Free tool
Generator Sizing Calculator
Work out the generator size in kVA and kW for a given load, allowing for power factor, starting surge from motors and air conditioners, and headroom for growth.
Why generators are sized in kVA, not watts
Watts are the power actually doing work. kVA is what the alternator has to supply, and the two differ by the power factor. A 5 kW load at 0.8 power factor needs a 6.25 kVA alternator before anything else is considered, which is why a generator rated in watts on the label is often rated optimistically.
The bigger trap is starting surge. A motor drawing 1500 W running can pull six times that for a second or two as it starts, so an air conditioner or a water pump decides the generator size far more than the total running load does. That surge figure is what the calculator checks against.
Oversizing has its own cost. A generator running under about 30 percent of its rating for long periods wet stacks, glazes the bores and fails early, so the aim is comfortably above the surge and not far beyond it. The 25 percent headroom default is the sensible middle.