Running watts and starting watts
Every appliance has a running (rated) wattage, the power it uses once it is going. Appliances with electric motors or compressors, such as refrigerators, freezers, sump and well pumps, furnace blowers and air conditioners, also need a short starting (surge) wattage when the motor starts, often two to three times the running figure for a second or two. Lights, electronics and heating elements have no significant surge.
A generator has two ratings to match: its running or rated output, which must cover everything running together, and its starting, peak or surge output, which must cover the running load plus the biggest single motor start. Because you can start motors one at a time, only the largest extra surge needs to be added.
Peak watts = total running watts + (starting − running watts of the largest motor)
Recommended size = peak watts × (1 + safety margin)
Worked example: keeping essentials on in an outage
The example list uses typical values: a refrigerator (700 W running, 2,200 W starting), a 1/2 hp sump pump (1,050 / 2,150 W), a 1/3 hp furnace fan (700 / 1,400 W), 300 W of lights and a 1,000 W microwave. Check your own labels: real figures vary by model.
- Total running watts: 700 + 1,050 + 700 + 300 + 1,000 = 3,750 W.
- Largest extra surge: the refrigerator adds 2,200 − 700 = 1,500 W when it starts.
- Peak: 3,750 + 1,500 = 5,250 W.
- With a 20% margin: 6,300 W peak and 4,500 W running, so look for a generator rated at least 6,300 starting watts and 4,500 running watts. The nearest common size is about 6,500 W.
Why add a safety margin?
Running a generator at its full rated output for hours shortens its life, and output falls at high altitude and in hot weather. A margin of 10–25% also leaves room for something you forgot, such as a phone charger, a TV or a second freezer. Inverter generators often have a lower continuous rating than their peak rating, so compare the running figure carefully.
Finding the wattage of your appliances
- The rating plate lists watts, or volts and amps: watts = volts × amps (see the watts to amps calculator).
- For motors, look for "LRA" (locked-rotor amps) or "starting amps" in the manual; starting watts ≈ voltage × LRA for a rough upper bound.
- For central air conditioners and well pumps, ask an electrician: their surge can be large, and a soft-start kit can reduce it.
Portable, inverter or standby?
Portable generators power a few essentials through extension cords. Inverter generators are quieter and produce cleaner power for electronics but are usually smaller. Standby generators are permanently installed and start automatically. Anything connected to your home wiring must go through a properly installed transfer switch or interlock, never by plugging into an outlet ("backfeeding"), which can kill utility workers and neighbours.
Safety
- Run generators outdoors only, far from doors, windows and vents. Generator exhaust contains carbon monoxide, which can kill within minutes. Fit CO alarms indoors.
- Keep the generator dry and let it cool before refuelling.
- Use extension cords rated for the load.
Fuel use and runtime
A generator burns more fuel at higher loads. Manufacturers list runtime at half or quarter load, for example "10 hours at 50% load" on a full tank. Running only essentials and staggering large loads, such as running the microwave while the sump pump is idle, reduces both fuel use and the size you need.
Common mistakes
- Adding every starting wattage together. Only the largest surge counts if motors start one at a time.
- Comparing your peak watts with the generator's running rating, or the reverse. Match both.
- Forgetting 240 V loads. Well pumps and central air need a generator with a 240 V output.
Estimates only. Check appliance labels and the generator manual, and have home connections installed by a licensed electrician.
Frequently asked questions
What size generator do I need to run a refrigerator?
Running watts plus its starting surge. With 700 W running and 2,200 W starting, you need at least 2,200 W peak for the fridge alone, more with other loads.
How do I calculate generator size?
Add all running watts, add the largest single extra starting surge, then add a 10–25% margin. 3,750 W running plus a 1,500 W surge with 20% margin is 6,300 W.
What is the difference between starting and running watts?
Running watts keep an appliance going; starting watts are the brief extra surge a motor needs to start.
Can I plug a generator into a wall outlet?
No. Backfeeding is dangerous and illegal in many places. Use a transfer switch or interlock installed by an electrician.
Sources
- US Consumer Product Safety Commission (cpsc.gov). Portable generator safety and carbon monoxide.
- US Occupational Safety and Health Administration (osha.gov). Portable generator hazards.
- Generator manufacturers' wattage worksheets and appliance rating plates.
Last reviewed 27 October 2026. Results are estimates; see our disclaimer.