Generator Specs: What Each Number Really Means

Reading a generator spec sheet

A generator spec sheet is a table of numbers that describe what the machine can do. The most important figures are watts, amps, decibels, and runtime. Each one answers a different question: how much power the generator can supply, how much current flows through the outlets, how loud it is, and how long it runs on a tank of fuel.

On this site, every listing is compared on stated details. That means the numbers you see come from the seller's description and the maker's page. The site does not measure output, runtime, fuel use, or noise. When you read a spec sheet, treat each figure as the maker's claim, and check the maker's manual for the conditions behind it.

Watts: starting and running

Watts measure electrical power. A generator listing usually shows two watt figures: starting watts and running watts. Starting watts, sometimes called surge watts, is the brief burst of power needed to start appliances with motors, like a refrigerator or a sump pump. Running watts is the steady power the generator can supply while those appliances run.

The first watt figure in a title is often the starting figure, because it is the larger number. For example, a listing that says "3,000 watts" may mean 3,000 starting watts and 2,500 running watts. Always look for the running watts figure on the maker's page, because that is the number that tells you what the generator can sustain.

Watts are grouped on this site into ranges, such as 3,000 to 5,000 watts and 5,001 to 9,000 watts. These groups help you compare models with similar capacity. For a deeper look at the two watt figures, see Running vs Starting Watts.

Amps: current at a voltage

Amps measure the flow of electrical current. On a generator spec sheet, amps are usually listed for each outlet, such as a 30-amp or 50-amp outlet. The amp rating tells you how much current that outlet can supply at a given voltage.

For example, a 120-volt outlet rated at 30 amps can deliver more current than a 15-amp outlet. The relationship between watts, volts, and amps is simple: watts equal volts times amps. So a 120-volt outlet at 30 amps can supply 3,600 watts, while a 240-volt outlet at 30 amps can supply 7,200 watts.

When you compare generators, look at the outlet types and their amp ratings. A 30-amp RV outlet is common for travel trailers, while a 50-amp outlet is used for larger RVs. For more on outlets, see Generator Outlets.

Decibels: sound level

Decibels (dB) measure sound pressure. A generator spec sheet may list a decibel rating, often measured at a certain distance, such as 7 meters. The lower the number, the less noise the generator makes, but the rating depends on the measurement conditions.

The site does not measure noise, and it does not promise that any generator is quiet. The decibel figure on a listing is the maker's stated value. When you compare models, use the decibel number as a relative guide, but remember that actual sound depends on the environment and the load.

For a fuller explanation of decibel ratings, see Generator dB Ratings.

Runtime: how long it runs

Runtime is the number of hours a generator can run on a full tank of fuel. The figure is usually stated at a specific load, such as 50 percent load. A generator that runs 10 hours at half load may run fewer hours at full load.

The site does not measure runtime or fuel use. The runtime figure on a listing is the maker's estimate under stated conditions. When you compare models, look for the load percentage that goes with the runtime number, and check the maker's page for the full picture.

For more on runtime, see Generator Runtime at Half Load.

Other numbers on the sheet

A spec sheet also lists fuel type, engine displacement, fuel tank capacity, and features like electric start or a CO sensor. Fuel type tells you what the generator burns: gasoline, propane, natural gas, or a combination. Engine displacement, measured in cubic centimeters, is a rough indicator of engine size, but the site does not rate performance from it.

Fuel tank capacity is the volume of fuel the tank holds, usually in gallons. It relates to runtime, but the site does not calculate fuel consumption. Features like electric start and a CO sensor are listed as stated details, and you can read about them in Electric Start or Recoil and Best Generator with CO Sensor (2026).

Comparing models by stated details

To compare generators, start with the watts group that matches your needs. Then look at the running watts figure, the outlet types and amp ratings, the decibel rating, and the runtime at a stated load. These numbers together give you a picture of what the generator can do.

The site's buying guides compare models on stated details, not on measured performance. Use the guides to build a shortlist, then read the maker's page for the full spec sheet. For a step-by-step approach, see how to use the guides.

What to pick for your situation

Questions

What is the difference between starting watts and running watts?

Starting watts, also called surge watts, is the brief burst of power needed to start appliances with motors. Running watts is the steady power the generator can supply while those appliances run. The starting figure is usually higher and is often the first number in a title.

How do I convert watts to amps?

Watts equal volts times amps. To find amps, divide watts by volts. For example, 3,600 watts at 120 volts is 30 amps. The spec sheet usually lists amps for each outlet, so you can compare directly.

What does the decibel rating mean?

Decibels measure sound pressure. A lower number means less noise, but the rating depends on the measurement distance and conditions. The site does not measure noise, so treat the decibel figure as the maker's stated value.

How long will a generator run on a full tank?

Runtime is the number of hours a generator runs on a full tank at a stated load, such as 50 percent. The actual runtime depends on the load and conditions. Check the maker's page for the runtime at different load levels.

What other specs should I look at?

Look at fuel type, fuel tank capacity, outlet types, and features like electric start or a CO sensor. These details affect how you use the generator and what you can plug into it.