Backup power
Generator size calculator
Add the running watts of everything you want on at once, then add the single largest starting surge (the extra watts a motor pulls for a second or two). Size the generator so the running total is no more than 80% of its rated running watts and the peak fits under its starting watts.
4,100 Wrecommended running watts
- Total running load
- 3,275 W
- Peak while the biggest motor starts
- 4,825 W
- Biggest single surge
- Gas furnace blower (1/2 hp) (+1,550 W)
- Current at 240 V / 120 V
- 13.6 A / 27.3 A
- Smallest common size that fits
- 7,500 W portable (L14-30 (30 A, 120/240 V))
Start motors one at a time. If everything with a motor started together, you would need 6,925 W of surge.
How the calculator sizes a generator
running total = Σ (running watts × quantity) peak = running total + largest (starting − running) recommended = running total ÷ 0.8
Two numbers matter on a generator's spec sheet. Running (rated) watts is what it can deliver for hours. Starting (surge) watts is what it can deliver for the second or two a motor needs to get spinning. Resistive loads like heaters, toasters and coffee makers draw the same at start-up as they do running. Anything with a compressor or an induction motor — refrigerators, sump and well pumps, furnace blowers, air conditioners — briefly pulls two to three times its running watts, and a central AC compressor can pull more.
The calculator assumes you switch motor loads on one at a time, which is how you should run a generator anyway. Only the biggest surge then lands on top of everything already running. The 80% margin keeps the engine out of its overload zone and leaves room for the loads you forgot.
Worked example: a typical outage kit
The defaults above are the loads most people keep on in a winter outage: refrigerator, ½ hp sump pump, gas furnace blower, LED lights, the internet router, phone charging and a microwave. Running total: 3,275 W. The biggest surge is the gas furnace blower (1/2 hp), adding 1,550 W, for a peak of 4,825 W. Divide the running total by 0.8 and you need about 4,100 running watts, so the smallest common size that fits is a 7,500 W portable.
Add a 3-ton central air conditioner and the answer changes completely: its compressor alone needs about 11,000 W to start. That is why central AC is the line between a portable generator and a standby one.
Running and starting watts used by the calculator
These are typical values for US appliances. Your nameplate beats them: look for watts, or multiply volts × amps. For motors, the nameplate may show LRA (locked-rotor amps); starting watts ≈ LRA × volts. Every value is editable in the calculator once you select the appliance.
| Appliance | Running W | Starting W | Surge |
|---|---|---|---|
| Refrigerator / freezer | 200 | 1,200 | 6.0× |
| Chest freezer | 100 | 600 | 6.0× |
| Sump pump (1/2 hp) | 1,050 | 2,150 | 2.0× |
| Well pump (1 hp, 240 V) | 1,500 | 4,000 | 2.7× |
| Gas furnace blower (1/2 hp) | 800 | 2,350 | 2.9× |
| LED lights (10 bulbs) | 100 | 100 | none |
| Internet modem + router | 25 | 25 | none |
| Phone and laptop charging | 100 | 100 | none |
| CPAP machine (with humidifier) | 60 | 60 | none |
| Microwave (1,000 W) | 1,000 | 1,000 | none |
| Coffee maker | 1,000 | 1,000 | none |
| Electric range, one burner | 1,500 | 1,500 | none |
| Dishwasher (no heated dry) | 300 | 300 | none |
| Window AC (10,000 BTU) | 900 | 2,700 | 3.0× |
| Central AC (3 ton) | 3,500 | 11,000 | 3.1× |
| Central AC (4 ton) | 4,700 | 14,000 | 3.0× |
| Space heater | 1,500 | 1,500 | none |
| Electric water heater (40 gal) | 4,500 | 4,500 | none |
| TV (55" LED) | 100 | 100 | none |
| Clothes washer | 500 | 1,200 | 2.4× |
| Gas clothes dryer (motor only) | 700 | 1,800 | 2.6× |
| Electric clothes dryer | 5,400 | 6,750 | 1.3× |
| Garage door opener (1/2 hp) | 875 | 2,350 | 2.7× |
| EV charging, Level 1 (12 A) | 1,440 | 1,440 | none |
Common generator sizes and what they carry
| Size | Running W | Starting W | Typical connection |
|---|---|---|---|
| 2,200 W inverter | 1,800 | 2,200 | 120 V outlets only |
| 3,500 W inverter | 3,000 | 3,500 | TT-30R (30 A, 120 V) |
| 5,000 W portable | 4,000 | 5,000 | L14-20 or L14-30 |
| 7,500 W portable | 6,000 | 7,500 | L14-30 (30 A, 120/240 V) |
| 9,500 W portable | 7,500 | 9,500 | L14-30 or 14-50 |
| 12,000 W portable | 9,500 | 12,000 | 14-50 (50 A, 120/240 V) |
| 14 kW standby | 14,000 | 14,000 | Hard-wired transfer switch |
| 18 kW standby | 18,000 | 18,000 | Hard-wired transfer switch |
| 22 kW standby | 22,000 | 22,000 | Hard-wired transfer switch |
| 26 kW standby | 26,000 | 26,000 | Hard-wired transfer switch |
A 30 A, 240 V inlet (L14-30) carries at most 7,200 W, and a 50 A inlet (14-50 or CS6365) carries 12,000 W. If your result is above 7,200 running watts, a 30 A transfer setup will be the bottleneck, not the generator. See L14-30 vs 14-50 inlets.
Whole house generator sizing
For a standby generator that powers the whole house automatically, size for every large load that could run together, not just the essentials. A practical way to start: add the running watts of the central AC or heat pump, the well pump, the electric water heater, range and dryer if you have them, and about 3–5 kW for lights, outlets and the kitchen; then add the largest motor's starting surge. Homes with gas heat and no central air usually land at 10–14 kW, homes with central air at 18–22 kW, and all-electric homes at 24 kW or more.
Two details change the number on the box. Air-cooled standby units are rated on propane and make roughly 10% less on natural gas (a 22 kW unit is about 19.5 kW on natural gas). And load-management modules can lock out the dryer or water heater while the AC runs, so a smaller generator covers a bigger house. The definitive answer is a load calculation under NEC 220.82 (existing homes) by an electrician.
Safety. Never run a generator indoors, in a garage, or near windows and vents: carbon monoxide kills within minutes. The U.S. Consumer Product Safety Commission advises at least 20 feet from the house with the exhaust pointed away. Connect to house wiring only through a transfer switch or an interlock kit installed under permit; plugging a generator into an outlet (backfeeding) can electrocute utility workers.
Common questions
What size generator do I need to run a refrigerator?
A modern refrigerator runs on about 150–250 W but needs 1,000–1,500 W for a second when the compressor starts. A 2,000 W inverter generator runs one comfortably, with room for lights and phone charging. See the full breakdown.
What size generator do I need to run a whole house?
Most US homes with gas heat and no central AC are covered by 7,500–9,500 running watts if you manage loads. With central air, an electric water heater or an electric range, you are usually in 18–26 kW standby territory, and an electrician should do a load calculation (NEC 220.83 or 220.87).
Should I size on running or starting watts?
Both. Running watts must cover everything you run at once with about 20% headroom; starting watts must cover that running load plus the biggest single motor start.
Is a 7,500 W generator enough?
For a refrigerator, freezer, sump pump, furnace blower, lights, TV and microwave, yes — that set peaks around 5,000–6,000 W with one motor starting. It will not start a 3-ton central AC without a soft-start kit.
Guides that use this calculator
- What Size Generator Do I Need for Central AC?
- Interlock Kit vs. Transfer Switch: Which One Do You Need?
- Generator Carbon Monoxide Safety: How Far From the House?