Inverter Size Calculator

Find the inverter size in kVA for your appliances, checked against the starting surge of motors, with the battery voltage, fuse and cable size.

How to calculate inverter size

• Add the running watts of every appliance that can be on at the same time.
• Divide by the power factor, usually 0.8, to get volt-amperes (VA).
• Add 20 to 30% headroom so the inverter isn't working at its limit.
• Check the starting surge. Fridges, freezers, ACs and pumps draw two to three times their running power for a moment when the motor starts, and most inverters can only give about twice their rating for a few seconds.
• Round up to the next standard size: 1, 1.5, 2.5, 3.5, 5, 7.5 or 10 kVA.

Worked example

Eight LED bulbs, three fans, a 43 inch TV, a decoder, a fridge, a laptop, a router and three phone chargers draw 609 W together.

• 761 VA × 1.25 headroom = 952 VA
• When the fridge starts the load jumps to about 909 W for a moment, well within what a 1 kVA inverter can surge to
• Result: a 1 kVA pure sine wave inverter, loaded to 76% of its 800 W rating

Battery voltage, fuse and cable

The inverter draws its power from the battery bank as direct current. At full output the battery current is the inverter's watts divided by the battery voltage and the inverter efficiency, so an 800 W inverter on 24 V draws about 37 A. The fuse or DC breaker should be about 25% above this, and the cable thick enough for the fuse rating. Higher battery voltages mean lower currents, which is why inverters of 5 kVA and above normally run on 48 V.

Pure sine wave or modified sine wave?

A pure sine wave inverter produces power like the grid and is safe for every appliance. Modified sine wave inverters are cheaper but make motors, fridges, fans and some chargers run hot and noisy. If you have any motor load, choose pure sine wave.

Frequently Asked Questions

Next, find out how many batteries you need with the Battery Backup Calculator, or size the whole system, including solar panels, with the Solar System Calculator.