Generators are sized by two numbers, and the bigger one is not the obvious one. Running watts is the steady total of everything switched on. Starting watts is that total plus the inrush of the largest motor as it spins up, which for an induction motor is commonly two to four times its running figure. A set that comfortably handles the running load can still stall when the well pump kicks in.
Generators are then rated in kVA rather than kilowatts, because alternator capacity is limited by current rather than by real power. Dividing watts by power factor converts between the two, and 0.8 is the conventional domestic assumption — resistive loads such as heaters are nearer 1.0, while motors and older fluorescent gear are worse.
A 3500 watt running load with a 1200 watt motor at a 3× surge peaks at 5900 watts, which at 0.8 power factor is 7.38 kVA. Adding 20 per cent headroom for future load and derating gives 8.85 kVA, so you would look at the next standard size above that. Sets also lose roughly 3 per cent of output per 300 metres of altitude and more in high ambient temperatures.
The formula
running W- Total steady load with everything on
motor W- Running watts of the single largest motor
surge- Starting multiple for that motor, typically 2-4×
PF- Power factor, 0.8 for mixed domestic load
headroom- Spare capacity for future load and derating
How it works, step by step
- Add up the running watts of everything that will be on at once.
- Enter the largest single motor separately and set its surge multiple.
- Set power factor: 0.8 for mixed load, closer to 1.0 for pure heating.
- Add headroom; the gauge shows the kVA rating to shop for.
Worked examples
3500 W of load with a 1200 W pump
Running load is 3500 W, but starting the pump at 3× adds 2400 W of inrush for a peak of 5900 W. At 0.8 power factor that is 7.38 kVA, and with 20 per cent headroom you want 8.85 kVA.
What a soft start saves
Fitting a soft starter that drops the surge to 2× cuts the peak to 4700 W and the requirement to 7.05 kVA — often one whole frame size smaller, which is far cheaper than the larger generator.
How to read your score
Frequently asked questions
What is the difference between kW and kVA?
kVA is apparent power, the product of volts and amps; kW is real power, the part that does work. Power factor is the ratio between them. Generators are rated in kVA because their limit is heating from current, so a 5 kVA set at 0.8 power factor delivers only 4 kW.
Do I really need to allow for starting surge?
Yes, for anything with an induction motor: fridges, freezers, pumps, compressors and air conditioners. The inrush lasts under a second but a set that cannot supply it will stall or trip, and repeated stalling is hard on both the generator and the motor.
Can I run several motors from one generator?
Usually, provided they do not all start together. The convention is to allow surge for the largest motor only, since simultaneous starts are unlikely, but stagger them deliberately if you can. If two large motors must start together, allow surge for both.
Why should the generator not run at full capacity?
Continuous running above about 80 per cent of rating shortens life and raises fuel consumption per unit of output. Sets also derate with altitude, roughly 3 per cent per 300 metres, and in high ambient temperatures. That is what the headroom input is for.
Is inverter or conventional better?
Inverter sets produce a cleaner sine wave suited to electronics, run quieter and throttle back at part load to save fuel. Conventional sets are cheaper per kVA and generally better suited to heavy motor starting where raw surge capacity matters more than waveform.
Generator sizing reference
| Running load | No motor | 1 kW motor at 3× | 2 kW motor at 3× | 3 kW motor at 4× |
|---|---|---|---|---|
| 1000 W | 1.50 kVA | 4.50 kVA | 7.50 kVA | 15.00 kVA |
| 2000 W | 3.00 kVA | 6.00 kVA | 9.00 kVA | 16.50 kVA |
| 3500 W | 5.25 kVA | 8.25 kVA | 11.25 kVA | 18.75 kVA |
| 5000 W | 7.50 kVA | 10.50 kVA | 13.50 kVA | 21.00 kVA |
| 7500 W | 11.25 kVA | 14.25 kVA | 17.25 kVA | 24.75 kVA |
| 10000 W | 15.00 kVA | 18.00 kVA | 21.00 kVA | 28.50 kVA |
Starting surge, not running load, sets the size on most domestic installations.