Generators
What Size Generator Do I Need? A Simple Sizing Guide
Short answer
List what you want to power, add the running watts, then add the extra starting surge of your largest motor (fridge, pump, air conditioner). Divide by 0.8 to get kVA and add 20% headroom. A typical home backing up lights, TV, fridge and a few sockets often fits a 5 to 10 kVA generator, but a load list for your own site gives the real answer.
Step 1: Make a load list
Write down each appliance you need during an outage and its running watts from the label. Motors, such as fridges, pumps and air conditioners, need two to three times their running power for a moment when starting.
Step 2: Add running loads and the biggest start-up surge
Total running watts, plus the extra start-up watts of the single largest motor, gives the peak demand. Motors seldom start at the same time, so you add the surge for only the largest one.
Step 3: Convert to kVA and allow headroom
kVA ≈ watts ÷ (1000 × 0.8). Add around 20% headroom, and never run a generator at its limit all day. Example: 6,000 W peak ÷ 800 = 7.5 kVA, plus 20% gives about 9 kVA, so choose a 10 kVA unit.
Indicative sizes
| Use | Typical size |
|---|---|
| Small shop or lights, TV, fridge | 3 to 5 kVA |
| Home with fridge, pump, several sockets | 5 to 10 kVA |
| Large home or small office | 10 to 30 kVA |
| Workshop with three-phase motors | 30 kVA and above |
Guides only. Choose from your own load list.
Other decisions
- Single or three-phase: match your supply and loads. See single vs three-phase.
- Automatic changeover (ATS): starts the generator and switches the load when the grid fails.
- Fuel and runtime: diesel for long runtimes, petrol for small portable units.
- Canopy and location: ventilation, noise and exhaust away from windows.
- Servicing: regular oil, filter and test-run schedule.
Generator or solar?
Many sites combine both: solar for daily use, a generator for long outages. See generator installation and solar.
Common questions
Why not buy a much bigger generator to be safe?
A generator running far below its rating wastes fuel and can suffer wet stacking. Size with headroom, not extreme oversize.
Can a generator damage my appliances?
A properly sized, well-maintained unit with a stable output is fine. Cheap or overloaded units can give unstable power, so use protection for sensitive electronics.
Does a generator need an ATS?
Not strictly, but an ATS gives automatic changeover and prevents unsafe back-feeding to the grid.