Off Grid Living WisdomGenerator Size Calculator
Use this free generator size calculator to answer the question every buyer asks: what size generator do I need? Tick the appliances you want running during an outage and it adds up the running watts, the starting surge and the daily energy, then shows the generator, power station or battery size your house actually needs.
How to read your results
- Running watts is the steady power everything you ticked needs at the same time. The calculator adds 20% headroom, because a generator running near its limit struggles and burns more fuel.
- Starting (peak) watts covers the extra burst a motor needs for about a second when it starts. You only need room for the single largest surge, because motors rarely start at the same moment.
- Watt-hours per day matters only for batteries and power stations, which store a fixed amount of energy.
Why fridges trip small generators
Pumps and blowers need two to three times their running watts to start. Fridge and freezer compressors often need five to ten times: a fridge that runs on 150 watts can briefly pull 1,200 or more. That's why a generator that looks big enough on paper can stall the moment the fridge kicks on. Check the label, or use the starting watts in the table.
What size generator do I need? Typical homes
| What you want to run | Daily energy | Usually enough |
|---|---|---|
| Essentials: fridge, lights, phones, internet, CPAP | 2–4 kWh | 2,000–3,500 W inverter generator |
| Essentials + furnace blower or well pump, freezer, microwave | 5–12 kWh | 5,000–8,000 W portable with a transfer switch |
| Whole house, including central AC and electric cooking | 20–40+ kWh | 14–26 kW home standby generator |
Most private well pumps run on 240 volts. If you need the well, buy a generator with a 240 V outlet (usually a 30 A or 50 A twist-lock), connected through a transfer switch.
Sized your generator? Now connect it safely and plan the fuel.
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Frequently asked questions
What size generator do I need to run a refrigerator?
A modern fridge uses 100–250 watts running but 800–2,200 watts to start. A 2,000–3,000 watt inverter generator runs a fridge plus lights, phones and internet. Check your fridge's label for its exact numbers.
Can a 5,000 watt generator run a house?
It can run the essentials of most homes, including a fridge, freezer, gas furnace blower, lights and electronics, but not central air, an electric range or an electric water heater. Connect it through a transfer switch or interlock installed by a licensed electrician.
What size generator do I need for a well pump?
A 1/2 HP well pump needs about 1,000–1,500 running watts and 2,000–4,000 watts to start, on 240 volts. Plan on a generator of 5,000 watts or more with a 240 V outlet.
What size generator do I need for my whole house?
Whole-house backup, including central AC and electric appliances, usually takes a 14–26 kW home standby generator. Most families only need the essentials, which a much smaller generator covers.
Is bigger always better?
No. An oversized generator costs more, burns more fuel at light loads and is harder to move. Size for your real running watts plus 20%, with enough starting watts for your largest motor.