Inverter Size Calculator

Estimate what size inverter you may need based on running watts, startup surge, battery voltage, inverter efficiency, and a planning buffer.

Enter Your Numbers


Presets are rough planning examples. Check the actual device label or manual when possible.


Use the normal running wattage of the device or combined loads.


Motors, fridges, pumps, and compressors may need extra startup power.


Higher-voltage battery banks usually draw less current for the same wattage.


Many real-world inverters are roughly 85% to 92% efficient.


Adds extra headroom so the inverter is not sized too close to the load.


Optional. Used to estimate battery energy used over time.


Pure sine wave is usually safer for sensitive electronics, motors, fridges, medical devices, and many modern appliances.


Your Results

Suggested continuous size

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Enter values and calculate.

Suggested surge rating

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For startup loads.

Estimated battery current

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At the running load.

Battery energy used

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For the entered use time.

This calculator gives a planning estimate only. Inverter choice also depends on device startup surge, waveform, battery capacity, cable size, fuse size, ventilation, temperature, and manufacturer limits.

How to Use This Inverter Size Calculator

Enter the running watts for the device or group of devices you want to power. If the device has a motor, compressor, pump, or heating element, also enter the startup or surge watts if you know it.

The calculator adds a planning buffer and suggests a common inverter size above the exact calculated number. This helps avoid choosing an inverter that is too close to the load.

Formula Used

Required continuous watts = running watts × (1 + buffer %)
Required surge watts = startup surge watts × (1 + buffer %)
Estimated DC battery current = running watts ÷ (battery voltage × inverter efficiency)
Battery energy used = running watts × hours ÷ inverter efficiency

Example Calculation

A device that uses 500 running watts with a 1,000-watt startup surge and a 25% buffer needs about:

500W × 1.25 = 625W continuous requirement
1,000W × 1.25 = 1,250W surge requirement

In that case, a common planning choice may be around a 750W inverter with at least a 1,500W surge rating, depending on the actual device and inverter specifications.

Common Inverter Sizing Mistakes

  • Only checking running watts and ignoring startup surge.
  • Choosing an inverter with no headroom above the calculated load.
  • Ignoring battery current draw on 12V systems.
  • Using thin cables or underrated connectors on high-current battery setups.
  • Assuming every appliance works well on a modified sine wave inverter.
  • Forgetting that battery capacity controls runtime, not just inverter size.

Important Safety Note

Use this calculator for planning only. It does not size battery cables, fuses, breakers, transfer switches, outlets, or permanent wiring. Do not use it as permission to overload an inverter, battery, cable, fuse, outlet, extension cord, generator, or electrical system.

For home backup wiring, transfer switches, breaker panels, permanent circuits, damaged wiring, sparks, heat, burning smells, or code-sensitive work, stop and contact a qualified electrician.