Generator Sizing Calculator - Free Calculators for DIY Projects

Goal

What do you want the generator to power?

Essentials = fridge, lights, furnace blower, well pump. Comfort adds AC, water heater, more outlets. Whole house powers everything simultaneously.

Loads to power

Check off the loads you need. Enter how many of each (most homes have 1 of each).

Heating and cooling (the big loads)

Central AC – 2 ton (3,800 W run / 11,000 surge)

Central AC – 3 ton (5,500 W run / 16,000 surge)

Central AC – 4-5 ton (7,500 W run / 22,000 surge)

Furnace blower / heat pump fan (1,000 W / 2,500)

Electric water heater (4,500 W run, no surge)

Well pump – 1/2 to 1 HP (1,500 W / 4,500 surge)

Kitchen and laundry

Refrigerator / freezer (700 W run / 2,200 surge)

Microwave (1,200 W, no surge)

Electric range (8,000 W, no surge)

Dishwasher (1,500 W, no surge)

Washer (1,200 W / 2,500 surge)

Electric dryer (5,400 W, no surge)

Lighting and electronics

LED lights total (10 W each, count individually)

TV + WiFi router (200 W combined)

Computer / laptop (200 W each)

Sump pump (800 W / 2,000 surge)

Garage door opener (700 W / 1,400 surge)

Generator size

Running watts

continuous load

Surge watts

peak startup load

Recommended size

kW running rating

Generator type

Portable

recommended for this load

Generators are rated by “running watts” (continuous load) and “surge watts” (startup). Motors (AC compressors, well pumps, fridges) draw 2-3x their running wattage for a few seconds on startup. Always size for the highest motor’s surge added to other running loads. Whole-house standby generators with auto transfer switches are best for medical equipment or extended outages.