What this calculator does
Generator sizing is driven by the worst moment, not the average one. Motors draw several times their running current for a second or two at startup, so a load that runs at 3,000 W but surges to 4,500 W has to be sized against the 4,500.
Two adjustments then apply on top. Power factor converts real watts into the apparent power the alternator actually has to supply, and a target operating capacity keeps the generator off its own limit, since running continuously at full rating shortens its life and leaves nothing for anything unexpected. At 0.8 power factor and 70% capacity, a 4,500 W surge calls for a generator of about 8,036 W.
The formula
The surge figure is divided by the power factor to give apparent power in volt-amps, which is what the alternator has to produce. That is then divided by the target operating capacity so the generator runs at the intended fraction of its rating rather than flat out. The headroom line checks the result against the running load, and the coverage line confirms the sizing actually holds it.
| Term | Meaning |
|---|---|
| Running watts | Steady draw once everything is up and running. |
| Surge watts | The brief peak at startup, driven by the largest motor starting, commonly two to three times its running draw. |
| Power factor | The ratio of real power to apparent power. 1.0 for heaters and incandescent lighting, around 0.8 for motors and mixed loads. |
| Operating capacity | The fraction of its rating you intend the generator to run at. 0.7 is a common target, leaving 30% in reserve. |
The inputs explained
| Field | What to enter |
|---|---|
| Total running watts (W) | Total steady watts of everything running at once. This is checked against the result rather than used to size it, see the FAQ. |
| Peak surge watts (largest startup load included) (W) | The highest instantaneous demand, which is the running total plus the extra startup draw of the single largest motor. Motors do not usually all start at the same moment. |
| Power factor | Power factor as a decimal. Use 1.0 for purely resistive loads, and around 0.8 where motors or electronics are involved. |
| Target operating capacity | The share of the generator rating you want to be using. 0.7 is a reasonable target for sustained running. |
When to use it
Sizing a backup generator for a house
The large motors, usually a fridge, freezer, air conditioner or pump, set the surge figure. Sizing against the biggest one starting while everything else runs gives a realistic peak rather than an unnecessarily pessimistic one.
Powering tools on a site
Compressors and saws have heavy startup draws relative to their running load, which is why a generator that handles them comfortably once running can stall when one is switched on.
Checking whether an existing generator is adequate
Running your actual loads through and comparing the recommendation against what you own shows whether it is genuinely sized for the job or merely getting away with it.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does the startup surge drive the generator size?
The same 3,000 W running load with an increasing startup surge.
| Surge watts | Recommended generator size | In kVA | Apparent power required |
|---|---|---|---|
| 3,500 W | 6,250 W | 6.25 kVA | 4,375 VA |
| 4,000 W | 7,143 W | 7.14 kVA | 5,000 VA |
| 4,500 W | 8,036 W | 8.04 kVA | 5,625 VA |
| 5,500 W | 9,821 W | 9.82 kVA | 6,875 VA |
| 7,000 W | 12,500 W | 12.50 kVA | 8,750 VA |
At what running load does the sizing stop being enough?
The surge held fixed while the steady running load rises, to show where the coverage check fails.
| Running watts | Recommended generator size | Headroom over running watts | Covers the running load? |
|---|---|---|---|
| 3,000 W | 8,036 W | 2,625 W | Yes |
| 4,000 W | 8,036 W | 1,625 W | Yes |
| 5,000 W | 8,036 W | 625 W | Yes |
| 5,625 W | 8,036 W | 0 W | Yes |
| 6,000 W | 8,036 W | -375 W | No, size up further |
Questions
Why does changing the running watts not change the recommendation?
Because the sizing is calculated from the surge figure, which is normally the larger constraint. The running load is used for the coverage check instead. If that check comes back as no, the surge is no longer what limits you and the generator needs sizing against the running load directly.
How do I work out my surge figure?
Take the total running watts and add the extra startup draw of the single largest motor, not of all of them. Motors rarely start simultaneously. A motor typically surges to two or three times its running draw, and the figure is sometimes given on the appliance plate as locked rotor amps.
What power factor should I use?
1.0 for purely resistive loads such as heaters, kettles and incandescent lamps, where watts and volt-amps are the same. Around 0.8 for anything with motors, transformers or switch-mode supplies, which is most mixed household or site loads. Generator ratings in kVA assume a power factor, usually 0.8.
Why not run the generator at its full rating?
Because the rating is normally a peak rather than a figure for sustained use, and running at it continuously runs hot, increases wear and leaves no margin for anything you did not plan for. Around 70% is a common target. Running very lightly loaded is not ideal either, particularly on diesel units, where it causes wet stacking.
For converting between watts, amp-hours and watt-hours, see the watt-hours calculator. For what running an appliance costs to power, see the electricity bill calculator.