Generator Size Calculator

Add up the running watts of everything you’ll power at once, then add the extra surge of the largest motor or compressor that could start while the rest is running — fridges, pumps and air conditioners briefly draw about three times their running watts. Convert to kVA at the generator’s power factor (usually 0.8), add 20–25% headroom, and round up to the next standard size.

kVA is apparent power — what the generator’s windings actually have to supply — while kW (and the running watts on a US generator label) is the real power delivered. They differ by the power factor: a 5 kVA generator at 0.8 PF delivers about 4 kW, or 4,000 running watts. Sizing in kVA at 0.8 PF is the industry convention, and it is what the calculator returns alongside the wattage.

Motors and compressors draw a large burst of current for a second or so when they start — typically about 3× their running watts for a fridge or pump, and more for machines that start under load. A generator must cover the running load plus the biggest single surge on top, or it will stall or trip when that appliance kicks in.

Generators shouldn’t run flat out: continuous operation is happiest around 70–80% load, fuel efficiency is best there, and you keep margin for the loads you forgot. Headroom also covers derating for altitude and heat. 20–25% is a sensible default — more if you expect the load to grow.

Only on a generator with decent voltage regulation. Cheap units produce rough power that can damage electronics; look for AVR (automatic voltage regulation) as a minimum, or an inverter generator for clean power. Better still, feed sensitive gear through a UPS or inverter so the generator never touches it directly.

Manage the load: stagger startups so only one motor starts at a time, switch water heating and other heavy resistive loads off while on generator power, and use soft-start kits on air conditioners — they cut the starting surge dramatically. The difference between “everything at once” and a managed load is often a full size class.