Sizing a generator for home backup is the one decision that determines whether your setup works in an outage or leaves you sitting in the dark with an oversized paperweight in the driveway. Get it right and a $1,500 dual-fuel unit will run your essentials reliably for years. Get it wrong — undersize by 1,500 watts or miss a motor-start spike — and the generator trips its breaker every time the well pump or AC compressor kicks on.
This is the method we'd use to size a generator for any house. It takes about 20 minutes with a notepad.
Step 1: Decide what "backup" actually means to you
Before counting watts, define the goal. There are three realistic home-backup tiers, and the difference between them is roughly $1,000 in generator cost.
- Essentials only: Refrigerator, freezer, furnace blower (or window AC in summer), a few lights, phone and laptop charging, internet. Typical sizing: 3,500 to 5,000 running watts.
- Essentials plus comfort: Add a microwave, coffee maker, well pump, sump pump, and a couple more outlets. Typical sizing: 5,500 to 7,500 running watts.
- Whole-house lite: Add central air conditioning or electric water heater (one at a time, manually managed). Typical sizing: 9,000 to 12,000 running watts, often a dual-fuel open-frame or inverter at the top of the portable range.
If you're at the third tier, also seriously consider whether a standby (whole-house) generator is the right call instead — we cover that comparison in whole-house vs portable.
Step 2: Inventory your actual loads
Pull the data tag (a small metal or sticker plate, usually on the back or bottom) of every appliance you intend to run. Write down the watts, or if only amps are listed, multiply by 120 (for standard household loads) or 240 (for dryers, ranges, water heaters, etc.). When a tag only lists volt-amps (VA), use that figure as your watts for sizing purposes — it's a slight over-estimate, which is what you want.
Here are realistic running-watt figures for common loads — but always verify against your specific appliances, because efficiency varies by model and year:
| Appliance | Running watts | Starting watts (motor surge) |
|---|---|---|
| Refrigerator (full-size) | 150–300 | 900–1,200 |
| Chest freezer | 100–200 | 500–900 |
| Furnace blower (1/2 HP) | 700–875 | 2,000–2,400 |
| Furnace blower (3/4 HP) | 875–1,000 | 2,500–3,000 |
| Sump pump (1/2 HP) | 800–1,050 | 2,000–3,000 |
| Well pump (1 HP, 240V) | 2,000–2,500 | 4,000–6,000 |
| Window AC (10,000 BTU) | 1,200 | 1,800 |
| Central AC (2-ton) | 2,200–2,800 | 4,500–6,000 |
| Microwave (1,000W cooking) | 1,500–1,800 | 1,800 |
| Coffee maker | 1,000–1,500 | 1,500 |
| LED lighting (whole-house) | 100–200 | — |
| TV + cable box | 150–250 | — |
| Internet router + modem | 20–40 | — |
| Phone/laptop chargers | 50–150 | — |
Step 3: Add up running watts (the easy part)
Sum every load you intend to run at the same time. This is the most important word in this whole article. You don't add the microwave and the coffee maker and the toaster if you're never running them simultaneously — you cycle them. A 4,000-watt generator that runs the coffee maker for five minutes and then the microwave for three minutes is doing exactly what 95% of users need.
For a typical essentials-only setup, the running-watt total often lands around 2,800 to 3,500 watts. That sounds like a 3,500W generator is enough. It's not, because of the next step.
Step 4: Add the largest starting-watt surge
Motors (refrigerator compressors, well pumps, sump pumps, AC compressors, furnace blowers) draw 2 to 4 times their running wattage for the first one or two seconds when they start. Your generator has to absorb this spike on top of whatever else is already running.
The right math: identify the single largest motor in your load list, and add only that one motor's starting-watt surge to the running-watt total of everything else. You don't add starting watts for every motor because they don't all start simultaneously — appliances cycle randomly. The realistic worst case is one motor surge on top of the steady-state baseline.
So if your running total is 2,800 watts with the fridge already running, and the well pump (4,000W surge) kicks on, peak load is 2,800 + 4,000 = 6,800 watts for about one to two seconds. Your generator needs at least a 6,800W starting rating, which translates to about 5,500 running watts in most product lines.
Read the full running watts vs starting watts piece if this section feels handwavy — the math gets clearer with concrete examples.
Step 5: Add 20% headroom
Running a generator at 100% of its rated capacity continuously is hard on the engine and gives you zero margin for the day you decide to plug in a heater. Plan to run the generator at no more than 80% of running-watt capacity in normal operation. So multiply the running-watt total from Step 3 by 1.25 to get your minimum generator size.
2,800W of essentials × 1.25 = 3,500W minimum running rating. Cross-check against Step 4's peak surge requirement, and the larger number wins.
Step 6: Match to a real product
For most homes — 1,800 to 2,800 square feet, gas or oil heat, no central AC backup — the sweet spot is a 5,500 to 7,500 running watt dual-fuel. That handles essentials plus comfort with headroom, runs on either gasoline or propane, and integrates cleanly behind a 30-amp or 50-amp transfer switch.
For larger homes with central AC backup, step up to a 9,500 to 11,500 running watt dual-fuel open-frame unit, often with electric start and remote start.
For apartment-dwellers or anyone with strict noise rules, a 2,200W inverter or 1,800W solar power station covers fridge + electronics during shorter outages — see our solar generators and power stations picks for that route.
Step 7: Plan the transfer connection
Picking the right wattage is half the project. The other half is getting the power into the house without backfeeding through the meter — that's both a code violation and a real risk to utility line workers. The right answer for most homes is a manual transfer switch or an interlock kit on the panel. We walk through the options in detail in how to connect a generator to your house.
A worked example
2,200 sq ft house, gas furnace, no central AC, well water, family of four. The owner wants fridge, freezer, furnace blower, well pump, lights, internet, and the ability to microwave food. Inventory:
- Fridge: 200W running, 1,000W surge
- Freezer: 150W running, 700W surge
- Furnace blower (1/2 HP): 800W running, 2,200W surge
- Well pump (1 HP, 240V): 2,200W running, 5,000W surge
- LED lights: 150W
- Internet + router: 40W
- Microwave (when used): 1,600W
Running total with everything except microwave: 200 + 150 + 800 + 2,200 + 150 + 40 = 3,540W. Largest motor surge: well pump at 5,000W. Peak: 3,540 - 2,200 + 5,000 = 6,340W for one to two seconds. With 20% headroom on the running side: 3,540 × 1.25 = 4,425W minimum running rating.
A 7,500 running / 9,500 starting watt dual-fuel generator covers this house comfortably — handles peak surge, has headroom for the microwave on top of essentials, and gives margin for a hot plate or kettle that wasn't in the original list.
That's the method. It's not glamorous, but it produces a sized generator that actually works when you need it.
Frequently Asked Questions
Should I size for everything I might ever want to run, or just what I'll actually run?
Size for what you'll actually run, plus 20% headroom. Oversizing leads to a generator that runs at light load constantly, which carbons up the engine and wastes fuel. A 5,500W unit running at 60% capacity will outlast a 9,500W unit running at 20% capacity every day of the week.
What if my appliance data tag is missing or unreadable?
Most major manufacturers publish power specifications in their user manuals, which are available as PDFs on the brand website. Search by the model number, which is usually printed elsewhere on the unit even if the wattage tag is gone. As a fallback, plug-in wattage meters like the Kill A Watt give accurate readings for any 120V appliance.
Do I add up starting watts for every motor?
No — you add the running watts of everything you'll have on at once, then add only the single largest motor's starting-watt surge on top. Multiple motors do not start simultaneously, so adding all surges produces a number 2 to 3 times higher than the actual worst case.
Is it OK to run a generator at 100% capacity?
Briefly, yes — generators are rated for that. Continuously, no. Most engines are designed to run at 50% to 80% of rated continuous load for best fuel efficiency and engine life. Plan around 80% as your normal ceiling, leaving room for a load you didn't expect.
What size generator do I need for a 2,000 square foot house?
Square footage by itself is a poor proxy because it doesn't tell you what's running. A 2,000 sq ft house with gas heat, a fridge, freezer, and lights needs maybe 4,000 running watts. The same house with central electric AC and an electric water heater could need 10,000+ running watts. Inventory your loads — don't guess from square footage.