SolarSizr

Inverter Size Calculator

Work out what size inverter you need from your appliances, including the startup surge that motors demand.

ApplianceWattsStartup surge

How to use this inverter size calculator

List the AC appliances you might run at the same time, with their running watts (check the label or manual). For anything with a motor, set a startup surge factor:

  • Electronics (×1) — lights, TV, laptop, phone chargers, resistive heaters.
  • Light motor (×2) — fans, small tools.
  • Heavy motor (×3) — fridges, freezers, pumps, air conditioners, large power tools.

The result is the continuous inverter rating to buy, sized for your running load plus the biggest startup surge.

How inverter sizing works

An inverter has two limits that both matter:

continuous load = sum of running watts (everything on at once)
surge demand = continuous load + biggest single startup surge
inverter (continuous) = max( continuous × 1.2 , surge demand ÷ 2 )

The 20% headroom keeps the inverter off its limit. The surge check matters because a motor briefly pulls far more than its running watts, and most inverters can only surge to about twice their continuous rating for a couple of seconds.

Two more rules: match the inverter's DC input to your battery voltage (12, 24 or 48 V), and choose a pure sine wave inverter for motors and sensitive electronics.

Frequently asked questions

Add the running watts of everything running at once, add ~20% headroom, and round up. If an appliance has a motor, the inverter must also cover its startup surge (2 to 3× running watts).
The brief spike an appliance draws at startup. Motors can pull 2 to 3× their running watts for a second or two. Inverters typically surge to about 2× their continuous rating to cover it.
Pure sine wave for motors, fridges, microwaves and sensitive electronics. Modified sine is cheaper but can cause buzzing, heat or malfunction. For off-grid, choose pure sine wave.
Yes, the inverter's DC input must match your battery bank (12V, 24V or 48V). Bigger systems use 24V or 48V to keep current lower.

More SolarSizr calculators

Sizing a whole system? Start with the off-grid solar calculator, then set your charge controller, wire size and panel angle.