Generator Size Calculator
Enter how many of each motor-driven appliance you want to keep running, then add lights, electronics and anything else in watts. The calculator adds running watts, allows for the largest starting surge and picks a generator size.
- Running and starting watts
- Motor surge built in
- 20% headroom
Typical wattages are used for each appliance. Check the label on your own equipment for exact figures.
Motors need two to three times their running power to start. A fridge that runs on 700 watts can pull 2,200 watts for a second or two as its compressor starts. A generator that only covers running watts will stall or trip when that happens. Size for the running total plus the biggest single surge.
Appliance wattage chart
Typical running and starting watts. Search for an appliance.
| Appliance | Running (W) | Starting (W) |
|---|---|---|
| Fridge | 700 | 2200 |
| Chest freezer | 500 | 1500 |
| Sump pump 1/2 hp | 1050 | 2150 |
| Well pump 1/2 hp | 1000 | 2100 |
| Furnace blower 1/2 hp | 800 | 2350 |
| Window AC 10,000 BTU | 1200 | 3600 |
| Central AC 3 ton | 3800 | 11400 |
| Microwave | 1000 | 1000 |
| Electric kettle | 2200 | 2200 |
| Space heater | 1500 | 1500 |
| LED lights (10) | 100 | 100 |
| TV and router | 200 | 200 |
| Laptop | 60 | 60 |
Appliances without motors, such as lights, TVs and toasters, start at their running wattage. Only motors and compressors have a surge.
Running load and generator size
With 20% headroom. Slide the running load.
Generators run most efficiently and quietly at 50% to 80% of their rating. Running one flat out shortens its life and leaves no room for motor surges.
How to size a generator
List, add, then allow for surge. Step through the method.
List essentials
What must run in a power cut: fridge, freezer, heating, pumps, lights.
A transfer switch lets the generator feed your home's circuits safely. Never backfeed through a socket, as it can electrocute line workers.
Portable, inverter or standby
Three ways to keep the power on. Compare the types.
Quiet and fuel-efficient with clean power. Best for electronics and camping.
Open-frame generators with more power for the money. Louder and thirstier.
Fixed outside, runs on gas or propane and starts itself in a power cut.
Inverter generators produce clean power that is safe for computers and modern boilers. Older open-frame generators can have voltage spikes that upset electronics.
Generator safety checklist
Generators are safe when used outdoors and connected properly. Tick each item.
Tick each item as you go.
Carbon monoxide from a generator can kill within minutes. Never run one in a garage, shed or near open windows, even with the door open.
What the result tells you
Power, surge and fuel. Each tile explains one result.
The standard size that covers your load.
Total watts with everything on.
Running load plus the largest motor start.
Amps at 240 V for the transfer switch.
Petrol for 8 hours at about half load.
Fuel use depends heavily on load. A 7,500 W generator at half load uses about 1.7 litres of petrol an hour, so plan fuel for at least 24 hours.
Frequently Asked Questions
No question matches that search.
What size generator do I need to run a house?
Most homes can run essentials on 5,000 to 7,500 W. Running central air and an electric range needs 15 to 22 kW.
What size generator for a fridge and freezer?
About 3,500 W covers both with room for lights and a phone charger.
What is the difference between running and starting watts?
Running watts are what an appliance uses steadily. Starting watts are the short surge when a motor starts.
Can a generator be too big?
It will work, but it uses more fuel and may run poorly at very light loads.
Can I run central air on a generator?
A 3 ton central AC needs about 3,800 W running and up to 11,400 W to start, so a 15 kW or larger unit, or a soft starter.
How long will a generator run on a tank?
Usually 8 to 12 hours at half load for a portable model.
Do I need a transfer switch?
Yes, to connect to your home's wiring safely. It stops power flowing back into the grid.
Stay powered when the grid goes down
List the appliances that matter, add their running watts and allow for the biggest motor surge. Pick a generator with headroom, connect it through a transfer switch and keep it running outside, and a power cut becomes an inconvenience rather than an emergency.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.