Generator Size Calculator
Find what size generator you need. Choose the appliances you want to run at the same time, or enter their running and starting watts, and the calculator adds them up the way generator sizing guides do: all the running watts, plus the largest extra surge to start one motor, plus headroom. Use it as a generator wattage calculator for a portable, standby, or RV generator.
- For each appliance you want to run at the same time, pick it from Appliance. The running and starting watts fill in with typical values. If the nameplate or manual of your appliance shows different watts, enter them.
- For an appliance that isn't listed, choose Custom, give it a name if you like, and enter its Running watts.
- Enter Starting watts for appliances with a motor, such as a refrigerator, a pump, or an air conditioner. This is the total power while it starts, including its running watts. If your manual only gives the additional starting watts, add the running watts to it. Leave it blank if the appliance has no motor.
- Set Quantity for several of the same appliance, such as light bulbs, and use Add appliance for the next one.
- Check the Headroom. It starts at 20%.
The watt fields accept whole numbers and decimals, such as
1,200 or 75.5, in watts. If the label only
shows amps and volts, multiply them: 120 V × 3 A = 360 W.
How generator size is calculated
A generator has to keep every appliance running and still have enough power when one motor starts. Motors draw a short surge for a few seconds as they start, so the calculator adds the largest extra surge to the total running watts:
Running watts = sum of (running watts × quantity)
Starting watts = running watts + the largest (starting watts − running watts) of one appliance
Recommended size = starting watts × (1 + headroom ÷ 100)
This assumes you start motors one at a time while everything else is running, which is how Champion Power Equipment's "What Size Home Standby Generator Do I Need?", Briggs & Stratton's "Choosing a Portable Generator", and the Know Generator Limits section of Generac's GP Series portable generator manual size a generator. The extra surge counts for one appliance only, even if you have two of the same, because they don't start at the same moment. Start the appliance with the biggest starting watts first.
The calculator picks the appliance with the largest difference between starting and running watts, not the one with the largest starting watts. They can be different appliances, and the largest difference is what the generator has to cover on top of everything that's already running.
Starting watts
The calculator reads starting watts (also called surge watts) as the total power while the appliance starts, including its running watts. Honda's Portable Generators 2023 Catalog lists the same number in both columns for appliances without a motor (800 starting and 800 running watts for a toaster), which only works if starting watts include running watts.
Some guides, such as Generac's Generator Wattage Worksheet, add the starting watts of one appliance straight to the total running watts. That counts the running watts of that appliance twice, so those guides come out higher than this calculator by that much.
Headroom
The recommended size adds 20% on top of the starting watts, so the generator doesn't run at its limit. Champion's standby guide adds a 20% safety cushion to its total, and Briggs & Stratton's "Motor Starting 101" suggests a safety margin of 20% to 30%. You can set it to 0% to see the minimum.
Compare the results with both ratings of a generator. Its running (rated) watts should cover your running watts, and its starting (surge) watts should cover your starting watts.
Generators make less power on some fuels. Champion's "What Size Generator Do I Need?" says propane gives about 10% less than gasoline and natural gas about 10% to 15% less, so compare against the rating for the fuel you'll use.
Appliance wattages
The presets come from Champion Power Equipment's "Generator Wattage Chart: Common Appliances & Tools" (Home & Backup Power, and the RV air conditioners from Outdoors & Recreation). The chart gives ranges, and the calculator uses the top of each range, because a generator that's too small won't start the appliance. A dash means the chart lists no extra starting watts, so the starting watts are the same as the running watts.
| Appliance | Running | Starting |
|---|---|---|
| Kitchen and laundry | ||
| Refrigerator | 400 W | 1,200 W |
| Freezer | 500 W | 1,000 W |
| Washing machine | 1,000 W | 2,300 W |
| Electric dryer | 6,000 W | — |
| Dishwasher | 2,400 W | — |
| Microwave | 1,200 W | — |
| Electric range (one burner) | 2,400 W | — |
| Heating and cooling | ||
| Window AC (5,000 BTU) | 600 W | 1,200 W |
| Window AC (10,000 BTU) | 1,200 W | 2,400 W |
| Central AC (2 ton) | 2,500 W | 4,500 W |
| Central AC (3 ton) | 3,500 W | 6,000 W |
| Central AC (4 ton) | 5,000 W | 8,000 W |
| Furnace fan (1/2 HP) | 800 W | 1,600 W |
| Space heater | 1,500 W | — |
| Window fan | 200 W | — |
| Ceiling fan | 75 W | — |
| Water | ||
| Electric water heater | 4,500 W | — |
| Sump pump (1/3 HP) | 800 W | 1,600 W |
| Sump pump (1/2 HP) | 1,050 W | 2,150 W |
| Well pump (1/2 HP) | 1,000 W | 2,000 W |
| Well pump (1 HP) | 2,000 W | 4,000 W |
| Lights, electronics, and medical | ||
| TV (32"–55") | 400 W | — |
| LED light bulb | 15 W | — |
| Phone or laptop charger | 100 W | — |
| CPAP (without heat) | 60 W | — |
| CPAP (with heated humidifier) | 120 W | — |
| Home security system | 40 W | — |
| Internet router or modem | 20 W | — |
| RV | ||
| RV air conditioner (13,500 BTU) | 2,000 W | 3,500 W |
| RV air conditioner (15,000 BTU) | 2,500 W | 4,500 W |
These are typical values, and manufacturers' charts differ a lot for the same appliance. For a 1/2 HP sump pump, Champion's chart gives 800–1,050 running and 1,300–2,150 starting watts, Honda's 2023 catalog gives 1,050 running and 3,000 starting watts, and Generac's GP Series manual gives 800 to 1,050 running watts for a sump pump of any horsepower and says to allow 3 times that to start. For a refrigerator, Champion's chart gives 150–400 running watts, while Champion's standby guide gives 700–800 for a refrigerator or freezer. Use the watts on your appliance's nameplate or in its manual when you have them. The U.S. Department of Energy notes that the nameplate wattage is the most power the appliance draws.
Examples
Backup power for the essentials
A refrigerator (400 W running, 1,200 W starting), a 1/2 HP sump pump (1,050 W, 2,150 W), 10 LED bulbs at 15 W, a TV at 400 W, and a router at 20 W run on 2,020 W. The sump pump has the largest extra surge, 2,150 − 1,050 = 1,100 W, so starting it takes 3,120 W. With 20% headroom, the recommended size is 3,744 W (3.74 kW).
A home with central air
Champion's standby guide adds the running watts of a refrigerator (800 W), a sump pump (1,050 W), a 3-ton central AC (3,500 W), and 500 W of lights and Wi-Fi, for 5,850 W. The AC starts at 10,000 W, so it adds 10,000 − 3,500 = 6,500 W to start, for 12,350 W. With 20% headroom, that's 14,820 W (14.82 kW).
Largest difference, not largest starting watts
One appliance runs at 3,000 W and starts at 4,000 W. Another runs at 500 W and starts at 2,000 W. The running watts are 3,500 W. The second one has the larger difference, 1,500 W, so starting it needs 5,000 W. Adding only the first one's extra 1,000 W would give 4,500 W, too little for the second one to start.
An RV
A 13,500 BTU RV air conditioner (2,000 W, 3,500 W), a microwave (1,200 W), and a phone charger (100 W) run on 3,300 W and need 4,800 W while the air conditioner starts. With 20% headroom, the recommended size is 5,760 W (5.76 kW).
FAQ
It depends on what you run, not the size of the house. A refrigerator, a sump pump, lights, a TV, and a router need about 3,744 W with 20% headroom. A home that also runs a 3-ton central air conditioner needs about 14,820 W.
Running watts are what an appliance draws while it runs. Starting watts are the higher draw for a few seconds while a motor starts. The calculator treats starting watts as the total, including the running watts.
No. Add only the largest extra surge of one appliance to the total running watts. Motors rarely start at the same moment, so the generator only has to cover one start at a time.
Multiply amps by volts. An appliance that draws 3 A at 120 V uses 360 W. Most US appliances use 120 V, and larger ones like dryers and electric ranges use 240 V.
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