Generator Sizing Calculator
Size a generator for both steady-state and motor-starting demand. Enter your total running load, the largest motor and its starting multiplier, power factor, and a headroom margin to get the recommended kW and kVA.
Connected Load
Generator Size
Peak = other loads running + largest motor's starting surge. kVA = kW ÷ power factor. Sizing guidance — confirm with the genset manufacturer for motor-start voltage dip.
How this calculator works
Generators fail their loads at two different moments: steady state, when everything is simply running, and the instant the largest motor starts while everything else is already on. The calculator models both and sizes to the worse one:
peak = (running − largest motor) + largest motor × start multiplier
kW = max(running, peak) × (1 + headroom%) · kVA = kW ÷ power factor
There is no code table behind the start multiplier — it is a stated assumption. Across-the-line induction motors typically pull on the order of 3× their running watts while accelerating (locked-rotor current is higher still — often 6× current — but at a collapsed power factor, so ≈3× is the common watts-based planning figure), and that is the calculator’s default. Soft starters and VFDs cut the multiplier sharply; hard-starting compressors can exceed it. The headroom percentage is deliberate spare capacity for load growth and derating (altitude, temperature), and the power factor converts kW to the kVA rating gensets are cataloged by — 0.8 is the standard three-phase genset rating point.
Worked example
A site with 8,000 W of running load whose largest motor is a 2,000 W pump, using the 3× starting assumption, 0.8 power factor, and 20% headroom — the calculator’s defaults:
- Everything except the pump: 8,000 − 2,000 = 6,000 W running.
- Pump starting surge: 2,000 × 3 = 6,000 W.
- Peak with the pump starting: 6,000 + 6,000 = 12,000 W — worse than the 8,000 W steady state, so it governs.
- With 20% headroom: 12,000 × 1.2 = 14,400 W → 14.4 kW.
- At 0.8 power factor: 14.4 ÷ 0.8 = 18.0 kVA.
Reproduce it above with the default inputs. Note what did the sizing: a pump that is a quarter of the running load set the generator size — motor starting almost always governs.
Why there’s no lookup table here
Unlike ampacity or box fill, generator sizing has no NEC table to cite — the code governs how standby systems transfer and protect, not how many kW you buy. The honest method is the one above: sum the running load, model the largest motor’s start as an assumption you state out loud, and add deliberate headroom. The step after this calculator is the manufacturer’s sizing tool, which knows each genset’s alternator surge capacity and voltage-dip limits — bring these numbers to that conversation.
Frequently asked questions
How do I size a generator for motor starting?
The largest motor sets the peak, not the total connected load. An across-the-line induction motor draws locked-rotor current of roughly six times its full-load amps while accelerating, which for genset sizing works out to roughly three times the motor's running watts — the default starting multiplier in this calculator. Peak demand is modeled as everything else running plus that one motor starting; motors on soft starters or VFDs start far gentler and can use a lower multiplier. These multipliers are estimates — confirm against the motor's code letter and the genset maker's motor-starting data.
What is the difference between running watts and starting watts?
Running watts are the steady-state draw once everything is up to speed — the sum of your continuous loads. Starting watts are the momentary peak when the largest motor kicks on while the rest of the load is already running, and they usually govern the generator size. A set sized only for running load will bog or trip on voltage dip every time the compressor or well pump starts.
Why does the calculator show both kW and kVA?
Generators are limited by both real power (kW, the engine) and apparent power (kVA, the alternator windings). The two are linked by power factor: kVA = kW ÷ PF, and this calculator uses the power factor you enter — 0.8 is the standard rating point for three-phase gensets. A load with poor power factor can max out the alternator kVA well before the engine runs out of kW.
How much headroom should I add when sizing a generator?
A planning margin of 10–25% over the calculated peak is common practice — it covers load growth, altitude and temperature derating, and keeps the set off its ceiling. As a rule of thumb, not a code value: diesel sets also dislike running lightly loaded for long stretches (wet stacking), so don't swing wildly oversized either. The genset manufacturer's sizing software is the final word for a specific model.
Can I use this to size a code-required standby generator?
Use it for planning, not for the permit. Optional standby systems fall under NEC Article 702 — with automatic transfer the source must carry the full calculated load or use load management, while with manual transfer it may be sized for the loads you select; emergency and legally required standby systems (Articles 700 and 701) carry stricter rules on capacity and transfer. The load side of those calculations comes from Article 220, not from adding up nameplates — verify the full design against the code edition adopted in your jurisdiction.
Method: peak demand modeled as the remaining load running plus the largest motor at its starting multiplier (≈3× running watts for across-the-line induction motors — an assumption, not a code value); recommended kW = the governing demand plus headroom; kVA = kW ÷ power factor. Single-largest-motor model — simultaneous motor starts, high-inrush loads (transformers, LED drivers at cold start), and harmonic-rich loads need the margin or a sequenced start. Sizing guidance only — confirm with the genset manufacturer for motor-starting voltage dip and step-load acceptance.
Get the motor numbers right first with the Motor FLA Calculator (full tables on the motor full-load current chart), the building demand with the Service & Load Calculator, and any step-down transformers with the Transformer Sizing Calculator.