Generator Size Calculator

Two numbers decide your generator size. The first is how many watts your appliances draw while they are simply running. The second is the extra kick a motor needs in the first half-second as it spins up, the starting or surge watts. A fridge that runs at 700 W can demand 2,200 W for a moment when the compressor kicks in.

Here is the part most sizing guides get wrong, motors do not all start at the same instant. Your fridge, your well pump and your furnace fan each surge for a fraction of a second, and the odds of two doing it in the same fraction of a second are low. So you add up every running watt, then add only the single biggest surge on top.

1. Step 1 Add up the running watts total_running = sum of running watts for everything you checked
2. Step 2 Find the biggest single surge largest_surge = max(starting watts − running watts) across your appliances Only the largest one, because motors rarely start simultaneously.
3. Step 3 That is your true minimum required = total_running + largest_surge
4. Step 4 Add 20% headroom recommended = required × 1.2 A generator run at 100% is loud, hot and short-lived. 20% spare keeps it in its efficient range and leaves room for the appliance you forgot.
5. Step 5 Round up to a size you can buy Next size in 2,000 / 3,500 / 4,500 / 6,500 / 8,000 / 10,000 / 12,000 / 15,000 / 18,000 / 22,000 / 26,000 / 32,000 / 38,000 / 48,000 W
6. Step 6 Or, from the house, NEC 220.82 general = 3 VA/sq ft + 4,500 + fixed appliance nameplates
7. Step 7 Apply the demand factor demand = first 10,000 VA at 100% + remainder at 40% The step that stops a nameplate total from becoming a nonsense answer.
8. Step 8 Add heating or cooling calculated = demand + max(heating, cooling) Never both. They do not run at the same time, and the code says so.

For example, a fridge (700 W running, 2,200 W starting), LED lights (150 W) and device charging (100 W) gives 950 W running. The fridge has the biggest surge delta at 1,500 W. So you need 2,450 W minimum, 2,940 W with headroom, a 3,500 W generator.

Sizing from the whole house instead works differently, because there is a published method for it. NEC 220.82, the optional calculation for a dwelling unit. It allows 3 VA per square foot for lighting and receptacles, adds 4,500 VA for the small-appliance and laundry circuits, adds the nameplate of every fixed appliance, then takes the first 10 kVA at 100% and everything above it at 40%. Heating or air conditioning is added afterwards, whichever is larger, never both, since they never run together.

The result is your calculated load, and NEC 702.4(B)(2) lets an optional standby system be sized to exactly that with no extra margin, because the demand factors already are the margin. That is why a gas-heated 2,000 square foot house comes out at 17,207 VA and an 18 kW generator rather than the 22 kW that adding a safety factor on top would suggest.

It is also why sizing on your service rating is wrong. A 200 A service can theoretically pass 48,000 VA, but the calculated load of the house behind it is typically 70 to 140 A. Quoting a generator against the breaker rather than the load routinely doubles the machine, and the price.

Two numbers decide your generator size. The first is how many watts your appliances draw while they are simply running. The second is the extra kick a motor needs in the first half-second as it spins up, the starting or surge watts. A fridge-freezer that runs at 150 W can demand 900 W for a moment when the compressor kicks in.

Here is the part most sizing guides get wrong, motors do not all start at the same instant. Your fridge, your borehole pump and your boiler pump each surge for a fraction of a second, and the odds of two doing it in the same fraction of a second are low. So you add up every running watt, then add only the single biggest surge on top.

European homes shift the balance towards resistive loads. A kettle, an oven or a washing machine heating its own water pulls 2–3 kW with no surge at all, which lands entirely on the running total. That is usually what decides the size, not the motors.

1. Step 1 Add up the running watts total_running = sum of running watts for everything you ticked
2. Step 2 Find the biggest single surge largest_surge = max(starting watts − running watts) across your appliances Only the largest one, because motors rarely start simultaneously.
3. Step 3 That is your true minimum required = total_running + largest_surge
4. Step 4 Add 20% headroom recommended = required × 1.2 A generator run at 100% is loud, hot and short-lived. 20% spare keeps it in its efficient range and leaves room for the appliance you forgot.
5. Step 5 Round up to a size you can buy Next size in 2,000 / 3,000 / 4,000 / 5,500 / 6,500 / 8,000 / 10,000 / 12,000 / 15,000 W

For example, a fridge-freezer (150 W running, 900 W starting), a boiler and its pump (150 W running, 450 W starting), LED lights (100 W) and device charging (100 W) gives 500 W running. The fridge-freezer has the biggest surge delta at 750 W. So you need 1,250 W minimum, 1,500 W with headroom, a 2,000 W generator. Add the kettle and it jumps to 4,000 W.

• Only one motor starts at a time. This is true in practice but not guaranteed, if your well pump and AC habitually cycle together, add the second surge delta manually by editing the wattages.
• A 20% headroom factor is applied. Running a generator continuously at full nameplate output shortens its life and makes it much louder.
• The wattages are typical values for common household appliances. Yours may differ by 20% or more. Check the data plate on the appliance and edit the numbers.
• Generator nameplate ratings are peak/starting watts. Continuous running output is usually 10–20% lower, which the headroom factor partially covers.
• Altitude and temperature derate engines. Above roughly 3,000 ft, expect to lose about 3% of output per 1,000 ft.
• Resistive loads (heaters, kettles, incandescent bulbs) have no surge; their starting and running watts are the same.
• This does not size a transfer switch or check your panel. Anything hardwired needs a licensed electrician.
• The whole-house mode uses NEC 220.82, the optional dwelling-unit calculation. It is a sizing method, not a measurement. It deliberately errs high so a service is never undersized, and your actual peak demand will be lower.
• Floor area drives only the lighting and receptacle allowance. Whether your heat, range, dryer and water heater are gas or electric moves the answer far more than square footage does, which is why two identical-looking houses can need generators twice apart.
• The service rating is used only for comparison. A generator is sized on your calculated load under NEC 702.4(B)(2), never on the rating of your main breaker.

• Only one motor starts at a time. This is true in practice but not guaranteed, if your borehole pump and heat pump habitually cycle together, add the second surge delta manually by editing the wattages.
• A 20% headroom factor is applied. Running a generator continuously at full nameplate output shortens its life and makes it much louder.
• The wattages are typical European figures. Yours may differ by 20% or more. Check the rating plate and edit the numbers.
• Generators sold in Europe are often rated in kVA rather than watts. Multiply kVA by about 0.8 to get usable watts, a 5 kVA set is roughly a 4,000 W generator.
• Everything is assumed to be 230 V single-phase, which covers almost all domestic appliances. Ovens, hobs and large heat pumps are sometimes wired 400 V three-phase, particularly in Germany, Austria and Scandinavia. Those are flagged in the result.
• Altitude and temperature derate engines. Above roughly 1,000 m, expect to lose about 10% of output per 1,000 m.
• Resistive loads (kettles, immersion heaters, ovens, hobs) have no surge; their starting and running watts are the same. They are also the largest single loads in most European homes.
• This does not size a changeover switch or check your consumer unit. Anything hardwired needs a qualified electrician.
• There is no whole-house sizing mode here, and that is deliberate. Square footage, "200 amp service" and the NEC 220.82 calculation on the US version of this page are all American; the European equivalent is a diversity assessment under BS 7671 Appendix A, which works differently. Whole-house standby generation is also rare in Europe, so the appliance list is the tool that fits the job.