Generator Size Calculator (Running & Starting Watts for Your House)
What size generator you need for a power outage: add the running watts of everything you will run at once, then add the single biggest motor starting surge. Enter how many of each item you plan to run at the same time.
Formula
Total running watts = Σ (quantity × running watts). Total starting watts = total running watts + the single highest additional starting watts among the items you run, because motors rarely start at the same instant. Amps at 240 V = running watts ÷ 240. Margin is an optional buffer you choose.
Worked example
Inputs: Refrigerator/freezer: how many (700 W running, +2,200 W to start) = 1; Window AC, 10,000 BTU: how many (1,200 W running, +1,800 W to start) = 1; Central AC, 24,000 BTU (2 ton): how many (3,800 W running, +4,950 W to start) = 0; Central AC, 40,000 BTU: how many (6,000 W running, +6,700 W to start) = 0; Furnace fan blower, 1/2 HP: how many (800 W running, +2,350 W to start) = 0; Sump pump, 1/2 HP: how many (1,050 W running, +2,200 W to start) = 0; Water well pump, 1/3 HP: how many (1,000 W running, +2,200 W to start) = 0; Microwave, 1,000 W: how many (1,000 W running) = 1; Coffee maker: how many (1,000 W running) = 0; Washing machine: how many (1,150 W running, +2,250 W to start) = 0; Electric water heater: how many (4,000 W running) = 0; Space heater: how many (1,800 W running) = 0; Box fan, 20 in: how many (200 W running) = 0; Light bulb, 60 W: how many (60 W running) = 4; Garage door opener, 1/2 HP: how many (875 W running, +2,350 W to start) = 0; Other running watts (TV, router, chargers: from nameplates) = 300 W; Extra starting watts of your largest other motor (0 if none) = 0 W; Safety margin (your choice) = 10 %.
| Total running watts | 3,440 |
|---|---|
| Largest starting surge watts | 2,200 |
| Total starting watts | 5,640 |
| Running watts with margin | 3,784 |
| Starting watts with margin | 6,204 |
| Running amps at 240v | 14.3 |
FAQ
What size generator do I need to run a refrigerator?
With the typical figures used here, a refrigerator/freezer runs on about 700 W but needs about 2,200 W extra for a few seconds to start, so plan on at least 2,900 starting watts for the fridge alone. Check the nameplate or manual for your model.
What size generator runs a whole house with central air?
A 24,000 BTU (2 ton) central AC is listed at about 3,800 W running and 4,950 W extra to start, so the AC alone needs roughly 8,750 starting watts before anything else. Whole-house standby units are sized by a licensed electrician from your actual loads.
Why add only one starting surge?
Starting watts last two or three seconds, and in most cases only one motor starts or cycles at a time, so the worksheet method adds the single largest surge to the running total. If you stagger loads you can run a smaller generator.
Where do I find the exact wattage?
On the data plate or in the owner's manual. If only amps are listed, watts ≈ volts × amps. Motor-driven items without a starting figure are often estimated at 1 to 2 times running watts.
Is it safe to run a generator in the garage?
No. Ready.gov says to run generators only outdoors, at least 20 feet from windows, doors and attached garages, because of carbon monoxide. Connecting to house wiring needs a transfer switch installed per code.
Sources
- Generator Wattage Worksheet (PDF): typical running watts and additional starting watts by appliance; method = total running watts + the single highest starting watts
- Ready.gov Power Outages: generator safety (use outdoors only, at least 20 feet from windows, doors and attached garages)
Related
- Wire Size & Voltage Drop Calculator (AWG, Copper/Aluminum)
- Appliance Energy Cost Calculator (kWh and $ per Year)
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Last reviewed 2026-10-14.