Generator Size Calculator
Size a home or job-site generator from an appliance list: running watts, the largest motor starting surge and headroom, with editable typical wattages.
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About this toolWhat is the Generator Size Calculator?
List what you need to run at the same time, tick the loads that will actually be on, and this calculator sizes a generator the way manufacturers' worksheets do: add up the running watts, add the single largest extra surge a motor needs to start, then add headroom so the generator is not running flat out. It suggests the next common portable or standby size.
The list starts with a typical home outage set: refrigerator, sump pump, furnace blower, lights and the TV and router. Each line has editable running and starting watts and a quantity, and you can add typical loads from a menu (well pumps, window and central air conditioners, water heaters, power tools) or your own nameplate figures.
The results show where the peak comes from, so if a central air conditioner or a 1 hp well pump forces you up two sizes, you can see it and decide whether a soft starter or load management is cheaper than a bigger generator.
- Editable appliance list with running and starting watts and quantities
- Typical wattages for common household and job-site loads
- Largest-start method with adjustable headroom
- Optional credit for the generator's surge rating
- Suggested common generator size and current at 120/240 V
How to use the Generator Size Calculator
- Untick anything that will not run during the outage or job, and set quantities.
- Replace typical wattages with nameplate values where you can. For a motor without a starting figure, use two to three times the running watts.
- Add loads from the typical list or as custom lines.
- Choose headroom on the running load (20 to 25% is usual) and whether to count the generator's own surge rating.
- Read the running total, peak at motor start, minimum rated output and the suggested generator size.
Worked examples
Essential home circuits
Input
Fridge, 1/2 hp sump pump, furnace blower, LED lights, TV and router; 20% headroom; no surge credit
Output
2,900 W running, 4,450 W peak, 5 kW generator
The furnace blower has the largest surge (+1,550 W), so it sets the peak.
Adding a 1 hp well pump
Input
Same list plus a 1 hp, 240 V well pump
Output
4,900 W running, 6,900 W peak, 7.5 kW generator
The pump's 2,000 W extra surge now dominates. It also needs a 240 V outlet on the generator.
Whole-house with central air
Input
Essentials plus a 3-ton central AC and a microwave
Output
7,900 W running, 14,900 W peak, 17 kW generator
The air conditioner's 7,000 W starting surge doubles the requirement. A soft starter can bring it down sharply.
Job site
Input
7-1/4 in circular saw, 1 hp compressor, lights
Output
3,100 W running, 6,000 W peak, 6.5 kW generator
If the compressor never starts while the saw is cutting, the surge-credit option lets a smaller inverter unit work.
Background and insight
Resistive loads such as heaters, kettles and incandescent lights draw the same power when they switch on as when they run. Induction motors in compressors, pumps and fans draw several times their running current for a second or two while they accelerate, which shows up as starting watts. The generator must ride through that surge without its voltage collapsing.
The largest-start method assumes motors start one at a time, so it adds only the biggest surge: required watts = sum of running watts + max(starting − running). For a refrigerator (700 W running, 2,200 W starting), a 1/2 hp sump pump (1,050 / 2,150 W), a furnace blower (800 / 2,350 W), lights and electronics, the running total is 2,900 W and the furnace blower adds the biggest surge of 1,550 W, so the peak is 4,450 W and a 5,000 W generator fits. Typical wattages here are drawn from generator makers' worksheets and vary a lot between models, so nameplate figures (watts = volts × amps) always win.
Portable generators list a running (rated) and a higher surge (starting) wattage; inverter models typically surge 10 to 30% above rating for a few seconds. Home standby units are rated in kW at 0.8 power factor and quote motor-starting ability separately. By default this calculator does not count on any surge rating, the conservative choice; you can let it use a 20% or 50% surge margin. However you connect it, the NEC requires transfer equipment for optional standby systems (702.5) so the generator cannot back-feed the utility line.
Common use cases
Buying a portable generator
Pick a size that keeps the fridge, sump pump and furnace running in an outage without tripping.
Specifying home standby
Compare essential-circuit and whole-house loads before talking to an installer.
Job-site power
Check that a generator can start a compressor while saws and lights are running.
RV and camping
See whether a 2,000 W inverter generator can start the rooftop air conditioner.
Tips and limitations
- Stagger motor starts. Letting the fridge start before switching on the well pump keeps the peak at one surge.
- Soft starters on air conditioners and well pumps cut starting current by 50 to 70% and can save a generator size.
- Check voltage: 240 V loads such as well pumps, dryers and central AC need a generator with a 240 V outlet or connection.
- Generators lose about 3 to 4% of output per 1,000 ft of altitude and in high heat; see the manual.
- Never back-feed a house through an outlet. Use a transfer switch or interlock installed by an electrician.
Frequently asked questions
What size generator do I need to run my house?
For essentials (fridge, sump pump, furnace blower, lights, router) most homes need about 5 to 7.5 kW. Running central air or electric heat pushes that to 15 to 22 kW or more. Add up your own loads with the largest-start method to be sure.
What is the difference between running watts and starting watts?
Running watts are what an appliance draws continuously. Starting (surge) watts are the brief extra power a motor needs to get moving, often two to three times its running watts.
How do I calculate generator size with starting watts?
Add the running watts of everything that will be on, then add the single largest difference between starting and running watts. Add 20 to 25% headroom on the running total.
Will a 5000 watt generator run a refrigerator and a sump pump?
Yes, typically. They draw about 1,750 W running together, and the larger of their surges adds about 1,500 W, for a peak near 3,250 W.
Do I need a transfer switch for a portable generator?
To power house circuits, yes: a listed transfer switch or panel interlock prevents back-feeding the utility line, which can injure line workers. Extension cords to individual appliances do not need one.
Why does a central air conditioner need such a big generator?
Its compressor can draw several times its running current while starting. A 3-ton unit running at 3,500 W can surge to about 10,000 W, unless fitted with a soft starter.
Is there an in-depth guide to the Generator Size Calculator?
Yes. “What Size Generator Do I Need? Running Watts, Surge and Headroom” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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