Formula
Running watts are the sum of running watts times quantity. Starting watts are that sum plus the single largest gap between starting and running watts. Motors rarely all start together, and this page assumes they do not. Recommended minimum is the greater of the starting total and the running total divided by one minus the headroom. That figure rounds up to the next common size: 2,000, 3,500, 5,000, 7,500, 10,000, 12,000, 15,000, 20,000, 22,000, or 24,000 watts.
running = Σ (watts × quantity)
starting = running + largest (start − run)
size = max(running / (1 − headroom), starting)
A soft-start device can cut a compressor’s surge. This page does not model one. Portable and standby generators are different machines. Altitude and heat reduce output. The manual has that derate. This page does not. No medical equipment is listed. Amps are a resistive approximation.
Worked examples
Example list: refrigerator 700 / 2,200, sump pump 800 / 1,300, furnace fan 800 / 2,350, LED lights 200, microwave 1,000. Running 3,500 W. Largest surge 1,550 W. Starting 5,050 W. With 20 percent headroom the size is 7,500 W. 3,500 W is 29.17 A at 120 V and 14.58 A at 240 V.
LED 200, TV 200, laptop 100. Starting 500 W. With 20 percent headroom, 625 W, which rounds up to 2,000 W.
Reference
Honda Power Equipment wattage estimation guide, approximate running and starting watts. A range is shown as a range. The field uses the top of the range until you edit it. A chest freezer, a circular saw, a central air conditioner, and a well pump are left off because that chart does not give a pair for them.
Questions
What size generator do I need for a house?
Add the running watts of what you will run at once, then add the largest single motor surge, and keep continuous load under the headroom you set. A whole house is a different list from a few essentials. The nameplate and the generator manual override this page.
How many watts does a refrigerator need to start?
Honda’s chart lists an Energy Star refrigerator at 132 to 192 watts running and 1,200 watts starting. Older refrigerators are often higher. The 700 and 2,200 watt pair on the example row is a planning example, not that chart. Read the nameplate.
Can a 5,000 watt generator run a central air conditioner?
Often no. A central compressor’s starting watts are commonly well above 5,000, and this page does not publish a central-air pair because the Honda chart used here does not list one. Enter the nameplate. A window unit on that chart is 1,500 watts running and 2,200 starting.
How do you calculate starting watts?
Add every running watt, then add only the largest gap between starting and running watts. The assumption is that motors do not all start at the same instant. If two can start together, add both gaps yourself.
What is the difference between running watts and starting watts?
Running watts are the load once the motor is turning. Starting watts are the brief extra a motor or compressor draws as it comes up to speed. A heater or a lamp usually has no gap between the two.
How many amps is a 7,500 watt generator?
7,500 divided by 120 is 62.5 amps, and 7,500 divided by 240 is 31.25 amps, as a resistive approximation. A motor load is not a resistor. This is not a wire size.
What size generator for a sump pump?
Honda lists a 1/3 horsepower sump pump at 800 watts running and 1,300 starting, and a 1/2 horsepower pump at 1,050 and 2,150. The pump still has to share the generator with everything else that is on.
Sources
- Honda Power Equipment, wattage estimation guide. Refrigerator or freezer (Energy Star) 132–192 W running and 1,200 W starting; sump pump 1/3 hp 800 / 1,300 and 1/2 hp 1,050 / 2,150; furnace fan 1/3 hp 700 / 1,400 and 1/2 hp 875 / 2,350; window air conditioner 10,000 BTU 1,500 / 2,200; 1,000 W microwave 1,500 / 1,500; flat screen 120 W and 190 W; laptop 200–250 W.
- U.S. Department of Energy, Energy Saver, “Estimating Appliance and Home Electronic Energy Use,” as reprinted by Virginia Cooperative Extension (BSE-301P). Portable heater 750–1,500 W. The same note says the nameplate wattage, or amps times volts, is the figure to use.
- CPSC carbon monoxide guidance: a portable generator outdoors, away from doors and windows, with carbon monoxide alarms. NEC 2023 Article 702 is the optional-standby article. This page does not apply it.