How to Size a Portable Generator (the Way an Installer Does It)
The four-step sizing method installers actually use: list must-run loads, separate running from starting watts, size for the biggest surge, then add headroom.
Updated July 15, 2026
Every week someone buys a 2,000-watt generator, gets it home, and discovers it won't start their well pump. And every week someone else spends $1,500 on a machine three times bigger than their actual outage load. Both mistakes come from skipping the same twenty minutes of arithmetic. Here's the method, the same one an installer runs on a clipboard before quoting you anything.
Step 1: List what must run — not what could run
Walk the house and write down only the loads that matter in an outage. For most homes that's the refrigerator, a freezer, the furnace blower (the fan — a gas furnace burns gas, but the blower is electric), a sump or well pump if you have one, the internet router, some lights, and phone chargers. The electric dryer, the oven, and the water heater are wants, not needs, and each of them alone can double the generator you have to buy.
Step 2: Separate running watts from starting watts
Anything with a motor or compressor pulls a brief surge — typically two to three times its running draw — for the second or two it takes to spin up. A refrigerator that runs at 700 watts can demand 2,200 at the moment the compressor kicks on. Resistive loads (space heaters, kettles, incandescent bulbs) have no surge at all. This distinction is the entire game in generator sizing.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Chest freezer | 500 | 1,500 |
| Gas furnace blower | 800 | 2,350 |
| Sump pump (1/2 HP) | 1,050 | 2,640 |
| Wi-Fi router | 20 | 20 |
| LED lights (×6) | 60 | 60 |
Step 3: Add running watts, then add the single largest surge
Sum the running watts of everything on your list. Then find the one appliance with the biggest gap between its starting and running watts, and add that gap on top. You don't add every surge together — motors don't all start in the same instant, and once a compressor is running it's back down to its running draw. For the table above: 3,130 running watts, plus the sump pump's 1,590-watt surge gap (the biggest of the bunch — bigger than the furnace blower's 1,550W or the fridge's 1,500W), for a peak of 4,720 watts.
Step 4: Buy 20–25% bigger than your number
A generator run flat-out at 100% load runs hot, drinks fuel, sags voltage when anything else blinks on, and wears out early. Installers size so the expected load sits at 75–80% of rated output. For the 4,720-watt example above, that lands around 5,650-5,900 watts — solidly in the mid-size portable class. The single most popular home-backup size overall is still the 7,500W class, since most real households' lists run a bit heavier than this simplified example once you add a second fridge, more lighting circuits, or a well pump instead of a sump pump.
That's the whole method. Our calculator runs these four steps for you with per-appliance published figures — pick your loads and it does the surge math and the headroom check against real generator spec sheets.
The two classic sizing mistakes
- Sizing on running watts only. The generator runs everything fine — until the well pump tries to start, the voltage collapses, and the overload breaker trips in the dark.
- Buying triple your load 'to be safe.' Big open-frame units burn roughly twice the fuel of a right-sized machine at the same actual load, and lightly-loaded engines carbon up. Bigger is not automatically safer — it's just heavier and thirstier.
Frequently asked questions
What size generator do I need for a typical house?
For essentials — refrigerator, freezer, furnace blower, a pump, lights, and electronics — most homes land between 5,000 and 7,500 running watts. Whole-house coverage including central air conditioning almost always means a standby unit: a 3-ton compressor's real nameplate locked-rotor surge runs well over 20,000 watts on its own, far more than a portable generator's starting-watts rating covers, soft starter or not. Run your actual appliance list through a sizing calculation rather than guessing from square footage.
Do I add up all the starting watts?
No. Add up running watts for everything, then add only the single largest surge (the biggest gap between one appliance's starting and running watts). Motors almost never all start in the same instant. Adding every surge together is how people end up buying twice the generator they need.
Will a 2,000-watt generator run a refrigerator?
Usually yes — a typical fridge runs at ~700 watts and surges to ~2,200 to start, which a 2,000-running / 2,200-plus-starting-watt inverter can handle as long as little else is drawing at that moment. It will not simultaneously start a second compressor like a freezer; give each motor a moment to settle before adding the next load.
Is it bad to oversize a generator?
Moderately oversizing (20–25% headroom) is correct. Wildly oversizing wastes money twice: big units cost more up front and burn substantially more fuel at light load, and engines run lightly loaded for long periods tend to carbon up. Size to run at roughly three-quarters of rated output.