Speedy Calculator

Generator Wattage Calculator

Everyday Life Updated October 25, 2026

Quick answer

The generator wattage calculator sizes a portable or standby generator from the running and starting watts of the appliances you want to power, and converts the result to kVA with a power factor and safe operating load.

%
Total running watts
— W
Peak watts (largest start-up added)
— W
If everything started at once
— W
Recommended generator size
— kVA
Equivalent generator rating
— kW

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

  1. Appliance 1 (Refrigerator) – running watts
  2. Appliance 1 – starting watts
  3. Appliance 2 (Sump pump) – running watts
  4. Appliance 2 – starting watts
  5. Appliance 3 (Lights) – running watts
  6. Appliance 3 – starting watts
  7. Appliance 4 (Microwave) – running watts
  8. Appliance 4 – starting watts
  9. Appliance 5 (Window AC) – running watts
  10. Appliance 5 – starting watts
  11. Power factor
  12. 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.

Tips

  • Use the figures from your own bills and product labels – prices and appliance ratings vary widely.
  • Look at yearly totals: small daily costs add up and make it easier to compare options.
  • Round quantities up to whole packs, rolls or bulbs when you shop.
  • Re-check the numbers when prices change; most results scale directly with the unit price.

Common mistakes to avoid

  • Confusing watts (power) with watt-hours or kilowatt-hours (energy).
  • Comparing prices without bringing them to the same unit first.
  • Ignoring one-off costs such as installation, delivery or replacement parts.

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 wattsPeak watts = running watts + the largest (starting − running) of any one applianceGenerator 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).

What formula does the Generator Wattage Calculator use?

Running watts = Σ running watts.

Cite this calculator

Speedy Calculator. (2026). Generator Wattage Calculator. Speedy Calculator. https://speedycalculator.com/calculators/everyday-life/generator-wattage/