Pick the appliances you want to keep running during an outage or on a job site, and the calculator adds up their running watts, finds the biggest starting surge, and tells you the generator size to buy — with a 20 % safety margin and the class of generator (inverter, portable or standby) that fits.
Tick everything you want to run at the same time
Kitchen
Home systems
Heating & cooling
Living
Laundry & tools
RV & camping
Custom appliances
Recommended generator
5,500 W
portable (4,000–5,500 W)
Running watts
2,680
7 items
Largest starting surge
+1,550
Furnace fan (gas furnace, 1/2 hp)
Peak (start) watts
4,230
= running + largest surge
Recommendation = peak watts × 1.2 headroom, rounded up to 500 W. Look for a generator whose running (rated) watts is at least 3,300 and whose starting (peak) watts is at least 4,230. Start motors one at a time — the surge figure assumes the largest motor starts while everything else is already running.
📌 Wattages are typical values from manufacturer sizing charts; your appliance labels are the real numbers (amps × volts = watts). Never back-feed a generator into house wiring — use a transfer switch or interlock, and run it outdoors 20 ft from windows.
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How generator sizing works
running load = Σ running watts of everything on at once
Worked example (storm outage essentials): refrigerator 700 W, sump pump 800 W, furnace fan 800 W, ten LED bulbs 100 W, TV 150 W, chargers 30 W, modem and alarm 100 W = 2,680 W running. The furnace fan has the largest surge (2,350 − 800 = 1,550 W), so the peak is 4,230 W. With 20 % headroom that is about 5,100 W → a 5,000–5,500 W portable, or a 4,500 W unit if you start the furnace before the sump pump.
Watts from a label that only shows amps: multiply by volts (120 V for ordinary outlets, 240 V for dryers, ranges, well pumps and central AC). A 12 A window unit on 120 V draws 1,440 W running. For the electric bill side, the electricity bill calculator converts the same watts into dollars, and the BTU calculator sizes the AC you are trying to power.
Generator classes and what they run
Class
Rated watts
Typically runs
Price (2026)
Small inverter
1,800–2,200
Fridge or a few electronics, camping, CPAP
$400–1,200
Mid inverter
3,000–4,000
Fridge + furnace fan + lights, RV AC
$700–2,000
Portable
5,000–7,500
Essentials plus well pump or window AC, job-site tools
$600–1,500
Large portable / dual-fuel
9,000–13,000
Most of a house except central AC, or a 2-ton AC
$1,000–2,500
Home standby
14,000–26,000
Whole house including central AC, automatic transfer
$4,000–8,000 + install
Frequently asked questions
Essentials during an outage — refrigerator, furnace fan, sump pump, lights, TV, phone chargers and Wi-Fi — total about 2,500 running watts with a 3,900 W peak when the furnace fan starts, so a 4,500–5,000 W portable covers them comfortably. Add a well pump and a window AC and you are at 7,500 W; running central air (3 ton) needs 12,000 W of surge, which is standby-generator territory (14–22 kW for a whole house).
Running (rated) watts is the continuous power an appliance draws. Starting (surge or peak) watts is the burst a motor needs for the first second or two — often 2–3 times the running figure for compressors and pumps (a refrigerator: 700 W running, 2,200 W starting). Generators quote both numbers; your running total must fit under the rated watts and the largest single surge on top of the running load must fit under the peak watts.
Because motors do not all start at the same instant. Standard sizing practice is to add up everything’s running watts, then add the starting surge of the single biggest motor — the worst realistic moment is “everything running and the AC compressor kicks on”. If you switch things on one at a time after the generator is running, that is the peak you will hit.
Yes, in most cases: a modern fridge draws about 700 W running and 2,000–2,200 W starting, so a 2,000–2,200 W inverter handles it alone, with room for lights and chargers once the compressor is running. It will not also start a sump pump or furnace at the same moment; for those choose 3,500 W or more.
A 5,000 W portable burns roughly 0.75 gallons of gasoline an hour at half load — about 18 gallons a day — and an inverter generator around 0.2–0.3 gal/h at low load. A 20 kW standby on natural gas uses about 200–300 cubic feet an hour at full load. Size close to your real load: running at 25 % of rated capacity wastes fuel and, for conventional generators, produces dirtier power.