Generator Size Calculator
Tick the appliances you need to run at the same time during an outage. You get the running watts, the largest motor's starting surge, the peak the generator must supply, and the standard generator size that covers it with headroom.
Tick everything you want running at the same time.
running / starting watts
Generator size
6,500 W
6.5 kW, next standard size with ~10% headroom
Running watts
3,850 W
everything ticked, running together
Largest surge
+1,550 W
Furnace fan (1/2 hp) starting
Peak demand
5,400 W
running + one motor starting
Method: sum running watts; add the single largest (starting − running) surge, since motors rarely start together; pick the next standard size 2, 3.5, 5, 6.5, 7.5, 9… kW with 10% headroom. Manage the rest by starting big motors one at a time.
Three typical outage scenarios
| Scenario | Running together | Running W | Peak W | Generator |
|---|---|---|---|---|
| Essentials | fridge, sump, furnace fan, lights, TV, microwave | 3,850 | 5,400 | 6.5 kW |
| Comfortable | essentials + well pump, window AC, coffee maker, garage door | 8,925 | 12,925 | 16 kW |
| Whole house | comfortable + central AC, water heater, range, washer, dryer | 23,825 | 31,025 | 26 kW |
Appliance wattage reference
Typical values from manufacturer sizing guides. Your nameplate wins; multiply amps by volts if only amps are listed.
| Appliance | Running W | Starting W | Surge |
|---|---|---|---|
| Refrigerator / freezer | 700 | 2,200 | +1,500 |
| Sump pump (1/2 hp) | 1,050 | 2,150 | +1,100 |
| Well pump (1 hp) | 2,000 | 6,000 | +4,000 |
| Furnace fan (1/2 hp) | 800 | 2,350 | +1,550 |
| Window AC (10,000 BTU) | 1,200 | 3,600 | +2,400 |
| Central AC (3 ton) | 3,800 | 11,000 | +7,200 |
| Heat pump (3 ton) | 4,000 | 10,000 | +6,000 |
| Electric water heater | 4,500 | 4,500 | — |
| Space heater | 1,500 | 1,500 | — |
| Lights (10 LED bulbs) | 100 | 100 | — |
| TV + internet router | 200 | 200 | — |
| Microwave (1,000 W) | 1,000 | 1,000 | — |
| Coffee maker | 1,000 | 1,000 | — |
| Electric range (one element) | 2,100 | 2,100 | — |
| Washing machine | 1,150 | 2,250 | +1,100 |
| Electric dryer | 5,400 | 6,750 | +1,350 |
| Dishwasher | 1,500 | 1,500 | — |
| Garage door opener (1/2 hp) | 875 | 2,350 | +1,475 |
| Laptop + monitor | 150 | 150 | — |
| EV charger, level 2 (32 A) | 7,700 | 7,700 | — |
Connecting it safely
- Transfer switch or interlock kit at the panel, installed by an electrician. It isolates your house from the grid so the generator cannot backfeed the utility lines.
- Outdoors, 20 ft from the house, exhaust pointed away from windows. Carbon monoxide from generators kills people every storm season; a CO alarm inside is not optional.
- Heavy extension cords for a portable without a transfer switch: 12 AWG for 20 A, kept short. The voltage-drop page shows why a 100 ft 16 AWG cord starves a fridge compressor.
- Fuel: a 7.5 kW portable burns 0.75-1 gallon of gasoline an hour at half load. Twelve hours a day for a three-day outage is 30+ gallons, stored somewhere.
Frequently Asked Questions
What size generator do I need to run a house?
Essentials -- fridge, sump pump, furnace fan, lights, TV, microwave -- come to about 3,850 W running with a 5,400 W peak, so a 6.5 kW portable. Add a well pump, a window AC and the garage door and you are at 16 kW. Whole-house with central air, water heater and dryer is 26 kW or more, which is standby-generator territory (Generac's 22 kW is the common pick).
What is the difference between running and starting watts?
Motors draw two to six times their running current for a second or two as they spin up. A refrigerator that runs at 700 W may need 2,200 W to start; a well pump at 2,000 W may need 6,000. A generator must supply the running total plus one motor's surge on top, or it will bog and trip. Resistive loads (heaters, lights, cooktops) have no surge.
Why only the largest surge, not all of them?
Because motors almost never start at the same instant, and you can make sure they don't: start the well pump, wait, then the fridge. The standard sizing method (Honda, Generac and the portable-generator makers all use it) is running watts plus the single largest starting delta. The 10% headroom in the result covers a second small motor overlapping.
Can I run central air on a portable generator?
A 3-ton unit needs around 11,000 W to start and 3,800 to run, so a 12 kW portable can do it with almost nothing else on, and only through a proper transfer switch or interlock. A soft-start kit on the compressor cuts the surge by 60-70% and makes a 7.5 kW generator workable. Never backfeed through a dryer outlet: it can kill a lineworker.
Inverter or conventional generator?
Inverter generators produce clean sine-wave power safe for electronics, throttle down at low load (quieter, a third of the fuel) and are the right choice up to about 7 kW. Conventional open-frame units are cheaper per watt above that and fine for motors, heaters and lights; put sensitive electronics on a UPS.
These calculators use National Electrical Code tables and industry sizing charts and are for planning. They are not a substitute for a licensed electrician, an HVAC contractor's Manual J load calculation, or your local code, which governs. Electrical work can kill you and may require a permit; when in doubt, hire it out.