# Generator Wattage Calculator

URL: https://speedycalculator.com/calculators/everyday-life/generator-wattage/
Last updated: 2026-10-25

> 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.

## Inputs

- 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

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

## 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
```

## Explanation

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.

### 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

FieldWhat to enterAppliance 1 (Refrigerator) – running wattsA numberAppliance 1 – starting wattsA numberAppliance 2 (Sump pump) – running wattsA numberAppliance 2 – starting wattsA numberAppliance 3 (Lights) – running wattsA numberAppliance 3 – starting wattsA numberAppliance 4 (Microwave) – running wattsA numberAppliance 4 – starting wattsA numberAppliance 5 (Window AC) – running wattsA numberAppliance 5 – starting wattsA numberPower factorA numberOperating capacityIn %

### 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 wattsRecommended generator size (kVA)35010.852511.1270011.4387511.741,05012.051,40012.68

## 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.

## FAQ

**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.