Generator Sizing Calculator
Find what size generator you need — check the appliances you want to keep running to get the total running and starting (surge) watts, the recommended kW class, and matching standby and portable models.
Prices vary by location. Verify with local suppliers.
What size generator do I need?
The right generator size comes down to one question: what do you want to keep running at once? Check those appliances in the calculator above and it adds up the running watts (the steady draw) and the largest starting surge (the extra power a motor needs for a split second to spin up), then recommends a size with a safety margin built in.
How generator sizing works
The calculator uses the standard trade method, in three steps:
- Add the running watts of every appliance you want on at the same time.
- Add the single largest starting surge. Motors — a fridge compressor, AC, well or sump pump — draw two to three times their running watts at start-up, but they don’t all start at the same instant, so you only add the biggest one.
- Add about 20% headroom and round up to the next generator size, so the unit isn’t running flat-out.
peak load = (sum of running watts) + (largest starting surge) · size = peak × 1.2
Typical appliance wattage
These are the planning figures the calculator uses — your own appliances may vary, so check the label where you can.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator / freezer | 700 | 2,200 |
| Furnace blower (gas, ½ HP) | 800 | 2,350 |
| Well pump (½ HP) | 1,000 | 3,000 |
| Sump pump (⅓ HP) | 800 | 2,000 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Central AC (3-ton) | 5,000 | 15,000 |
| Electric water heater | 4,500 | 4,500 |
| Lights (10 × LED) | 400 | 400 |
Portable vs. home standby
Below roughly 12,000 watts of peak load, a portable generator is usually the practical choice — most people size one to cover essentials (fridge, furnace blower, a pump, lights and Wi-Fi). Above that, or whenever you want central AC and hands-off, whole-house backup, a home standby generator in the 18–26 kW range with an automatic transfer switch is the right call. Compare the leaders on the best whole house generators and best portable generators pages.
This is a planning estimate. A whole-house standby install must be sized with a proper load calculation and wired by a licensed electrician through a transfer switch (NEC). Never run a portable generator indoors or in a garage — exhaust contains carbon monoxide.
Frequently asked questions
What size generator do I need for my house?
It depends on what you want to keep running. For essentials only — refrigerator, furnace blower, lights, Wi-Fi and a sump or well pump — most homes need about 5,000–7,500 running watts, which a mid-size dual-fuel portable covers. To run a whole house including central air conditioning, you typically need a 18–26 kW home standby generator. The calculator above adds up the exact running and starting watts for the appliances you pick and names the size.
How do I calculate what size generator I need?
Add the running (rated) watts of everything you want on at once, then add the single largest starting surge — because only one motor spins up at a time. That total is your peak load; add about 20% headroom and round up to the next generator size. For example, a fridge (700 W run) plus lights and Wi-Fi (500 W) plus a furnace blower that surges to 2,350 W gives roughly 1,200 running watts and a peak near 2,850 W, so a 3,500–4,000 W generator fits. The calculator does this math for you.
Do I add starting watts or running watts?
Both, but not the way most people expect. Use the running watts of every appliance for the continuous load, and add only the biggest single starting (surge) figure on top — motors like a well pump, AC compressor or fridge draw two to three times their running watts for a split second at start-up, but they don’t all start at the same instant. Sizing to the running total plus the largest surge, with headroom, is the standard method.
What size generator will run a refrigerator?
A refrigerator or freezer draws about 700 running watts but surges to around 2,200 watts when the compressor kicks in, so a generator of at least 2,000–2,500 running watts handles a fridge plus a few lights. To also cover a chest freezer, Wi-Fi and phone charging during an outage, step up to about 3,500 watts. Select those items in the calculator to see the exact figure.
Will a 22 kW generator run a whole house?
For most homes, yes. A 22 kW air-cooled home standby generator supplies about 22,000 running watts, which typically covers central air conditioning, the kitchen, well and sump pumps, and general household loads at the same time. Very large homes with multiple AC units or electric heat may need 24–26 kW. A licensed electrician sizes the exact unit with a load calculation and wires it through an automatic transfer switch.
Is it better to get a portable or a standby generator?
A portable generator is cheaper and covers essentials — fridge, furnace blower, a pump and lights — but you have to roll it out, refuel it and run cords or an interlock. A home standby generator is permanently installed, starts automatically within seconds of an outage, runs on natural gas or propane, and can power the whole house, but it costs several times more and needs professional installation. If you mainly want to protect food and heat during occasional outages, portable is enough; for hands-off whole-house backup in a storm-prone area, standby is worth it.