Generator Wattage Calculator
Pick what you'd actually run during an outage — not everything you own — and the calculator adds up the running watts, finds the one appliance with the biggest starting surge, applies a safety margin, and names a generator class that covers both. It also tells you whether you can get by with extension cords or need a transfer switch or interlock installed. Every wattage figure comes from a manufacturer chart (Honda, Generac, Champion, Kohler, Westinghouse) and is listed with its source on the methodology page.
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Data last updated: 2026-08-24
How this calculator works
Two numbers matter for a generator: the running watts your appliances draw once they're going, and the starting watts a motor or compressor pulls for a fraction of a second when it kicks on — often two to three times its running draw. Every major manufacturer sizes the same way: add up the running watts of everything that will be on at once, then add the single largest extra starting surge, on the assumption that your fridge and your sump pump won't both start in the same instant. Generac's worksheet puts it as "add to the total running watts the single highest starting watts requirement"; Briggs & Stratton says to take "the ONE ITEM with the highest number of additional starting watts" and add it to the total.
We add a margin on top (20 % by default) because a generator run at its nameplate rating continuously runs hot, burns more fuel per watt, and regulates voltage worse. The result is compared against generic size classes — a 3,500-watt inverter, a 7,500-watt conventional portable, a 14 kW home standby — rather than any specific model, because ratings differ between brands and change yearly.
Reading the result
Running total. Everything ticked, times quantity. If a load is hard-wired (furnace, well pump, central AC, water heater, dryer), you can't feed it with an extension cord no matter how small the number is — that alone means a transfer switch or interlock.
Largest surge. The one appliance whose starting draw exceeds its running draw by the most. Sump and well pumps, central AC compressors, and 240 V dryers usually win. A "soft-start" kit on a central AC compressor cuts its surge sharply and can move you down a generator class; that's an installer conversation.
Recommended class. The smallest tier whose running and peak rating clear your sized numbers. If it lands on the 12,000 W portable class, ask yourself whether a 14 kW standby makes more sense — that's the point where the portable stops being portable and the fuel logistics get serious.
Connection. Under roughly 4,500 W with only plug-in loads, cords are fine (outdoors, 20 feet from the house, exhaust away from openings — CPSC and CDC). Anything larger or hard-wired needs either a manual transfer switch (a separate small panel with pre-selected circuits) or an interlock kit (a mechanical slide on your existing panel that prevents the main and generator breakers from being on together). Both must be installed by a licensed electrician and both exist to prevent backfeeding, which Generac's own support page calls illegal under the NEC and lethal to line workers. For an automatic home standby, NEC Article 702 requires the unit to carry the full load the automatic transfer switch hands it — the dealer does that calculation, and this tool is a sanity check for it.
Typical outage set-ups, in watts
These are the presets, so you can see what's inside them.
Outage essentials (fridge, sump pump 1/2 HP, furnace blower 1/2 HP, a lighting circuit, Wi-Fi, phone chargers, TV): about 3,650 W running, largest surge from the furnace blower (+1,475 W), so a peak need around 5,100 W — with 20 % margin that's a 7,500 W class conventional portable. Add a 1,000 W microwave (it draws about 1,500 W) and you cross into the 9,500 W class — or just run it while the furnace is idle.
Essentials plus central AC (3 ton): adds 3,500 W running and a surge of up to 6,000 W. You're now at roughly 7,200 W running / 9,700 W peak, about 8,600 / 11,600 with the margin — the 12,000 W class, the top of the portable range, or a 14 kW home standby. This is why "can I run my AC on a generator" is usually a standby-generator question.
Whole house, typical 2,000 sq ft (essentials + electric water heater + well pump 1 HP + electric dryer): past 15,000 W running. Kohler's bands put that in "20–26 kW: designed for larger homes or whole-home backup," and a dealer load calculation is mandatory anyway.
Things the calculator can't know
- Your actual appliances. The chart values are typical for the class. The nameplate on the back of your unit wins; use the "edit watts" control to enter it.
- 240 V loads on small generators. Inverter generators under about 4,000 W usually have no 240 V outlet; the calculator flags 240 V appliances if you pick a small class, but it can't see your outlets.
- Altitude and heat. Engine-driven generators lose output with elevation and extreme heat; every engine maker publishes a derating in the manual. If you're at altitude, size up a class.
- Fuel. Consumption depends on load and fuel; dual-fuel units are rated lower on propane than on gasoline — the spec sheet lists both. A separate page on fuel consumption is coming; the electricity cost calculator covers the grid side.
Embed this calculator
The badge below links to this page and can be dropped into a blog post, a forum signature, or a prep checklist. The snippet is on the page under the tool.
Parts and supplies mentioned on this page
The hardware side of "connection": an interlock kit or transfer switch (installed by an electrician), the cord that connects to it, and a CO alarm for the house.
- Generator interlock kit (match to your panel brand)Check price on Amazon
- Manual transfer switch, 30 A, 6–10 circuitsCheck price on Amazon
- L14-30 generator cord, 25 ftCheck price on Amazon
- Battery-powered CO alarmCheck price on Amazon
Frequently asked questions
What size generator do I need to run a house?
For essentials — refrigerator, sump pump, furnace blower, lights, electronics — most homes total 3,500–5,000 running watts, which with the largest surge and a margin calls for a 7,500 W-class portable. Add central air and you're at the 12,000 W class and into home-standby territory. A whole-house standby for a typical 2,000 sq ft home is 18–22 kW; Kohler's published bands are 10–14 kW for essentials, 14–20 kW for most systems including central AC, and 20–26+ kW for whole-home.
Do I add up starting watts for every appliance?
No. Add the running watts of everything, then add only the single largest starting surge (starting minus running for that one item). Generac, Honda, Champion, and Briggs & Stratton all size this way.
How many watts does a refrigerator use on a generator?
Manufacturer charts put a 16–20 cu ft frost-free refrigerator/freezer at about 700 W running and 1,800–2,200 W starting; a modern ENERGY STAR unit runs closer to 150–200 W with a surge of about 1,200 W. Your nameplate gives the exact figure.
Can I plug my generator into a wall outlet to power the house?
No. That's backfeeding: it energizes the utility line, can kill a line worker, and Generac's support page says it violates the National Electrical Code. Use a transfer switch or an interlock kit installed by a licensed electrician.
Interlock kit or transfer switch — which is better?
An interlock is cheaper and lets you choose any circuits in your panel, but it's a manual process and must be a kit listed for your panel. A manual transfer switch is a separate box with pre-selected circuits — more expensive, simpler to use. Both prevent backfeed; both need an electrician.
How far from the house should a generator be?
CPSC and CDC: at least 20 feet, outdoors, with the exhaust pointed away from windows, doors, and vents, and a battery-powered CO alarm inside.