Inverter Size Calculator
Size a power inverter from the appliances you will run — continuous watts, surge watts for motors and compressors, power factor and efficiency — with DC input current, fuse and battery-cable sizing.
How it works
Continuous rating ≥ Σ running watts × (1 + margin); surge rating ≥ (Σ running + largest motor’s extra start-up) × margin; DC amps = watts ÷ efficiency ÷ battery volts
An inverter has two ratings that matter: the continuous power it can supply indefinitely and the surge it can deliver for a second or two while a motor starts. Fridges, pumps, air conditioners and power tools draw three to seven times their running power at start-up, so a 700 W fridge can need over 2 kW for a moment — but since motors rarely start simultaneously, sizing for the running total plus the single largest surge is the usual practice. On the DC side the numbers get large fast: 1,000 W from a 12 V battery through an 88%-efficient inverter is almost 95 A, demanding thick cable and a properly rated fuse, which is why larger systems move to 24 or 48 V. Pure sine-wave inverters cost more but run motors cooler and keep sensitive electronics happy.
Worked example
A 700 W fridge (×3 start), 150 W TV and 60 W of lights: 910 W running, 2,310 W at compressor start. With 20% margin choose a 1,500 W inverter (3,000 W surge); at 12 V it draws about 86 A — a 125 A fuse and 4 AWG cable.
Frequently asked questions
Why is my 1,000 W inverter tripping on an 800 W fridge?
The compressor’s start-up surge exceeds the inverter’s surge rating, or the battery voltage sags under the 80 A draw and triggers low-voltage cut-off. Larger inverter, better cables or a soft-start kit fix it.
12 V or 24 V?
Above about 1,500 W, 24 V halves the current and cable size; above 3–4 kW, 48 V is standard. Match the inverter to your battery bank voltage.
Related calculators
Educational estimate. Mains voltages and stored energy can kill — have electrical work checked by a qualified electrician and follow local codes.