# Generator sizing: how many watts?

Generator sizing from your devices: the total running watts plus the highest additional starting watts gives the starting watts a portable or standby generator must supply, with an optional margin.

- Page: https://www.acalculator.org/construction/generator-size-calculator
- JSON spec: https://www.acalculator.org/construction/generator-size-calculator.json
- Version: 4e0b1f569ee7

## Default answer

Example with the default inputs (Devices you run at the same time [Running watts 400, Additional starting watts 1,200; Running watts 1,050, Additional starting watts 2,150; Running watts 1,200, Additional starting watts 2,400], Extra margin 0%): Look for a generator rated at least 2,650 W running and 5,050 W starting.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| devices | Devices you run at the same time | Each device’s running watts and additional starting watts, from its label or manual. |
| m | Extra margin | A safety margin added to both totals, such as 20%. 0 adds none. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| starting | Starting watts needed | Total running watts + the highest additional starting watts, plus the margin. |
| running | Running watts needed | The total running watts of every device, plus the margin. |
| totalRunning | Total running watts | The sum of the running watts, with no margin. |
| surge | Highest additional starting watts | The largest additional starting watts of one device. |
| kw | Starting kilowatts | The starting watts needed ÷ 1,000. |
| largest | Largest starting load | The device with the highest starting watts. |
| summary | Answer | The generator rating to look for. |

## Method

Running watts = sum of running watts × (1 + margin); starting watts = (sum of running watts + highest additional starting watts) × (1 + margin).

## Assumptions

- Only one motor starts at a time, so only the highest additional starting watts is added.
- Starting watts are the additional watts on top of running watts, as Champion and Firman list them.
- List only the devices that run at the same time.

## Worked examples

1. devices = {"running":400,"starting":1200} or {"running":1050,"starting":2150}, m = 0% gives totalRunning = 2,650, surge = 2,400, starting = 5,050, largest = Device 3. Source: Champion Power Equipment, What Size Generator Do I Need? (add all running watts; identify the highest starting wattage; add that starting wattage to the total running watts; starting watts are the additional burst a device needs to start; refrigerator 150–400 running, 800–1,200 starting; ½ HP sump pump 800–1,050 and 1,300–2,150; 10,000 BTU window AC 900–1,200 and 1,800–2,400), https://www.championpowerequipment.com/news-updates/what-size-generator-do-i-need/ (retrieved 2026-10-02).
2. devices = {"running":400,"starting":1200} or {"running":1050,"starting":2150}, m = 20% gives running = 3,180, starting = 6,060, kw = 6.06. Source: FIRMAN Power Equipment, Buying Guide (add the running watts of all items run at the same time; identify the device with the highest additional starting watts; total running watts + highest additional starting watts = total starting watts required), https://firmanpowerequipment.com/pages/buying-guide (retrieved 2026-10-02).
3. devices = {"running":60} or {"running":150,"starting":0}, m = 0% gives totalRunning = 1,010, starting = 2,310, largest = Device 3. Source: FIRMAN Power Equipment, Buying Guide (add the running watts of all items run at the same time; identify the device with the highest additional starting watts; total running watts + highest additional starting watts = total starting watts required), https://firmanpowerequipment.com/pages/buying-guide (retrieved 2026-10-02).

## FAQ

### How do I size a generator?

Add the running watts of everything you will run at the same time. Find the one device with the highest additional starting watts and add that number to the total. A refrigerator (400 W running, 1,200 starting), a sump pump (1,050 and 2,150) and a window AC (1,200 and 2,400) need 2,650 running watts and 2,650 + 2,400 = 5,050 starting watts.

### Why add only one starting wattage?

A motor needs its extra starting watts for only a moment, and devices seldom start at the very same time. Generator makers such as Champion and Firman add only the single highest starting surge to the running total.

### What is the difference between running and starting watts?

Running watts are what a device draws once it is going. Starting (surge) watts are the extra burst a motor or compressor needs to start, Champion puts it at typically 1 to 2 times the running watts, and sometimes over 3 times. A generator lists both ratings; your devices must fit under each.

### How much margin should I add?

The makers advise some headroom for unexpected surges and future needs but do not fix one number. Type the margin you want, such as 20%, and both totals go up by that share.

### Where do I find a device's watts?

On its label, its manual or the maker's website. If the label lists only volts and amps, multiply them: a 120 V, 5 A device draws 600 W. Starting watts for motors are often in the manual; lights and electronics have none.

### Does this size a whole-house standby generator?

It gives the watts the loads you list need. For a standby generator wired to your panel, an electrician sizes it from the circuits it will power, including large loads such as a central air conditioner or a well pump.

## Sources

- Champion Power Equipment, What Size Generator Do I Need? (list the devices; add all running watts; identify the highest starting wattage; add that starting wattage to the total running watts; starting watts are the additional burst certain devices need to start; refrigerator 150–400 running and 800–1,200 starting watts; ½ HP sump pump 800–1,050 and 1,300–2,150; 10,000 BTU window AC 900–1,200 and 1,800–2,400; choose a generator that exceeds the total by a safe margin). https://www.championpowerequipment.com/news-updates/what-size-generator-do-i-need/ (retrieved 2026-10-02)
- FIRMAN Power Equipment, Buying Guide (record the running and additional starting watts of the devices run at the same time; add the running watts; identify the highest additional starting watts; total running watts + highest additional starting watts = total starting watts required). https://firmanpowerequipment.com/pages/buying-guide (retrieved 2026-10-02)