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Inverter Size Calculator

Work out inverter size instantly with clear inputs, formula shown and shareable results.

About Inverter Size Calculator

Inverters are limited both by continuous watts and by surge capability. Continuous rating comes from the running load plus margin, converted to VA by the power factor; surge must cover motor starting, which is typically three to six times a motor's running power for a second or two.

Formula

Inverter sizing

VA = running load x (1 + margin) / power factor; DC current = W / efficiency / battery voltage

Worked example

With the default values below, Inverter Size Calculator returns Inverter rating required of 2,647. Change any input above and every figure here is recalculated in your browser.

Inputs used

Total running load
1,800 W
Largest motor surge
2,400 W
Load power factor
0.85
Design margin
25 %

Results

Inverter rating required
2,647 VA
Continuous watts required
2,250 W
Surge capability required
3,375 W
DC input current at 48 V
50.95 A

Step by step

  1. 1Values used
  2. 2Inverter sizing
  3. 3Inverter rating required= 2,647 VA
  4. 4Continuous watts required= 2,250 W
  5. 5Surge capability required= 3,375 W
  6. 6DC input current at 48 V= 50.95 A

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why VA and not watts?

The inverter's output stage is current limited, so a reactive load consumes rating without doing useful work. Both figures must be checked.

Do I need a pure sine wave inverter?

For motors, transformers, audio equipment and many appliance electronics, yes. Modified square wave causes extra heating and can damage sensitive loads.

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