

Enter your numbers

Battery capacity (Ah)

Example: 150

Battery voltage (V)

Example: 12

Load (watts)

Example: 300

Inverter efficiency percent

Example: 85

Depth of discharge percent

Example: 60

Result

Currency

Figures are computed on your device; nothing is sent anywhere.

Formula

usableWh = batteryAh * batteryVoltage * (depthOfDischargePercent/100) * (efficiencyPercent/100); backupHours = usableWh / loadWatts

How the calculation works

A battery stores energy in watt hours, found by multiplying its amp-hour rating by its voltage, so the two ratings must always be used together.

Not all of that energy is available, because the inverter loses some as heat and a lead-acid battery should not be drained fully.

Dividing the usable energy by the load in watts gives the hours of backup, which falls quickly as more appliances are switched on.

Common mistakes

Using the full battery capacity and ignoring depth of discharge, which overstates backup by nearly half for a lead-acid battery.
Adding the wattage of appliances that never run together, such as a fridge compressor and an iron.

When to use it

Use it to size an inverter and battery for a chosen set of lights, fans and a router during load shedding.
Do not use it for lithium batteries with a different safe discharge limit, or for loads with a high start-up surge.

Worked example

A 150 Ah 12 V battery at 85 percent efficiency and 60 percent depth of discharge gives 918 watt hours, which runs a 300 watt load for 3.06 hours.

Usable energy (Wh) 918.00

Backup hours 3.06

Common questions

Why is depth of discharge so important?

A lead-acid battery is only meant to give up part of its charge before recharging, so using the full capacity would shorten its life and overstate the backup.

Last updated: 2026-10-04