Work out what size power inverter you need. List the appliances you will run with their running and surge (start-up) watts, and get the continuous rating with a safety margin, the peak surge to cover, a standard inverter size and the current it will draw from the battery.
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Surge and quantity are optional. Motors (fridges, pumps, compressors, power tools) typically surge 2–7× their running watts.
%
20–25% is typical so the inverter is not run at 100%
× continuous
Many inverters handle about 2× their rating for a few seconds
× running
%
Enter your values to see the result.
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How to use the Inverter Size Calculator
List your appliances, one per line: name, running watts, surge watts, quantity. Surge and quantity are optional.
Set the safety margin (25% is typical) and your inverter's surge rating.
Tick the box if several motors may start at the same time.
Choose the battery voltage and efficiency to see the DC current.
Read the recommended size, the peak load and the load breakdown.
How inverter size is calculated
Running load = sum of running watts × quantity of all appliances used at the same time.
Peak load = running load + the extra start-up watts of the largest motor (or the sum of all surges if several can start together).
Inverter size = the larger of the continuous rating and peak load ÷ inverter surge factor, rounded up to a standard size.
Battery current = running watts ÷ (battery voltage × inverter efficiency).
Typical appliance wattages
Appliance
Running watts
Typical surge watts
Refrigerator / freezer
100–250 W
600–1,500 W
Chest freezer
60–150 W
500–1,000 W
Microwave (1,000 W cooking)
1,000–1,500 W
about the same
Coffee maker
800–1,200 W
about the same
Window air conditioner (10,000 BTU)
900–1,200 W
2,000–3,000 W
Well or sump pump (1/2 hp)
700–1,000 W
2,000–3,000 W
Furnace blower
300–800 W
1,000–2,000 W
LED TV (50 in)
60–150 W
about the same
Laptop
45–100 W
about the same
Circular saw
1,200–1,500 W
2,000–3,000 W
These are typical ranges, not exact values. For the most accurate result use the wattage on the appliance's label, or measure it with a plug-in power meter.
Frequently asked questions
How do I calculate what size inverter I need?
Add up the running watts of everything that will be on at the same time, add a safety margin of about 20–25%, and make sure the inverter's surge rating covers the running load plus the start-up surge of the largest motor. Then round up to the next standard size.
What is surge or starting wattage?
Motors and compressors (fridges, pumps, air conditioners, power tools) draw several times their running power for a fraction of a second when they start. Typical surge is 2–3× running watts for fridges and 3–7× for some pumps and compressors. The appliance label or manual gives the exact figure.
How much surge can an inverter handle?
Many inverters can supply about twice their continuous rating for a few seconds; some low-frequency inverters handle 3× or more. Check the peak or surge rating on the data sheet and set it in the calculator.
How many amps will my inverter draw from the battery?
DC current = AC watts ÷ (battery voltage × efficiency). A 1,000 W load on a 12 V battery with a 90% efficient inverter draws about 1,000 ÷ (12 × 0.9) ≈ 93 A.
Do I need a pure sine wave inverter?
Pure sine wave inverters are recommended for motors, compressors, medical and audio equipment, chargers and anything with sensitive electronics. Modified sine wave units are cheaper but can make motors run hot and noisy.
Is a bigger inverter always better?
Not always. An oversized inverter costs more and has a higher idle (no-load) consumption, which drains batteries in small systems. Size for your realistic peak use plus a reasonable margin.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.