Required Generator Power Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/10/2026
The minimum generator power is calculated with (resistive appliances + motor appliances × startup coefficient) × 1.25. For 1500 W resistive and 800 W of motors with a medium startup (×3), you need a generator of at least 4875 W.
Explanation
Choosing a generator based only on the sum of appliance nameplate powers is the mistake that stalls the engine on the first startup: a motor appliance (refrigerator, pump, compressor, power tool) draws a current surge of 2 to 5 times its normal running power for a fraction of a second when it starts. This surge must be covered by the generator, otherwise it trips or gets damaged. This calculator applies the recommended sizing method: add the power of non-motor appliances (lighting, heating, electronics, hobs), add the nameplate power of motor appliances multiplied by a startup coefficient (×2 for a light startup like a refrigerator, ×3 for a medium startup like a pump, ×4 for a heavy startup like a large compressor under load), then apply a 25% safety margin. The result is also expressed in kVA, the unit shown on most generators, assuming a power factor of 0.8. The resistive load to enter includes items such as the heating appliances sized by our room heating power calculator, which draw a steady current with no startup surge. To limit the required power, a good practice is to never start two large motors at the same time — the same logic our required radiator count calculator applies when splitting a heating load across several units rather than one oversized appliance.
Example: 1500 W resistive + 800 W of motors (medium startup)
Inputs
Non-motor appliances: 1500 W. Motor appliances: 800 W. Startup difficulty: medium (×3).
Calculation
Startup power = 1500 + (800 × 3) = 1500 + 2400 = 3900 W. With the margin: 3900 × 1.25 = 4875 W, about 6.1 kVA.
Result
You need a generator of at least 4875 W (about 6.1 kVA) for this set of appliances.
Frequently asked questions
Why add a 25% margin after already accounting for the startup surge?
The 25% margin covers several uncertainties: a generator's actual power drops with altitude and temperature, a combustion engine loses power as it ages, and running a generator at 100% of its capacity continuously sharply reduces its lifespan. Sizing with a 25% reserve lets the generator run in a comfortable range, around 70-80% load.
What does the kVA conversion mean?
Generators are usually rated in kVA (kilovolt-amperes), the apparent power, while appliances consume active power in watts. The ratio between the two is the power factor, close to 0.8 for mixed use: a 6.1 kVA generator therefore supplies about 4875 W of usable active power. Always check the power in watts (or kW) on the generator's spec sheet, not just the kVA.
Should I count all appliances or only those used at the same time?
Only appliances likely to run simultaneously should be added: if you never use the angle grinder and the welder at the same time, count only the more demanding of the two. However, a refrigerator that can restart at any moment must always be included, with its startup coefficient, even if you're not actively using it.