CalculateY
Quick Answer
A home powering a refrigerator, sump pump, lights, TV, and microwave needs approximately 4,480 starting watts, which with a 25% safety margin points to a 7,500-watt generator.
Check the appliances you plan to power and adjust quantities as needed.
| Appliance | Running (W) | Starting (W) | Qty | |
|---|---|---|---|---|
| 150 | 400 | |||
| 100 | 350 | |||
| 800 | 2,000 | |||
| 1,200 | 3,600 | |||
| 3,500 | 7,000 | |||
| 4,500 | 4,500 | |||
| 1,000 | 1,000 | |||
| 800 | 800 | |||
| 1,100 | 1,100 | |||
| 500 | 1,200 | |||
| 5,000 | 6,000 | |||
| 50 | 50 | |||
| 80 | 80 | |||
| 200 | 200 | |||
| 750 | 2,000 | |||
| 550 | 1,100 | |||
| 1,500 | 1,500 | |||
| 2,500 | 2,500 |
Custom appliances
Common Examples
| Input | Result |
|---|---|
| Refrigerator + Lights + TV | Approximately 530 running watts, 3,500W generator recommended |
| Refrigerator + Sump Pump + Lights + Microwave | Approximately 3,200 starting watts, 5,000W generator recommended |
| Central AC + Refrigerator + Lights | Approximately 7,200 starting watts, 10,000W generator recommended |
| Refrigerator + Well Pump + Washing Machine + Lights | Approximately 2,700 starting watts, 5,000W generator recommended |
| Full house: Central AC, Refrigerator, Water Heater, Dryer, Lights | Approximately 16,700 starting watts, 22,000W generator recommended |
How It Works
The formula
Generator sizing uses two wattage values for each appliance:
- Running watts (also called rated watts): the continuous power an appliance draws during normal operation
- Starting watts (also called surge watts): the brief spike of power required when a motor-driven appliance first turns on, typically 2 to 3 times the running wattage
The calculation follows three steps:
Step 1: Total running watts
Add the running watts for every appliance you plan to power at the same time.
Step 2: Total starting watts
Because motor-driven appliances do not all start at the same instant, the standard approach adds only the single largest starting surplus (starting watts minus running watts) to the total running watts.
Total Starting Watts = Total Running Watts + Largest (Starting - Running) Among All Appliances
Step 3: Safety margin and recommended size
Multiply total starting watts by 1.25 to add a 25% safety margin. Then round up to the nearest common generator size (2,000W, 3,500W, 5,000W, 7,500W, 10,000W, 12,000W, 15,000W, 20,000W, or 22,000W).
Resistive vs. motor loads
Appliances with heating elements (toasters, space heaters, electric water heaters) have identical running and starting wattages because they do not contain motors. Appliances with motors (refrigerators, sump pumps, air conditioners, washing machines) draw a surge of power at startup, sometimes 2 to 3 times their running wattage. This surge typically lasts only a few seconds but the generator must be capable of delivering it.
Worked example
Suppose you want to power a refrigerator (150W running, 400W starting), a sump pump (800W running, 2,000W starting), lights (50W running, 50W starting), and a microwave (1,000W running, 1,000W starting).
Step 1: Total running watts = 150 + 800 + 50 + 1,000 = 2,000W.
Step 2: The starting surplus for each appliance is: refrigerator 250W, sump pump 1,200W, lights 0W, microwave 0W. The largest surplus is 1,200W (sump pump). Total starting watts = 2,000 + 1,200 = 3,200W.
Step 3: Safety margin = 3,200 x 1.25 = 4,000W. The next standard generator size at or above 4,000W is 5,000W.
The recommended generator for this load is approximately 5,000 watts (5 kW).