Generator Size Calculator
Check everything you want to keep running during an outage. The calculator totals your running watts, covers the biggest motor surge, adds 20% headroom, and lands on a standard generator size — the same math an electrician would do on paper.
A small portable or inverter generator in this class covers the essentials — fridge, heat, lights, and communications — for most homes.
Fuel note: at this size, a gasoline or dual-fuel inverter is the usual pick. Propane stores indefinitely and runs cleaner; gasoline is cheaper per hour and easier to refill mid-storm.
How generator sizing actually works
Generators are sized in watts, but a single watt number hides the thing that actually trips people up: there are two wattages in play. Running watts are the steady draw of everything operating at once. Starting watts are the brief spike — two to three times the running draw, for a few seconds — when a motor kicks on. A compressor, pump, or blower that cannot get its starting watts simply stalls, and the generator bogs down with it.
So the sizing rule has two conditions. The generator's running-watt rating must cover everything you run simultaneously, and its starting (surge) rating must cover your biggest single motor start on top of the loads already running. The calculator above applies exactly that: it sums your running watts, finds the appliance with the largest gap between its starting and running watts, adds that one surge to the total, and then applies 20% headroom before rounding up to a standard size.
Why only one surge? Because motors almost never start at the same instant. Sizing for every surge added together is the most common way people overspend — you end up paying for, and feeding fuel to, capacity that exists for a coincidence that never happens. Our full generator sizing guide walks through the same method with worked examples for 1,500 to 3,000 sq ft homes, including the wattage table this calculator's figures come from.
Why the 20% headroom is not optional padding
The headroom looks like a fudge factor. It is not. Motors draw more starting current as they age, so the ten-year-old well pump on your list is hungrier than its nameplate suggests. Everyone forgets a load — the second fridge in the garage, the dehumidifier, the aquarium heater — and discovers it during the outage. And a generator's rated output assumes mild conditions: heat and altitude both derate it. Running at 50–80% of capacity is also where generators are happiest — quieter, more fuel-efficient per kilowatt-hour, and longer-lived than a unit pinned at 100% whenever the AC cycles.
Going too far the other way has a real cost too. A 22kW standby loafing along at 4kW of load still burns roughly 1.5–2 gallons of propane an hour, while a right-sized portable at the same load sips about half a gallon of gasoline. Size to your math plus the headroom — not double your math.
Portable, inverter, or standby? Let your number decide
If your recommended size lands at 5,500 W or under, you are in essentials territory: a portable or inverter generator keeps food cold, the furnace blower turning, lights on, and phones charged. Between 7,500 and 12,000 W, a large portable adds a window AC, a sump pump, and a microwave to the picture — the sweet spot for most suburban homes without central air. Past roughly 14,000–15,000 W, or the moment central AC enters your list, you are pricing standby generators: permanently installed, automatic, and fueled by natural gas or a large propane tank so there is nothing to refuel at 2 a.m. Our standby generator cost guide breaks down what those systems run installed, by size and brand.
One lever worth knowing about before you jump a size class: a soft starter on a central AC cuts its starting surge by 60–70% for about $300–$400 installed. On the calculator, try your list with the AC included, note the recommended size, then imagine that surge mostly gone — for many homes it is the difference between a large portable and a five-figure standby install.
Finally, however you power the house, the connection matters as much as the generator. Anything wired into your panel goes through a transfer switch or interlock kit, installed by a licensed electrician — never a backfed dryer outlet. Our guide to connecting a generator to your house covers the three legal methods and what each costs installed.
Frequently asked questions
How accurate is this generator size calculator?
It uses the same mid-range wattage figures electricians use for a first-pass sizing, taken from our appliance wattage table. Your exact appliances can draw 10–20% more or less — check the nameplate on the back or bottom of each unit for its real running watts, and use the higher number if it differs. The built-in 20% headroom absorbs most of that variation.
Why does the calculator add 20% headroom?
Three reasons: motors draw more starting current as they age, everyone forgets a load or two (the garage fridge, the dehumidifier), and generators lose output in heat and at altitude. A generator running at 50–80% load also lasts longer and burns less fuel per kilowatt-hour than one pinned at maximum.
Can a 7,500-watt generator run central air conditioning?
Not reliably. A 3-ton central AC draws about 3,250 running watts but needs roughly 10,500 watts to start its compressor — on top of everything else already running. That pushes you into 20kW standby territory. A soft starter ($300–$400 installed) cuts the AC starting surge by 60–70% and can bring a large portable back into play.
Should I add up the starting watts of every appliance?
No — motors almost never start at the exact same second. Correct sizing covers your total running watts plus the single largest starting surge. Adding every surge together oversizes the generator, wastes fuel, and costs you hundreds of dollars for capacity you will never use.
My result falls between two generator sizes. Which should I buy?
Round up — especially if a well pump, sump pump, or AC compressor is on your list, because a stalled motor start is the failure that matters. The exception is standby generators: the price jump between sizes (say 14kW to 22kW) is often only $800–$1,200 installed, so rounding up there buys real capability cheaply.