A generator has to do two jobs at once: carry everything that is running, and absorb the surgewhen the biggest motor kicks in. Size it for the running load plus the largest single starting surge, convert to kVA, add headroom, and round up. Miss the surge and the generator stalls the moment the fridge or pump starts.
Step 1: add the running watts
List everything you will power at once and add its running watts — lights, fridge, wifi, a few sockets, maybe a pump. This is the steady load the generator carries all evening.
Step 2: add the biggest starting surge
Motors and compressors draw a burst of current for a second or so when they start — a fridge, pump or air conditioner briefly pulls about 3× its running watts. Add the surge of the single largest motor on top of the running total (only one starts at a time if you are careful), not every motor’s surge at once.
| Appliance | Running W | Starting W |
|---|---|---|
| Fridge / freezer | 150–250 | ~700 |
| Water / borehole pump | 750–1,500 | 2,000–4,000 |
| Window / small A/C | 1,000–1,500 | ~3,000 |
Step 3: convert to kVA and add headroom
Generators are rated in kVA (apparent power) — what the windings must supply. Divide the watts by the power factor (usually 0.8): a 4,000 W peak load is 4,000 ÷ 0.8 = 5 kVA. Then add 20–25% headroom — generators run happiest at 70–80% load — so budget about 6 kVA and round up to the next standard size.
Generator Size Calculator
Build the load from an appliance list, allow for surge watts and power factor, add headroom — it returns the running wattage and kVA to buy.
Why kVA, not kW?
kVA is apparent power; kW (the running watts on a US 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.
How to get away with a smaller generator
Manage the load. Stagger startups so only one motor starts at a time, switch heavy resistive loads (water heating, kettles) off while on generator power, and fit soft-start kits to air conditioners — they cut the starting surge dramatically. The gap between “everything at once” and a managed load is often a full size class.
Protect sensitive electronics
Cheap generators produce rough power that can damage electronics. Look for AVR (automatic voltage regulation) at a minimum, or an inverter generator for clean power — or feed sensitive gear through a UPS so the generator never touches it directly.