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Generator Sizing Calculator

Check the appliances you need to run at once. Get the recommended generator size in watts, total running watts, and peak starting (surge) watts.

Inputs

What will you run?

Tick each appliance and set how many. Quantity multiplies that load.

Essentials

700 / 2,200 W
500 / 1,500 W
800 / 1,300 W
550 / 1,100 W

Water

1,000 / 3,000 W
4,000 / 4,000 W

Heating & cooling

800 / 2,350 W
1,200 / 3,600 W
3,000 / 8,000 W
1,500 / 1,500 W

Kitchen

1,000 / 1,000 W
1,500 / 1,500 W

Lighting & electronics

600 / 600 W
400 / 400 W
300 / 300 W

Laundry & cleaning

1,200 / 2,300 W
1,500 / 1,500 W

Medical

200 / 200 W
Custom load (optional)

Not in the list? Enter its nameplate running and starting watts.

Result

Adjust the inputs to see your result.

How the math works

A generator has to do two things at once: carry the steady draw of everything you're running, and absorb the brief spike when a motor starts. Get either wrong and the generator stalls or trips — usually at the worst moment, mid-storm.

The calculator sums the running watts of every appliance you check (times its quantity). Then it finds the single largest starting surge among them — because in practice only one motor spikes at a time — and adds just that one surge on top of the running total. That sum is your peak starting watts: the minimum a generator must be able to deliver. We round it up to the next common generator class so you're loaded around 50–80%, where generators run cleanest and last longest.

Why one surge, not all of them

Add up every appliance's starting watts and you'd buy a generator twice the size you need. Motors don't all start at the same instant — your fridge compressor and your well pump cycle independently. Sizing for the running total plus the worst single surge is the method every manufacturer's sizing guide uses, and it's why a 7,500 W unit can back up a house whose appliances list adds to 15,000 starting watts.

Portable vs. standby — the real line

The crossover is about 8,500 running watts. Below it, a portable generator on a few extension cords (or a manual transfer switch for hardwired circuits) covers you. Above it — typically once you add central AC or an electric water heater to the list — you're into standby-generator territory: a permanently installed unit wired to your panel, often on natural gas or propane. The calculator flags which side of the line you land on.

Common mistakes

  • Sizing by running watts only. Skip the surge and the generator trips the instant the well pump kicks on. Always size to peak starting watts.
  • Adding up every surge. The opposite error — it oversizes badly and wastes money and fuel. Only the largest single surge counts.
  • Forgetting the transfer switch. Hardwired loads (furnace, well pump, AC) can't run off a cord. Backfeeding the panel without an interlock is a lethal hazard to line workers.
  • No headroom for later. Buy for today's list and you'll regret it the first time you add a freezer. Step up one size now.

When this calculator is the wrong tool

For a permanently installed whole-home standby generator, an electrician should run a panel load calculation per the National Electrical Code — that accounts for demand factors, the largest motor rule, and your service size in ways this appliance-list shortcut can't. Use this tool to size a portable for outages, plan an essentials circuit, or sanity-check a quote before you call.

Typical running/starting watts from Generac, Westinghouse, and Honda public sizing guides. Your nameplate ratings override these — always check the label.

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FAQ

Questions, answered

What size generator do I need for a house?
It depends on what you actually run at once, not your home's square footage. A few essentials — fridge, sump pump, a furnace blower, and lights — usually fit a 5,000–7,500 W portable. Add a well pump or central AC and you're at 9,000–15,000 W, which is standby-generator territory. Check the boxes above to size yours.
What's the difference between running and starting watts?
Running (rated) watts are the continuous draw once an appliance is up to speed. Starting (surge) watts are the brief spike when a motor kicks on — for a fridge or well pump that can be 2–3× the running figure. A generator must supply enough running watts for everything at once, plus the largest single surge.
Can a generator run central air conditioning?
A 2-ton central AC draws about 3,000 running watts but surges to roughly 8,000 W on startup. To run it alongside the essentials you generally need a 12,000–15,000 W generator — almost always a standby unit, not a portable. A soft-start kit on the compressor can cut that surge and let a smaller generator handle it.
Do I need a transfer switch?
To power hardwired circuits — furnace, well pump, central AC — yes. A manual transfer switch (or an interlock kit) safely connects the generator to your panel and prevents backfeeding the utility line, which is a lethal hazard for line workers. Only plug-in appliances can run off extension cords without one.
How much should I oversize a generator?
Size to your peak starting watts, then step up to the next class so continuous load sits around 50–80% of the rating — generators run cleanest and last longest there, and you leave room for loads you'll add later. The next-size-up rounding this calculator does already bakes in that headroom.