Last verified: October 1, 2026. Use this calculator to estimate what size generator your home needs before you talk to an installer. Pick the appliances you want to keep running during an outage; the calculator adds up their running watts, accounts for the start-up surge of motors (refrigerators, pumps, air conditioners) and suggests a standby size and the minimum portable generator rating.
Generator size calculator
Tick what you want to run during an outage and set how many of each. Typical wattages are pre-filled; if you know your appliance’s nameplate rating, it will be more accurate.
Central air conditioning / heat pump
How it works: running load = sum of running watts. Peak start-up load = running load plus the largest single motor start-up surge (motors rarely start at the same moment). Standby recommendation = smallest common size that covers both 120% of the running load and the peak start-up load. This is an estimate; a licensed electrician should confirm with a load calculation.
How to read the results
- Running load is what your selected appliances draw once they are running. A generator should not run near 100% of its rating for hours, so the standby recommendation adds 20% headroom.
- Peak start-up load is the brief surge when a motor starts. Refrigerators, well and sump pumps, furnace blowers and especially central air conditioners can draw two to three times their running watts for a second or two.
- Standby size is rounded up to common home standby sizes: 10, 14, 18, 22, 24 and 26 kW.
- Portable generators are rated with two numbers: running (rated) watts and starting (surge) watts. You need both numbers at or above the calculator’s result.
Typical results by home type
These examples use the calculator’s default wattages. Your own appliances may differ.
| What you want to run | Running load | Peak start-up | Standby size | Portable option |
|---|---|---|---|---|
| Essentials only: fridge, sump pump, furnace blower, lights, TV/Wi-Fi | 3,225 W | 4,725 W | 10 kW | Yes: at least 3,600 W running / 4,800 W starting |
| Essentials + 1 HP well pump + microwave | 6,225 W | 8,225 W | 10 kW | Yes, a large portable: 6,900 W running / 8,300 W starting |
| Essentials + microwave, washer, dishwasher + 3-ton central AC (no soft starter) | 10,375 W | 16,375 W | 18 kW | Not realistic |
| Same home with a 5-ton central AC (no soft starter) | 12,675 W | 21,875 W | 22 kW | Not realistic |
Ways to need a smaller (cheaper) generator
- Add a soft starter to your central AC. It limits the start-up surge, which is often what pushes homes into a bigger generator size.
- Leave big resistive loads off. Electric water heaters, electric ranges and dryers use 4,500–5,400 W each. Running them one at a time, or not at all, changes the answer a lot.
- Use load management. Many automatic transfer switches can shed large loads (like AC) when another big appliance starts. Ask installers about it.
- Back up essentials only. A partial-home transfer switch that powers 8–16 critical circuits is cheaper to install than whole-home coverage.
Before you buy
- Check the nameplate of your biggest motors (AC condenser, well pump). The label shows running amps and locked-rotor amps (LRA); give these to the installer.
- Ask for a written load calculation. A good installer measures, not guesses.
- Remember fuel: a larger generator burns more natural gas or propane per hour, even at partial load.
- Installation must follow local code and be done by a licensed electrician. Never backfeed a generator through a household outlet.
Related guides will be linked here as they are published: what size generator you need, whole house generator costs, and standby vs portable generators.
Disclaimer: the calculator gives estimates based on typical appliance wattages. Actual loads vary. It is not a substitute for a professional load calculation. See our Disclaimer.