Generator Size Calculator
A generator that is too small trips its breaker the moment the fridge compressor kicks in; too large and you overpay for capacity and fuel you never use. This calculator sums your essential loads and adds the surge headroom motors demand.
Select what must run during an outage, add any extra wattage, and get a recommended generator size in watts and kVA.
Enter valid values to see the results.
Formula
Motor-driven appliances (fridge, AC, pumps) draw 2–3× their running watts for a second at startup — the calculator adds a 1,500 W surge allowance when any motor load is selected. The 1.25 headroom keeps the generator under 80% load for longevity. kVA = W ÷ 800 (power factor 0.8).
How to measure
- List only what must run during an outage — not everything you own.
- Check appliance labels for real wattage; the presets here are typical values.
- For central AC or electric water heaters, read the nameplate — these often need a dedicated large generator or a transfer-switch strategy.
Worked example
Practical tips
- Prioritise: fridge, heating/cooling, lights, communications, medical devices, then comfort loads.
- Stagger motor starts — turn the fridge breaker back on after the generator stabilises.
- Inverter generators cost more but run quieter, use less fuel and produce clean power for electronics.
- Never backfeed through a wall outlet; use a proper transfer switch or interlock kit.
Waste allowance
The 1.25 headroom is the built-in allowance — do not add further margin, or you will buy a generator that runs inefficiently at low load.
Typical appliance wattages
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | 700 | 2,100 |
| Window AC 12k BTU | 1,500 | 4,500 |
| Sump pump 1/2 HP | 800 | 2,400 |
| Microwave | 1,200 | 1,200 |
| LED lighting (whole house) | 400 | 400 |
Common mistakes
- 1 Summing only running watts and forgetting motor starting surge — the number one reason small generators trip.
- 2 Running a generator continuously above 80% load, which shortens engine life.
- 3 Buying by peak (starting) watts rating instead of rated (running) watts — compare like with like.
- 4 Forgetting fuel: a 5 kW petrol generator burns 2–3 litres per hour at half load — plan storage.
Frequently asked questions
What size generator do I need to run a house?
For essentials only (fridge, lights, electronics, one small AC or heater): 4–6 kW. Whole-house with central AC and electric cooking: 12–20 kW standby unit.
What is the difference between running and starting watts?
Running watts are the continuous draw; starting (surge) watts are the 2–3× spike motors need for a second at startup. Generators list both — size by running watts plus the largest surge.
Will a 2,000 W generator run a refrigerator?
Yes, barely: a fridge runs at ~700 W but surges to ~2,100 W at compressor start. A 2,000 W inverter generator with 2,200 W surge handles a fridge plus lights, but little else.
What does kVA mean vs kW?
kW is real power; kVA is apparent power including reactive load. For generators, kW ≈ kVA × 0.8 (power factor). A 5 kVA unit delivers about 4 kW of usable power.
Petrol, diesel or dual-fuel?
Petrol for occasional portable use, diesel for frequent or long outages (better economy and lifespan), dual-fuel/propane where fuel storage and shelf life matter.