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Inverter Sizing Calculator

Enter your running load, surge factor, and inverter efficiency to size the right inverter.

How it is calculated

Enter the total running power of everything you will plug in (P, watts), a surge multiplier for motor loads (k) and the inverter efficiency (η, percent). The tool returns three figures:

Recommended size = P × 1.25

Peak surge demand = P × k

Input power = P ÷ (η ÷ 100)

The recommended size is a continuous rating with a fixed 25% margin. The peak figure is what the inverter must tolerate for a moment at start-up, so its surge rating should be at least that high. Input power is the DC power drawn from the battery. If the surge factor is left empty or zero it is treated as 1, and an efficiency that is empty, zero or above 100 is replaced by 90. Load must be above zero. All results are rounded to whole watts.

Worked example

Running loads total 800 W, including a fridge whose compressor surges to roughly 3×, with an inverter of 92% efficiency.

  1. Recommended size: 800 × 1.25 = 1,000 W.
  2. Peak surge: 800 × 3 = 2,400 W.
  3. Input power: 800 ÷ 0.92 = 869.6, shown as 870 W.

You would look for an inverter rated about 1,000 W continuous that can also handle 2,400 W surge, and a battery bank that can supply roughly 870 W of DC power.

Try this example →

Common mistakes

  • Applying the surge factor to everything. Only motor and compressor loads surge; if only one 300 W fridge in an 800 W total surges, using a 3× factor on the whole 800 W overstates the peak. Add the surge of that one load to the rest of the running load instead.
  • Entering the efficiency as a decimal such as 0.92. The field is in percent, so type 92. A value of 0.92 is read as under 1% and gives an enormous input power.
  • Choosing an inverter by its continuous rating and ignoring the peak. A unit with a high enough continuous rating can still trip if its surge rating is below the peak figure shown.
  • Leaving the efficiency blank and expecting the real value. The tool then assumes 90%, so enter your inverter's figure from its data sheet.
  • Sizing the battery from the AC load. The battery has to supply the input power, which is larger than the load because of conversion losses.

About this calculator

An inverter has to handle two different demands from the loads it powers: the continuous (running) wattage it must sustain indefinitely, and a much higher surge wattage that motor-driven loads — refrigerator compressors, power tools, well pumps — draw for a fraction of a second at startup, often 2-3× their running wattage. Sizing an inverter only for running watts risks it tripping or shutting down the moment a compressor kicks on; the inverter's own conversion efficiency (typically 85-95%) also means the DC input power it draws from a battery is always somewhat higher than the AC power it delivers.

This calculator takes your total running load, a surge factor for motor-driven equipment, and the inverter's efficiency, then recommends a continuous wattage rating, the expected peak surge demand, and the DC input power required. RV and van-life owners sizing a power system, off-grid solar installers, and homeowners planning backup power for outages use it to pick an inverter that won't nuisance-trip under real-world startup surges while avoiding paying for far more capacity than the loads actually need.

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Comments (1)

  • Editorial team

    The input power matters on the battery side, so turn it into current: divide it by the battery voltage. The default 1,333 W input is about 111 A on a 12 V system, 56 A on 24 V and 28 A on 48 V, which is why cable size and fuses change so much with voltage. This is a quick sizing aid, not an installation design. What battery voltage are you planning to run?

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