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Generator sizing: how many watts?

List the devices you want to run at the same time, each with its running watts and additional starting watts. The generator size calculator adds the running watts, adds the one highest starting surge, and gives the running and starting watts your generator must supply, with an optional safety margin.

Your numbers

Devices you run at the same time
Row 1
Row 2
Row 3
Starting watts needed
5,050

Look for a generator rated at least 2,650 W running and 5,050 W starting.

Running watts needed
2,650
Total running watts
2,650
Highest additional starting watts
2,400
Starting kilowatts
5.05
Largest starting load
Device 3
Answer
Look for a generator rated at least 2,650 W running and 5,050 W starting

Starting watts needed: 5,050. Look for a generator rated at least 2,650 W running and 5,050 W starting.

How to calculate

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.

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.

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

  • 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.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Devices you run at the same time 400 1200; 1050 2150; 1200 2400, Extra margin 0% gives Total running watts 2,650, Highest additional starting watts 2,400, Starting watts needed 5,050, Largest starting load 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 you run at the same time 400 1200; 1050 2150; 1200 2400, Extra margin 20% gives Running watts needed 3,180, Starting watts needed 6,060, Starting kilowatts 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 you run at the same time 60; 150 0; 800 1300, Extra margin 0% gives Total running watts 1,010, Starting watts needed 2,310, Largest starting load 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)

How it works

For devices 1 to n run at the same time, each with running watts Rₖ and additional starting watts Sₖ (0 when left empty), and a margin of m%:

  • Total running watts R = R₁ + R₂ + … + Rₙ
  • Highest additional starting watts S = the largest Sₖ
  • Running watts needed = R × (1 + m ÷ 100)
  • Starting watts needed (headline) = (R + S) × (1 + m ÷ 100)
  • Starting kilowatts = starting watts needed ÷ 1,000
  • Largest starting load = the first device with the highest Sₖ ("Device 3"), or "None: no device has starting watts" when every Sₖ is 0

A generator fits when its running (rated) watts are at least the running watts needed and its starting (surge) watts are at least the starting watts needed.

Assumptions

  • Default devices, from Champion's typical values (the top of each range): a refrigerator at 400 running and 1,200 additional starting watts, a ½ HP sump pump at 1,050 and 2,150, and a 10,000 BTU window air conditioner at 1,200 and 2,400. Replace them with your own devices' numbers.
  • Starting watts are the extra watts above running watts, as Champion and Firman list them.
  • Only one motor starts at a time.

Rules

  • 1 to 30 devices; running and starting watts from 0 to 1,000,000 each; margin 0% to 100%.
  • The running watts must add up to more than 0 (starting watts are extra, so they need a running device); otherwise there is no answer.
  • Watts are added as the exact decimals typed.

Output format. Watts show as whole numbers, kilowatts with 2 decimals. The answer reads "Look for a generator rated at least 2,650 W running and 5,050 W starting", each rounded to a whole watt.

Worked examples by hand

Champion's three loads. R = 400 + 1,050 + 1,200 = 2,650 W. S = 2,400 (the window AC, device 3). Starting watts needed = 2,650 + 2,400 = 5,050 W (5.05 kW).

The same with a 20% margin. Running 2,650 × 1.2 = 3,180 W; starting 5,050 × 1.2 = 6,060 W (6.06 kW).

A light, a TV and a sump pump. R = 60 + 150 + 800 = 1,010 W; S = 1,300; starting = 2,310 W.

Other questions people ask

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.