Generator size calculator: running and starting watts
List the appliances you want to run at the same time. The calculator adds up their running watts, works out the brief starting peak when the biggest motor kicks on, and gives the running and starting ratings to look for on a generator. It works for gas, propane and dual-fuel generators, and for an inverter generator too.
How the calculation works
Starting watts = running watts + (largest starting watts − its running watts)
Generator running rating ≥ running watts × (1 + headroom)
Generator starting rating ≥ starting watts
Only one motor usually starts at any moment, so the calculator adds the extra starting draw of the single biggest motor, not of every motor. If two motors could start together, for example a well pump and a sump pump on the same storm night, add the second motor's extra draw yourself or stagger them.
Generators list two numbers. The running (or rated, or continuous) watts are what it can supply for hours. The starting (peak or surge) watts are what it can deliver for a few seconds while a motor gets going. Your running total must fit under the first, and your starting peak under the second. The 20% headroom keeps the generator from running flat out, which helps with voltage stability, noise and fuel economy, and leaves room for something you forgot.
Worked example: keeping the essentials on in an outage
The calculator starts with this list. The values are typical figures, so use your own labels.
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | 150 | 450 |
| Sump pump, 1/3 HP | 800 | 2,400 |
| Furnace blower, 1/3 HP | 600 | 1,200 |
| 6 LED bulbs | 54 | 54 |
| Wi-Fi router and modem | 20 | 20 |
| TV | 80 | 80 |
| 2 phones charging | 20 | 20 |
| Total | 1,724 |
- Running: 150 + 800 + 600 + 54 + 20 + 80 + 20 = 1,724 W.
- Starting: the sump pump has the biggest extra draw, 2,400 − 800 = 1,600 W. So the peak is 1,724 + 1,600 = 3,324 W.
- Running rating with 20% headroom: 1,724 × 1.2 ≈ 2,070 W.
- Starting rating: at least 3,330 W.
Any generator listing at least about 2,100 running watts and 3,400 starting watts covers this list. Add a 1,500 W microwave or space heater and the running total jumps to 3,224 W, so you'd need about 3,870 running watts, or you'd run those only while other loads are off.
240 V appliances
Well pumps, electric water heaters, electric dryers, ranges and central air conditioners usually run on 240 V. A generator can only power them if it has a 240 V outlet (often a 30 A or 50 A twist-lock) and the house has a transfer switch or interlock. The calculator flags the 240 V items in its list. Electric water heaters, ranges and dryers draw so much that most portable generator plans leave them off.
Connecting safely
- Never plug a generator into a wall outlet to feed the house ("backfeeding"). It can energize the utility lines and injure line workers, and it bypasses your breakers. Use extension cords rated for the load, or a transfer switch or interlock installed by a licensed electrician.
- Carbon monoxide. Fuel-burning generators produce carbon monoxide, which can kill within minutes. The U.S. Consumer Product Safety Commission advises running them outdoors only, at least 20 feet from the house, with the exhaust pointed away from doors, windows and vents. Never run one in a garage, basement or porch, even with the doors open, and keep battery CO alarms on every level of the home. See the CPSC carbon monoxide page and the CDC's guidance.
- Fuel. Let the engine cool before refueling, and store fuel away from the house in approved containers.
Generator or battery power station?
For a fridge, router, lights and phones, a large portable power station can work indoors with no fumes, as long as its battery holds enough watt-hours. For a sump pump, well pump or several days without power, a generator's fuel supply usually wins. Compare the two in our power station vs. gas generator guide, or size a battery for the same list with the power station size calculator.
Frequently asked questions
What size generator do I need to run a refrigerator?
A typical modern refrigerator runs on roughly 100–200 W and usually needs well under 1,500 W while its compressor starts (check its label), so almost any portable generator runs one. The question is what else runs at the same time. Add the fridge to your list above with everything else you want on.
Is starting watts the same as surge watts or peak watts?
Yes. Makers use "starting", "surge", "peak" and "maximum" watts for the brief output above the running rating. "Running", "rated" and "continuous" watts all mean the sustained output.
Can a small inverter generator run my house?
Small inverter generators, often around 2,000 W running, can run a fridge, lights, electronics and a modest motor or two, one at a time. They're quieter and give cleaner power for electronics. Most of them can't start a large pump or run 240 V loads.
How do I find an appliance's starting watts?
Check the manual or nameplate. Some list starting watts, or "locked rotor amps" (LRA) for compressors and pumps. Multiply amps by the voltage for an estimate. A clamp meter with an inrush function can measure it.