What Size Generator Do You Need for Your Home? A Wattage Guide
Size a generator by adding the running watts of everything it must carry, checking the largest motor's starting surge, and adding 20 percent headroom. Essentials-only backup typically lands at 5 to 8 kilowatts; whole-home standby coverage runs 12 to 22 kilowatts depending on the house.
What Size Generator for Home Backup? The Formula First
Working out what size generator for home use comes down to three numbers, and none of them are guesses. Add the running watts of everything the generator must carry at once. Identify the largest motor and make sure its starting surge fits on top of the rest. Then add roughly 20 percent headroom so the machine is not living at its ceiling. That sum is your size, and everything else in the buying process is just matching a unit to it.
The tiers that calculation typically lands in:
| Backup goal | Typical size | What it covers |
|---|---|---|
| Bare essentials | 3.5 to 5 kW | Fridge, sump pump, lights, phones |
| Comfortable essentials | 5 to 8 kW | Add furnace blower, freezer, well pump, more circuits |
| Most of the house | 10 to 14 kW | Everything but the biggest luxury loads at once |
| Whole home, no compromises | 18 to 22+ kW | Central AC, electric cooking, all of it together |
Running Watts: Build Your Actual List
Generic lists get you close, but your house has nameplates, and they outrank any blog table including this one. Still, the common essentials look like this: a refrigerator around 150 to 300 running watts, a chest freezer similar, a furnace blower near 500, a sump pump 800 to 1,050, a well pump around 1,000, lighting a few hundred across LED fixtures, and electronics a modest pile more.
Sum a realistic essentials list and most homes land near 2,500 to 3,500 running watts, which sounds smaller than the generators recommended for it. The gap is the next section’s fault, and it is where undersized purchases go wrong.
The discipline that matters when building the list: count what runs simultaneously, not what exists. A microwave used for three minutes while the kettle waits is a management decision, not a sizing requirement. Winter matters too, since the furnace blower and sump pump are both non-negotiable in a January outage, while summer’s equivalent question is whether air conditioning makes your essential list at all.
Starting Surges: The Number That Stalls Undersized Plans
Motors are greedy for the first second. A sump pump running at 1,000 watts can demand 2,000 to 4,000 to spin up; a well pump running at 1,000 often wants 2,000 to 3,000; furnace blowers and fridge compressors surge in smaller multiples. Starting watts do not stack, since motors rarely start in the same instant, but your generator must cover the full running load plus the single largest surge on top.
Run the worked example: essentials totalling 2,750 running watts, largest surge the sump pump at an extra 2,000. Peak requirement 4,750 watts, plus 20 percent headroom brings you to about 5,700, so a 6,500-watt unit fits with room to breathe. That is the arithmetic behind the 5-to-8 kilowatt essentials tier, and you can rerun it with your own nameplates in ten minutes.
Surge behaviour is also why generator specifications list two numbers, running and peak, and why the marketing headline is always the bigger one. Size against running watts with surge accounted for, never against the peak number alone.
Whole-Home Sizing Is a Different Calculation
Once the goal shifts from essentials to everything, the method changes from appliance lists to the same demand calculation used for service and panel sizing, because the generator is now standing in for the utility. Gas-heated homes with gas water heating mostly land in the 10 to 14 kilowatt band with load management shedding a luxury circuit or two at peak. Homes with central air that must run during outages, electric ranges, or any electric heating push into the 18 to 22 kilowatt tier and occasionally past it.
Two modern loads deserve explicit decisions rather than assumptions. An EV charger is technically just another circuit, but pausing charging during outages is a rational choice that saves thousands in generator capacity. Heat pumps are the same conversation with higher stakes, and their cold-weather draw belongs in any whole-home calculation for an electrified house.
Load-shedding modules blur the tiers helpfully: a mid-size standby with smart shedding can serve a house that naive addition says needs the next size up, by making sure the dryer and the AC simply never run in the same moment on generator power.
Already Own a Generator? Audit It Against the Math
Plenty of readers asking what size generator for home coverage already own one, inherited, borrowed, or bought in a pre-storm panic, and the same formula audits it. Find the running and peak watts on its nameplate, stack your essentials list against them, and check the largest surge. Three outcomes are possible, and all three are useful to know on a calm day.
If the unit clears your list with headroom, your project is not a generator, it is the safe connection: an interlock or transfer switch so it feeds real circuits instead of a spiderweb of cords. If it covers the running load but not the sump pump’s surge, it can still serve with discipline, starting the big motor loads one at a time before adding the small stuff, though that discipline must survive being executed in the dark by whoever is home. And if it misses the list outright, better to learn that in July than mid-outage; a right-sized unit plus proper connection often costs less than people fear once the panic pricing of storm week is out of the equation.
The audit takes fifteen minutes and a flashlight, and it converts a machine of vague reassurance into a plan with numbers.
Fuel, Runtime, and the Numbers Nobody Puts on the Box
Size determines more than coverage; it drives consumption. A portable near its limit drinks gasoline fast, and an extended outage turns fuel storage into its own logistics problem, which is worth weighing when the sizing decision hovers between tiers. Natural gas standby units sidestep storage entirely, one reason the whole-home tiers are almost always standby machines, a comparison covered properly in our standby versus portable breakdown.
The connection method scales with size too. Essentials-tier portables pair with an interlock or manual transfer switch and a labelled short-list of circuits; standby tiers pair with automatic transfer equipment integrated at the panel. Every version of that connection is licensed work under an ESA permit, and a professional generator installation treats sizing, transfer gear, and permit as one designed package rather than three purchases.
Your Ten-Minute Sizing Homework
Walk the house with your phone tonight. Photograph the nameplate of the furnace, sump pump, well pump if you have one, fridge, and freezer, then write the one-line answer to the only strategic question: essentials, most of the house, or everything. Add your outage history, how often and how long, since a house that loses power twice a decade budgets differently than one that drops monthly.
Run your numbers through the formula at the top and you will land within one size of your answer before talking to anyone. Bring the photos and the number to us and the reply is the confirmed size, the right transfer equipment, and a flat quote to make the next outage boring.
Frequently Asked Questions
What does a 7,500 watt generator actually run?
Comfortably: a refrigerator, freezer, sump pump, furnace blower, lights, and electronics, with margin for a well pump's start. Not comfortably: central air conditioning or electric heat on top. It is the classic essentials tier, not a whole-home number.
Why do motors need extra starting watts?
An electric motor draws a surge of two to three times its running wattage in the moment it spins up. A generator that covers running watts but not the largest starting surge will stall or trip exactly when the sump pump tries to save your basement.
Is it bad to oversize a generator?
Beyond wasting money, yes, somewhat. Standby units running at a small fraction of capacity operate less efficiently, and lightly loaded engines can wet-stack over time. Sizing to your real calculated load with sensible headroom beats buying the biggest unit on the pallet.