About the Inverter Size Calculator
This inverter size calculator works out what size power inverter you need to run a set of appliances from a battery bank — in an RV, van, boat, cabin, off-grid solar system or home backup setup. List each appliance with its running watts and, for anything with a motor or compressor, its starting (surge) watts.
The calculator adds up the continuous load, applies a safety margin so the inverter is not run flat out, and checks that the inverter’s surge capacity can handle the largest motor starting while everything else is running. It then rounds up to a common inverter size and shows the DC current your batteries and cables must supply.
It assumes only one motor starts at a time, which is typical in practice. Check appliance nameplates or manuals for accurate wattage; motor-driven appliances such as fridges, pumps and air conditioners often need 3–7 times their running power for a fraction of a second at start-up.
With the default inputs, the recommended inverter size is 2,000 W. Change any value above to recalculate instantly.
How to use the inverter size calculator
- 1List every appliance you will run at the same time, one per line.
- 2Enter running watts, and starting watts for motors and compressors.
- 3Choose your battery system voltage.
- 4Set a safety margin and your inverter’s surge rating.
- 5Pick an inverter at or above the recommended size and check the DC current for cabling.
Formula and method
The continuous rating must cover the sum of all running watts plus a safety margin, because inverters run hot and lose capacity when loaded near their limit. The surge requirement assumes one motor starts at a time: it is the total running load plus the extra start-up watts of the single biggest motor.
The recommended size is the smallest common inverter rating that meets both the continuous requirement and the surge requirement divided by the inverter’s surge ratio. The DC current drawn from the battery is the AC watts divided by battery voltage and inverter efficiency — use it to size fuses, cables and the battery bank.
- ΣW
- Sum of running watts of all appliances used together
- margin
- Safety headroom, typically 20–25%
- V
- Battery bank voltage (12, 24 or 48 V)
- η
- Inverter efficiency (about 0.85–0.95)
Worked examples
RV essentials on 12 V
The appliances draw 1,375 W together. With a 25% margin you need 1,719 W continuous, so a 2,000 W inverter is the next common size. The fridge compressor adds 1,050 W at start-up for a 2,425 W peak, well within a 2× surge rating. At 12 V and 90% efficiency the battery supplies about 127 A.
Cabin with a well pump on 24 V
Three 100 W lights, a 200 W fridge and a 1,000 W pump run at 1,500 W, so 1,875 W with margin. The pump’s 2,000 W start-up surge makes the peak 3,500 W, which a 2,000 W inverter with 2× surge (4,000 W) handles. At 24 V the battery current is about 69 A.
Surge-limited: a single deep-well pump
The pump only needs 750 W to run (938 W with margin) but 3,500 W to start. Divided by a 2× surge ratio that requires a 1,750 W inverter, so the start-up surge — not the running load — pushes the choice up to 2,000 W.
Kilowatt entries on a 48 V system
A 1.5 kW heater is read as 1,500 W; with a 50 W fan the running load is 1,550 W and there is no motor surge, so peak equals running. The 25% margin gives 1,937.5 W, so the 2,000 W size fits. At 48 V and 90% efficiency the battery supplies about 35.9 A, or 46.3 A at the inverter’s full rating.
Frequently asked questions
What size inverter do I need to run a refrigerator?+
A typical fridge runs at 100–200 W but needs 800–1,500 W for a moment when the compressor starts. A 1,000 W pure sine wave inverter with 2× surge is usually enough for the fridge alone; add the other loads you plan to run at the same time.
Should I buy a pure sine wave or modified sine wave inverter?+
Pure sine wave inverters produce clean power like the grid and are recommended for fridges, pumps, microwaves, medical devices and electronics. Modified sine wave units are cheaper but can make motors run hot and some devices buzz or fail.
How many amps does an inverter draw from a battery?+
Divide the AC load in watts by the battery voltage and inverter efficiency. A 1,000 W load on a 12 V system at 90% efficiency draws about 93 A; on 24 V it draws about 46 A, which is why larger systems use higher voltages.
Can an inverter be too big?+
An oversized inverter works but costs more and consumes more idle (no-load) power, often 10–50 W, which drains batteries around the clock. Size for your real loads plus a sensible margin rather than the biggest unit available.
What is the difference between continuous and surge rating?+
The continuous rating is the power the inverter can deliver indefinitely. The surge (peak) rating is how much it can supply for a fraction of a second to start motors — usually about twice the continuous rating.