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How to Size a Generator for Your Home
A generator that’s too small trips and stalls the moment the fridge and furnace kick on together—and one that’s too big just burns fuel and money while it idles. Sizing is the whole game, and it comes down to one honest tally of what you actually need to run when the power quits.
The short answer
Add up the running watts of everything you need on at once, then add the single largest motor’s starting surge on top. For most homes, a 3,500–5,000-watt portable covers the essentials (fridge, furnace blower, lights, phones, a few outlets). Add a well pump, sump, or window AC and you’re into 7,500–9,000 watts. Want the whole house to run like nothing happened? That’s a 14,000–24,000-watt standby wired to the panel by an electrician.
Running watts vs. starting watts—the number that fools people
Every generator lists two figures: running (or rated) watts and starting (or peak/surge) watts. Running watts is what it delivers continuously. Starting watts is the brief extra kick it can supply for a second or two—and that spike matters, because anything with a motor draws two to three times its running load the instant it turns on. A refrigerator that sips 200 running watts can demand 1,200 to start. A well pump running at 1,000 watts can surge past 3,000.
So the math isn’t “add everything up.” It’s: total the running watts of every device you’ll have on at the same time, then add the single largest starting surge—because motors rarely all lurch to life in the same instant. Size the generator so its running rating covers your continuous load and its starting rating clears that peak with headroom.
Tip: never plan to run a generator at 100% of its rating for hours. Aim to load it to about 75% max. That’s your safety margin, and it’s where the engine runs cleanest and lasts longest.
Build your watt list
Grab a notepad and walk the house. For each thing you truly need during an outage, jot the running watts (it’s often on a nameplate sticker, or search the model). Here are the usual suspects so you can ballpark fast.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator / freezer | 150–800 | 1,000–2,200 |
| Furnace blower (gas, 1/2 hp) | 600–900 | 1,500–2,350 |
| Sump pump (1/3 hp) | 800–1,050 | 1,300–2,900 |
| Well pump (1/2 hp) | 1,000–1,500 | 2,100–4,000 |
| Window AC (10,000 BTU) | 1,200 | 1,800–2,900 |
| Space heater | 1,500 | 1,500 |
| Microwave | 1,000–1,500 | 1,000–1,500 |
| Lights (LED, per room) | 10–60 | same |
| TV / router / phone chargers | 150–400 | same |
| Well-being: CPAP, medical device | 50–200 | same |
Add your running watts. Then find the item with the biggest starting surge, and add that surge on top of your running total. That final number is the minimum starting-watt rating you should shop for. Example: fridge (700) + furnace blower (800) + lights and electronics (400) = 1,900 running. The furnace surge is the largest at ~2,350, so your peak is roughly 1,900 + (2,350 − 800) = about 3,450 starting watts. A solid 3,500–4,000-watt unit handles it with room to spare.
Match the size to the job
Once you have a watt target, generators fall into clean tiers. Here’s how the ranges map to real coverage.
| Size | What it runs | Type |
|---|---|---|
| 1,000–2,000 W | Phones, laptops, a light, a small fridge—or a campsite | Inverter (portable, quiet) |
| 3,000–4,000 W | Fridge, furnace blower, lights, electronics—the true essentials | Inverter or open-frame portable |
| 5,000–7,500 W | Above plus a well pump or sump, a window AC, more circuits | Portable with 240V outlet |
| 9,000–12,000 W | Most of a modest home through a transfer switch, including central appliances | Large portable / small standby |
| 14,000–24,000 W | Whole home, central AC, automatic—runs like the grid never blinked | Standby (permanently installed) |
Brands worth a look span the range: DeWalt, Ryobi, and Champion make dependable portables and inverters; Honda and Yamaha lead on quiet inverter power; Generac, Westinghouse, and Kohler dominate the standby world. A portable in the right size, paired with a proper transfer switch and heavy-duty generator cord sets, gets most households through an outage without a hardwired install.
Fuel, transfer switches, and the safety non-negotiables
Fuel type shapes how you live with the machine. Gasoline units are cheapest and everywhere, but gas goes stale and you have to store it. Dual-fuel models run on gas or propane, and propane stores indefinitely—keep spare tanks and a stack of clean fuel cans and funnels on hand. Standby units typically tie into natural gas or a large propane tank and never need refilling by hand.
How power gets into the house matters just as much as the generator. Running extension cords through a cracked window works for a fridge and a lamp, but to power circuits—furnace, well pump, hardwired lights—you need a transfer switch or an interlock kit installed by an electrician. This isolates your home from the utility line so you can never backfeed the grid and electrocute a lineman.
- Never run a generator indoors, in a garage, or near open windows. Carbon monoxide is silent and lethal—keep it 20+ feet from the house, exhaust pointed away. Buy a battery CO alarm.
- Never backfeed by plugging the generator into a wall outlet. Use a transfer switch or interlock, full stop.
- Use the right cord. Match gauge to load and length—too-thin cords overheat and rob voltage. See our extension-cord guide below.
- Protect electronics with surge strips; conventional (non-inverter) generators put out dirtier power. Keep a few surge strips and outlet blocks ready.
- Keep backups for the backup: stock flashlights, fresh batteries, and a couple of work lights so you’re never fumbling in the dark before the generator’s running.
Pick by household
APARTMENT / ESSENTIALS
No well, no central heat to worry about? A 2,000–3,500W inverter keeps the fridge cold, devices charged, and a room lit—quietly, on a couple gallons a day.
SUBURBAN HOME
Fridge, gas furnace, sump, lights, and Wi-Fi through a transfer switch calls for 5,000–7,500W. Dual-fuel gives you flexibility when the gas station lines are long.
WELL & CENTRAL AC
A well pump plus central air pushes you to a large portable or a 14,000W+ standby. Standby wins if outages are frequent and you want hands-off, automatic power.
Shop the whole project
Generators & the gear that keeps them running
Every generator and accessory here ships from authorized manufacturer channels—the same trusted brands your neighborhood hardware store carries. See our guarantee.
Frequently asked questions
What size generator do I need to run my whole house?
For true whole-house coverage including central AC, plan on a 14,000 to 24,000-watt standby generator wired to your panel by an electrician. Most homes that only need the essentials—fridge, furnace, lights, and a well or sump pump—do fine on a 7,500 to 12,000-watt unit through a transfer switch.
What’s the difference between running watts and starting watts?
Running watts is the power a generator delivers continuously. Starting watts is the brief surge it can supply for a second or two when a motor turns on. Motors draw two to three times their running load at startup, so you size a generator by adding your total running watts plus the single largest starting surge.
Will a 5,000-watt generator run my refrigerator and furnace?
Yes, comfortably. A refrigerator runs on 150 to 800 watts and a gas furnace blower on 600 to 900, so together with lights and electronics you are well under 5,000 running watts with plenty of starting headroom. That size also leaves room for a sump pump or a few extra circuits.
Can I plug a generator into a wall outlet to power my house?
No. Plugging a generator into a wall outlet is called backfeeding and it is extremely dangerous—it can send power back onto the utility line and electrocute a lineman, and it bypasses your home’s breakers. Always use a transfer switch or interlock kit installed by a licensed electrician.
How far from the house does a generator need to be?
Keep a portable generator at least 20 feet from the house with the exhaust pointed away from doors, windows, and vents. Never run it indoors, in a garage, or in an enclosed space. Carbon monoxide is odorless and deadly, so also install a battery-powered CO alarm inside.
Size it once, size it right, and a generator turns a scary outage into a minor inconvenience. Not sure which tier fits your setup? Your local independent hardware store can walk your watt list with you—or ask us here and we’ll point you to the right size.
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