Generator & Inverter Sizing
Running and peak load of a list of appliances and motors with their starting surge, in W and kVA at your power factor and margin; DC amps for a battery inverter
The running load and the peak when the largest motor starts, both in watts and kVA with your margin, the load that sets the peak, and the battery current an inverter would draw.
Example: A fridge (150 W, ×3 to start), a pump (750 W, ×3) and 200 W of lights run at 1,100 W and peak at 2,600 W when the pump starts — 3.25 kVA at PF 0.8, 4.06 kVA with 25 % margin.
Size for the start,
run on the sum.
How running and peak load are added up, where the starting multiplier comes from, why kVA and power factor matter for a generator, and what the nameplate still decides.
Running and peak
The running load is the sum of every load's running watts × quantity. The peak is the running load plus the largest single starting surge — one unit of the load whose (starting − running) is greatest, started while everything else is already running. That is the usual sizing case; if two motors can start together, add both surges. A margin you set (0 = none) is applied to both figures.
Starting multiplier
Motors, compressors and pumps draw several times their running current for a moment at start. The multiplier is the load's own: locked-rotor amps ÷ full-load amps from the nameplate, or the manufacturer's starting watts. This page does not assume one per appliance type — a soft-started pump and a direct-on-line pump differ by a factor of three. Resistive loads (heaters, incandescent lamps) have no surge; 1 means none.
kVA, power factor and inverters
Generator sets are rated in kVA at a stated power factor — 0.8 lagging is the usual rating basis — so the kW figure on a nameplate is kVA × 0.8. Enter the set's power factor and the page reports both. For a battery inverter the same running and peak watts apply to its continuous and surge ratings, and the DC current is watts ÷ efficiency ÷ battery voltage — the figure that sizes the DC cables and fuse.
What the number does not settle
A set's ability to ride a surge (its motor-starting kVA, alternator design and governor), derating for altitude and temperature, harmonic-rich loads, fuel and transfer switching are the manufacturer's and the installer's. The appliance figures are yours — a nameplate maximum and a measured average are different numbers. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.