How do I calculate what size generator I need?

Add the running watts of every appliance that will run at the same time, then add the largest extra starting wattage among them (its starting watts minus its running watts). For the example that is 2,340 W plus 1,000 W, or 3,340 W. Then choose a generator with a running (rated) wattage at or above that figure.

What is the difference between running watts and starting watts?

Running (rated) watts are what an appliance draws continuously. Starting (surge) watts are a brief, higher draw when a motor or compressor starts; electric motors can need roughly two to three times their running watts to start. The typical refrigerator / freezer here runs at 200 W and starts at 1200 W, and the sump pump (1/3 hp) runs at 800 W and starts at 1300 W. Lights, TVs, laptops and heaters have little or no surge.

Why add only the largest starting surge?

The standard method assumes motors do not all start in the same instant. The generator must carry everything already running plus the biggest start, so the calculator adds the single largest extra wattage once. If several large motors may start together, for example after power returns, size above this minimum or use a soft-start device.

Why does the calculator suggest more than the minimum?

Running a generator near its limit leaves no margin if a wattage is underestimated, and the engine runs hard. As a guideline, many people size so the continuous load is at most about 80% of the rating: 3,340 W becomes 4,175 W. At 90% it would be 3,712 W and at 70% 4,772 W. This is guidance, not a manufacturer rule; follow your generator's manual.

How many amps is my load at 120 and 240 volts?

Amps are watts divided by volts. The example minimum of 3,340 W is 27.8 A at 120 V and 13.9 A at 240 V. Each 120 V receptacle on a generator has its own amp limit, so also check that no single outlet or circuit is overloaded.

How much do heavy loads add to the size?

A window air conditioner (typically 1500 W running, 2200 W starting) added to the example raises the minimum from 3,340 W to 4,840 W, and a space heater on high (1500 W) adds its full 1500 W with no surge. Electric heating and air conditioning are the loads that most often decide the generator size.

Where do the typical wattages come from?

From generator wattage charts published by Honda and Champion Power Equipment, rounded. Charts differ from one another, and some give ranges, so the table on this page says which end was used. They are typical values, not specifications of your appliance. Every number in the calculator can be edited, and the nameplate (or volts times amps) is the better source.

Can I plug a generator into a wall outlet or my panel?

No. Feeding a generator into a home outlet or panel (backfeeding) can send power out onto the utility lines and endanger line workers and neighbors, and can damage the generator or wiring. Plug appliances directly into the generator with suitable extension cords, or have a licensed electrician install a transfer switch or interlock that meets local code.

Is it safe to run a generator in a garage or near a window?

No. Generators produce carbon monoxide, a colorless, odorless gas that can kill within minutes. The U.S. Consumer Product Safety Commission and the CDC say to run a generator only outdoors, well away from the house (the CDC says at least 20 feet from doors, windows and vents), never in a garage, basement or enclosed space, and to use battery-operated carbon monoxide alarms. Opening doors or windows does not make indoor use safe.

Is anything I type stored or sent?

No. The calculation runs in your browser and nothing you enter leaves your device.

Generator Size Calculator

To size a generator, add up the running watts of everything you want to power at once, then add the extra starting watts of the single appliance that needs the biggest surge. The example list of 6 appliances runs on 2,340 W, and the refrigerator needs 1,000 W more to start, so the minimum generator size is 3,340 W. Allowing the generator to run at no more than 80% of its rating, a guideline rather than a rule, suggests about 4,175 W. Pick appliances from the list or type your own running and starting watts, set the quantity, and every figure, including amps at 120 and 240 volts, updates as you type.

Updated 2026-10-12Runs in your browser: nothing you enter is uploadedHow this tool works

3,340 W

Minimum generator size: 2,340 W running plus 1,000 W for the largest start. About 4,175 W keeps the load at or below 80%.

Running watts and extra starting watts for each appliance
Appliance	Qty	Running	Extra to start
Refrigerator / freezer	1	200 W	1,000 W
Sump pump (1/3 HP)	1	800 W	500 W
Furnace fan (1/2 HP)	1	800 W	800 W
LED light bulb	10	100 W	None
Television (46 inch flat screen)	1	190 W	None
Laptop computer	1	250 W	None

Total running watts: 2,340 W
Largest starting surge (extra): 1,000 W (Refrigerator / freezer)
Minimum generator size (running + surge): 3,340 W
Suggested size at 80% load: 4,175 W
Running load, amps at 120 V / 240 V: 19.5 A / 9.8 A
Minimum size, amps at 120 V / 240 V: 27.8 A / 13.9 A
Suggested size, amps at 120 V / 240 V: 34.8 A / 17.4 A

Generator safety

Carbon monoxide kills. Run a generator only outdoors, well away from the house (the CDC says at least 20 feet from doors, windows and vents), never in a garage, basement or other enclosed space, and use battery-operated CO alarms (CPSC, CDC).
Never plug a generator into a wall outlet or the breaker panel (backfeeding). It can energize utility lines and hurt line workers. Use a transfer switch or interlock installed by a licensed electrician, or plug appliances straight into the generator.

The example load, appliance by appliance

Running watts add up; only the largest extra starting wattage is added once.
Appliance	Qty	Running	Extra to start
Refrigerator / freezer	1	200 W	1,000 W
Sump pump (1/3 HP)	1	800 W	500 W
Furnace fan (1/2 HP)	1	800 W	800 W
LED light bulb	10	100 W	None
Television (46 inch flat screen)	1	190 W	None
Laptop computer	1	250 W	None
Total running watts		2,340 W
Largest extra start (Refrigerator / freezer)		1,000 W
Minimum generator size		3,340 W

Typical wattages used in the list

Typical figures, rounded, from published charts by Honda and Champion. Real appliances vary with size, age and brand; use the nameplate, or edit the numbers in the calculator.
Appliance	Running watts	Starting watts	Extra to start	Source and note
Refrigerator / freezer	200	1,200	1,000	Honda. Honda: 132 to 192 W running, 1,200 W starting; upper end, rounded.
Sump pump (1/3 HP)	800	1,300	500	Honda. Honda chart, 1/3 HP.
Furnace fan (1/2 HP)	800	1,600	800	Champion. Champion: 300 to 800 W running, 800 to 1,600 W starting; upper end.
Window air conditioner (10,000 BTU)	1,500	2,200	700	Honda. Honda figure as quoted in search results, not read on the chart itself. Window units vary widely (about 700 to 1,300 W running, 2,000 to 3,000+ W starting): use the nameplate.
Well pump (1 HP)	1,000	2,000	1,000	Champion. Champion: 1,000 to 2,000 W running, 2,000 to 4,000 W starting; low end used, so check the nameplate.
Microwave oven	1,000	1,000	0	Champion. Champion: 600 to 1,200 W, no extra starting surge.
LED light bulb	10	10	0	Bulb label. Honda says to use the wattage printed on the bulb; 10 W is a common LED.
Incandescent light bulb	60	60	0	Bulb label. Use the wattage printed on the bulb.
Television (46 inch flat screen)	190	190	0	Honda. Honda chart, 46 inch flat screen.
Laptop computer	250	250	0	Honda. Honda (New Zealand) chart.
Space heater (high setting)	1,500	1,500	0	Product label. Portable electric heaters are commonly rated up to 1,500 W; a resistive load with no surge.

Watts to amps at 120 and 240 volts

Amps are watts divided by volts, taking the power factor as 1.
Watts	Amps at 120 V	Amps at 240 V
1,000 W	8.3 A	4.2 A
2,000 W	16.7 A	8.3 A
3,500 W	29.2 A	14.6 A
5,000 W	41.7 A	20.8 A
7,500 W	62.5 A	31.3 A

How headroom changes the suggested size

The same example load (3,340 W minimum) at different maximum continuous loads. The percentages are guidance, not a code or manufacturer requirement.
Maximum continuous load	Suggested generator size
100%	3,340 W
90%	3,712 W
80%	4,175 W
70%	4,772 W

How to size a portable generator

List what you must power. Choose an appliance from the list and press Add, or add a blank row and type your own. Only include what must run at the same time; a load you can switch off while another runs does not need to be counted.
Check the running and starting watts. Look at the nameplate or manual. A motor such as a refrigerator compressor or sump pump draws more to start than to run: the typical refrigerator here runs at 200 W and starts at 1200 W. Leave starting watts blank for loads with no surge, such as lights.
Set the quantity. Running watts are multiplied by the quantity. The example has 10 LED bulbs at 10 W each.
Read the minimum size. The calculator adds all running watts (2,340 W) and the largest single extra starting wattage (1,000 W, from the refrigerator / freezer) for 3,340 W.
Allow some headroom. A common guideline is to load a generator to no more than about 80% of its rating for continuous use. For the example that suggests 4,175 W. Change the percentage to see other values, then round up to the next generator size sold.

Frequently asked questions

How do I calculate what size generator I need?: Add the running watts of every appliance that will run at the same time, then add the largest extra starting wattage among them (its starting watts minus its running watts). For the example that is 2,340 W plus 1,000 W, or 3,340 W. Then choose a generator with a running (rated) wattage at or above that figure.
What is the difference between running watts and starting watts?: Running (rated) watts are what an appliance draws continuously. Starting (surge) watts are a brief, higher draw when a motor or compressor starts; electric motors can need roughly two to three times their running watts to start. The typical refrigerator / freezer here runs at 200 W and starts at 1200 W, and the sump pump (1/3 hp) runs at 800 W and starts at 1300 W. Lights, TVs, laptops and heaters have little or no surge.
Why add only the largest starting surge?: The standard method assumes motors do not all start in the same instant. The generator must carry everything already running plus the biggest start, so the calculator adds the single largest extra wattage once. If several large motors may start together, for example after power returns, size above this minimum or use a soft-start device.
Why does the calculator suggest more than the minimum?: Running a generator near its limit leaves no margin if a wattage is underestimated, and the engine runs hard. As a guideline, many people size so the continuous load is at most about 80% of the rating: 3,340 W becomes 4,175 W. At 90% it would be 3,712 W and at 70% 4,772 W. This is guidance, not a manufacturer rule; follow your generator's manual.
How many amps is my load at 120 and 240 volts?: Amps are watts divided by volts. The example minimum of 3,340 W is 27.8 A at 120 V and 13.9 A at 240 V. Each 120 V receptacle on a generator has its own amp limit, so also check that no single outlet or circuit is overloaded.
How much do heavy loads add to the size?: A window air conditioner (typically 1500 W running, 2200 W starting) added to the example raises the minimum from 3,340 W to 4,840 W, and a space heater on high (1500 W) adds its full 1500 W with no surge. Electric heating and air conditioning are the loads that most often decide the generator size.
Where do the typical wattages come from?: From generator wattage charts published by Honda and Champion Power Equipment, rounded. Charts differ from one another, and some give ranges, so the table on this page says which end was used. They are typical values, not specifications of your appliance. Every number in the calculator can be edited, and the nameplate (or volts times amps) is the better source.
Can I plug a generator into a wall outlet or my panel?: No. Feeding a generator into a home outlet or panel (backfeeding) can send power out onto the utility lines and endanger line workers and neighbors, and can damage the generator or wiring. Plug appliances directly into the generator with suitable extension cords, or have a licensed electrician install a transfer switch or interlock that meets local code.
Is it safe to run a generator in a garage or near a window?: No. Generators produce carbon monoxide, a colorless, odorless gas that can kill within minutes. The U.S. Consumer Product Safety Commission and the CDC say to run a generator only outdoors, well away from the house (the CDC says at least 20 feet from doors, windows and vents), never in a garage, basement or enclosed space, and to use battery-operated carbon monoxide alarms. Opening doors or windows does not make indoor use safe.
Is anything I type stored or sent?: No. The calculation runs in your browser and nothing you enter leaves your device.

How it works

Each row contributes quantity times running watts to the running total. A row’s extra starting watts are its starting watts minus its running watts; if starting watts are left blank, or are lower than running watts, no extra is counted. The surge added to the total is the largest single extra among rows with a quantity above zero, counted once, because the standard method assumes motors start one at a time. The minimum generator size is the running total plus that surge. The suggested size is the minimum divided by the chosen maximum continuous load (default 80%, allowed 50 to 100%), rounded up to a whole watt; the percentage is guidance and not taken from a standard. Amps are watts divided by 120 or 240 volts with the power factor taken as 1, so real amps for reactive loads can be somewhat higher. Typical wattages are rounded figures from published Honda and Champion charts, are starting points only, and are not specifications of any particular appliance. Generator ratings are also reduced at altitude and in heat, and surge capacity differs between models. This tool does not replace an electrician’s load calculation. Everything runs in your browser and nothing you type is sent anywhere.

Sources

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