Every calculator on the internet guesses at your loads. A weekend with a $25 meter replaces the guess with a measurement.
Run the energy audit →Generator sizing goes wrong in one of two directions. Either somebody buys a 3,500 W unit because it was on sale and discovers the well pump trips it every time the toilet is flushed, or somebody buys 12,000 W of screaming gasoline engine to run a refrigerator and a few lamps. The right answer is a number you calculate, and the calculation takes about twenty minutes once you know what your house draws.
Two budgets, not one
Every generator has two ratings. Running watts, sometimes called rated or continuous, is what it can sustain indefinitely. Starting or surge watts is what it can deliver for a second or two while a motor spins up. A unit sold as 7,500/9,500 W means 7,500 continuous and 9,500 peak.
You need to satisfy both. Your continuous total is the sum of everything running at once. Your surge requirement is that same total, plus the single largest inrush spike on top, because motors do not politely start one at a time unless you make them.
The well pump sets the floor
On a house with land, the well pump is almost always the load that decides the size. It is a 240 V induction motor sitting a couple of hundred feet down a hole, and it starts under pressure. A half-horsepower submersible settles around 800 to 1,000 W running but asks for 2,000 to 3,000 W on start. Three-quarter horsepower is roughly 1,200 to 1,500 W running with inrush that can touch 3,500 to 4,500 W.
Two consequences follow. First, the generator has to produce 240 V split-phase, not just 120 V, which rules out a lot of small inverter units. Second, that surge has to fit alongside whatever else is already running. This is why houses on a well rarely end up below about 5,000 W of continuous capacity, and why the same house on city water might be fine at 3,000 W. If your well is the piece you are worried about, what happens when a well loses power covers the rest of that problem.
The method, step by step
- List only the loads you intend to run simultaneously. Not everything you own. Fridge, freezer, furnace blower, well, a few lights, the router.
- Write running watts next to each. Measured numbers if you have them, nameplate ranges if you do not.
- Add them up. That is your continuous requirement.
- Find the largest single surge on the list, usually the well pump, and add the difference between its surge and its running figure to the continuous total. That is your peak requirement.
- Add roughly 20 to 25 percent headroom to the continuous number. Engines lose output at altitude and in heat, and you will inevitably plug in something you forgot.
- Buy the smallest generator whose continuous rating clears the padded total and whose surge rating clears the peak.
A worked example
Take a typical house on a well with a gas furnace, in a January outage.
| Load | Running | Surge |
|---|---|---|
| Refrigerator | 150 W | 900 W |
| Chest freezer | 120 W | 700 W |
| Gas furnace, PSC blower | 600 W | 2,000 W |
| Well pump, 1/2 HP | 900 W | 2,700 W |
| Lights, router, chargers | 150 W | – |
| Continuous total | 1,920 W | – |
| Peak requirement | – | ~3,720 W |
Under 2,000 W continuous. Add 25 percent headroom and you are at about 2,400 W. So a 3,500 W continuous unit with 4,000 W or more of surge covers this house, provided it produces 240 V for the pump. A 5,000 to 6,500 W unit gives you room to add a microwave, a window unit or a second circuit without thinking, and that is where most people who did the math actually land.
Where the online calculators mislead you
They add up everything in the house rather than the loads you would genuinely run at the same time, they use nameplate maximums instead of measured draw, and they are frequently hosted by people selling generators. The result is a recommendation one or two sizes too big. Oversizing is not free: a bigger engine burns more fuel at the same load, costs more to service, and weighs enough to matter when you have to wheel it out of the shed in an ice storm.
The opposite mistake is quieter but worse. A generator that trips out under surge is not a generator, it is a heavy box that makes noise. If your numbers land near the edge of a size class, go up one.
Fuel is the other half of the sizing question
Watts tell you what starts. Gallons tell you how long you last. A 7,500 W portable at about half load burns roughly three quarters of a gallon to a gallon of gasoline per hour, which is 15 to 20 gallons a day if you run it continuously. Nobody stores that casually, and gas stations do not pump without power.
Two practical fixes. Run the generator in cycles rather than continuously: four hours on, four off keeps a full freezer cold and cuts consumption in half. Or move to propane, which stores indefinitely and does not gum a carburetor, at the cost of roughly 10 percent less power output from the same engine. A dual-fuel portable is usually the sensible compromise for a house that already has a propane tank.
Safety: A portable generator produces carbon monoxide in quantities that kill in minutes indoors. It never runs in a garage, a basement, a crawlspace or a covered porch, even with the door open. Keep it well away from the house and upwind of doors, windows and vents.
Two pitfalls after the purchase
Running a large diesel or gas generator at very light load for long periods causes incomplete combustion and carbon buildup, which shortens engine life. If you bought big for surge headroom, load it properly when you run it. And a generator you cannot connect to the house safely is only an extension-cord machine. Sizing is step one; a transfer switch or an interlock kit is step two, and skipping it is both illegal and dangerous.
The size I would buy
For a house on a well with a gas furnace, a 5,000 to 7,500 W dual-fuel portable with 240 V output hits the sweet spot: it starts the pump, runs everything critical with margin, costs a fraction of a standby unit, and fits a real fuel plan. Go bigger only if you have an electric water heater, a well pump above three quarters of a horsepower, or central air you refuse to live without, and in that case price out a permanently installed standby unit before you commit, because at that point the gap has narrowed.
Frequently Asked Questions
Will a 3,500 watt generator run a well pump?
A half-horsepower pump, usually yes, if the generator produces 240 V and has surge capacity around 4,000 W and little else is starting at that moment. A three-quarter or one horsepower pump is a stretch without a soft starter.
Do I need a 240 volt generator?
If you have a submersible well pump, an electric dryer, a well-fed water heater or you want to feed the panel through a transfer switch, yes. If your critical list is all 120 V plug loads, a 120 V-only inverter generator is quieter and cheaper.
How much headroom should I leave?
About 20 to 25 percent above your continuous total. That covers derating for heat and altitude plus the loads you did not think of. More than 50 percent spare is money spent on fuel consumption you will not use.
Does a soft starter really help sizing?
Yes, meaningfully. Fitted to a well pump or an air conditioner it ramps the motor instead of slamming it, cutting inrush substantially and often letting a smaller generator do the job. It is a couple of hundred dollars plus installation and is usually cheaper than the next generator size up.