Motors in fridges, pumps and air conditioners need two to three times their running power for a second or two when they start. Generators are sized for the total running load plus the biggest single start-up surge, because appliances rarely start at the same moment. Running a generator at no more than 70–80% of its rating extends its life.
How to use the Generator Wattage Calculator
- Appliance 1 (Refrigerator) running watts
- Appliance 1 starting watts
- Appliance 2 (Sump pump) running watts
- Appliance 2 starting watts
- Appliance 3 (Lights) running watts
- Appliance 3 starting watts
- Appliance 4 (Microwave) running watts
- Appliance 4 starting watts
- Appliance 5 (Window AC) running watts
- Appliance 5 starting watts
- Power factor
- Operating capacity
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the running and starting watts of up to five appliances (zero for unused rows), then the power factor and operating capacity.
Inputs explained
| Field | What to enter |
|---|---|
| Appliance 1 (Refrigerator) running watts | A number |
| Appliance 1 starting watts | A number |
| Appliance 2 (Sump pump) running watts | A number |
| Appliance 2 starting watts | A number |
| Appliance 3 (Lights) running watts | A number |
| Appliance 3 starting watts | A number |
| Appliance 4 (Microwave) running watts | A number |
| Appliance 4 starting watts | A number |
| Appliance 5 (Window AC) running watts | A number |
| Appliance 5 starting watts | A number |
| Power factor | A number |
| Operating capacity | In % |
What each result means
- Total running watts = [Appliance 1 (Refrigerator) running watts] + [Appliance 2 (Sump pump) running watts] + [Appliance 3 (Lights) running watts] + [Appliance 4 (Microwave) running watts] + [Appliance 5 (Window AC) running watts]
- Peak watts (largest start-up added) = [Total running watts] + max([Appliance 1 starting watts] – [Appliance 1 (Refrigerator) running watts], [Appliance 2 starting watts] – [Appliance 2 (Sump pump) running watts], [Appliance 3 starting watts] – [Appliance 3 (Lights) running watts], [Appliance 4 starting watts] – [Appliance 4 (Microwave) running watts], [Appliance 5 starting watts] – [Appliance 5 (Window AC) running watts], 0)
- If everything started at once = [Appliance 1 starting watts] + [Appliance 2 starting watts] + [Appliance 3 starting watts] + [Appliance 4 starting watts] + [Appliance 5 starting watts]
- Recommended generator size = [Peak watts (largest start-up added)]÷[Power factor]÷([Operating capacity]÷100)÷1000
- Equivalent generator rating = [Peak watts (largest start-up added)]÷([Operating capacity]÷100)÷1000
Example values
How the main result changes when Appliance 1 changes and everything else stays at the example values above:
| Appliance 1 (Refrigerator) running watts | Recommended generator size (kVA) |
|---|---|
| 350 | 10.8 |
| 525 | 11.12 |
| 700 | 11.43 |
| 875 | 11.74 |
| 1,050 | 12.05 |
| 1,400 | 12.68 |
Formula
Running watts = Σ running watts
Peak watts = running watts + the largest (starting − running) of any one appliance
Generator size (kVA) = peak watts ÷ power factor ÷ operating capacity
Worked example
Inputs: Appliance 1 (Refrigerator) – running watts: 700; Appliance 1 – starting watts: 2,200; Appliance 2 (Sump pump) – running watts: 800; Appliance 2 – starting watts: 1,300; Appliance 3 (Lights) – running watts: 300; Appliance 3 – starting watts: 300; Appliance 4 (Microwave) – running watts: 1,000; Appliance 4 – starting watts: 1,000; Appliance 5 (Window AC) – running watts: 1,200; Appliance 5 – starting watts: 3,600; Power factor: 0.8; Operating capacity: 70 %.
Results: Total running watts = 4,000 W; Peak watts (largest start-up added) = 6,400 W; If everything started at once = 8,400 W; Recommended generator size = 11.43 kVA; Equivalent generator rating = 9.14 kW.
Frequently asked questions
What size generator do I need to run a house?
Essentials (fridge, freezer, sump pump, lights, furnace fan) usually need 5,000–7,500 W; whole-house standby units are 15–25 kW.
What is the difference between running and starting watts?
Starting (surge) watts are the brief extra power motors need to start; running watts are what they draw continuously.
What is kVA?
Apparent power; kW = kVA × power factor (0.8 for most generators).