What size generator do I need?
Check the appliances you want to keep running. This worksheet adds the running watts and the single largest startup surge the right way, then recommends a size with headroom. Everything stays in your browser.
Generator sizing worksheet
running + peak watts · propane runtime- Never plug a generator into a wall outlet to power your house. That is called backfeeding, and it can electrocute a lineman and start a fire. A licensed electrician installs a transfer switch, which is the only safe way to feed house circuits.
- Never run a generator indoors, in a garage, or near a window. The exhaust is carbon monoxide: invisible, odorless, and deadly. Keep it outside and well away from the house.
A size-based number is rough. Backup power is really sized by the loads you run, so the appliance worksheet gives a truer answer.
Pick the appliances you need to run, and the size updates as you go.
Assumes a propane generator running near the load above. Real fuel use depends on the unit and how hard it is working. Gallon-rated tanks are figured at the safe 80% fill.
This is a planning estimate, not an electrical load calculation. A licensed electrician sizes the real install and the transfer switch. Motor loads surge at startup, so the peak is the running total plus the single largest surge, never the sum of every surge. Check each appliance's data plate for its own running and starting watts.
How this sizes a generator
Sizing a generator comes down to two numbers. The first is the running watts: what every appliance draws steadily once it is on. The second is the surge: the brief spike a motor pulls the instant it starts. A refrigerator that runs at 700 watts can jump to 2,200 for a fraction of a second as the compressor spins up.
The trap most calculators fall into is adding every appliance's surge together. Motors do not start at the same moment, so that number is far too high. The honest method, and the one this worksheet uses, is the running total plus the single largest surge. That covers the worst real instant: everything running while the one biggest motor kicks in.
We then add about a quarter as headroom so the generator is not pinned at full output, and round to a standard size. Manufacturers commonly suggest 10 to 20 percent; we default to 25 to keep the estimate on the safe side. Past a certain load a portable cannot keep up, and the answer becomes a permanently installed standby unit wired through a transfer switch.
Portable or whole-house standby
A portable generator is cheaper, rolls out when you need it, and runs an extension-cord list of essentials or a handful of circuits through a manual transfer switch. It is the right call for keeping food cold, the furnace blowing, and the sump pump running through an outage.
A standby generator is permanently installed, starts itself when the power drops, and can run the whole house, central AC included. It costs far more and needs professional installation, but it is the hands-off option for homes that lose power often or cannot go without. The generator sizing guide walks through the whole decision step by step.
Running and starting watts, common appliances
A sample of the full set, from the same manufacturer wattage worksheets. Motor loads carry a surge; resistive loads like heaters and toasters do not. Your own appliance's data plate is the final word.
| Appliance | Running (W) | Starting (W) | Motor? |
|---|---|---|---|
| Refrigerator or fridge-freezer | 700 | 2,200 | yes |
| Chest or upright freezer | 500 | 1,500 | yes |
| Sump pump (1/3 HP) | 750 | 1,500 | yes |
| Well pump (1/2 HP) | 1,000 | 2,100 | yes |
| Gas furnace blower (1/2 HP) | 800 | 2,350 | yes |
| Window AC (12,000 BTU) | 1,300 | 2,400 | yes |
| Microwave (countertop) | 1,000 | none | no |
| Electric clothes dryer | 5,500 | 6,750 | yes |
Common questions
What size generator do I need for a house?
It depends on what you want to run, not on the square footage. Add up the running watts of everything you need on at once, then add the single largest starting surge on top of that. Backing up the essentials (a fridge, a freezer, a well or sump pump, the furnace blower, and some lights) usually lands between 5,000 and 7,500 running watts, which a mid-size portable covers. Running the whole house, including central AC, is standby-generator territory, usually 18 to 24 kW.
What is the difference between running watts and starting watts?
Running watts are what an appliance draws once it is going. Starting watts, also called surge watts, are the brief spike a motor pulls in the first fraction of a second as it starts. A fridge might run at 700 watts but spike to 2,200 as the compressor kicks in. You size a generator to handle the running total plus the single largest surge, because motors do not all start at the same instant.
Why not add up every appliance's starting watts?
Because they never all surge at once. Only one motor starts at a time, and the spike lasts a fraction of a second. Adding every surge together gives a number far bigger than you need and pushes you toward an oversized, more expensive generator. The correct method is the running total plus the one biggest surge, which is what this worksheet does.
What size generator runs a well pump or sump pump?
Pumps are motor loads, so the surge matters. A 1/2 HP well pump runs at about 1,000 watts but can surge to about 2,100. A 1/3 HP sump pump runs near 750 watts and surges to about 1,500. Add the pump to the worksheet above with everything else you need, and it will fold the surge in correctly. Always check your pump's data plate, since the numbers vary by model.
How long will a propane tank run a generator?
It depends on the load and the tank. A 20 lb grill cylinder holds roughly 430,000 BTU, so a small load runs several hours; a 500 gallon tank filled to the safe 80% holds about 36 million BTU and runs a whole-house load for days. The propane panel in the worksheet estimates runtime for the load you build, at the standard 80% fill for gallon-rated tanks.