How to read a generator spec sheet

Generator listings pack a lot into a few lines. Here is what each common spec means and which ones to compare first.

Running watts (rated watts)

The power the generator can supply continuously. This is the most important number. Compare it with your running total plus headroom from the calculator.

Starting watts (peak or surge watts)

The power it can supply for a moment to start motors. It is always higher than running watts and is the number printed biggest on many boxes. It must cover your running total plus the largest motor surge.

Fuel type and dual-fuel ratings

Dual-fuel generators run on gasoline or propane. Their running and starting ratings are usually lower on propane; spec sheets list both. Compare the rating for the fuel you plan to use.

Run time

Usually given as hours on a full tank at a stated load, for example 25% or 50%. A figure at 25% load looks longer than one at 50%, so compare like with like. Our fuel run-time estimator converts it to your load.

Outlets

• 5-20R: standard household duplex outlets (120 V).
• TT-30R: 30-amp RV outlet (120 V).
• L5-30R: 30-amp twist-lock (120 V).
• L14-30R: 30-amp twist-lock with 120/240 V, common for transfer switch inlets.
• 14-50R: 50-amp RV outlet (120/240 V).

If you want to feed a transfer switch or power 240 V loads, you need an L14-30R or larger 240 V outlet.

THD (total harmonic distortion)

A measure of how closely the output matches a clean sine wave. Lower is cleaner. Inverter generators often advertise low THD; some conventional generators list it too. If equipment you plan to power specifies clean power, compare this figure.

Noise (dB)

Decibel ratings are usually measured at a specific distance (often 7 meters or 23 feet) and load (often 25%). Because decibels are logarithmic, a few dB is a noticeable difference. Compare figures measured the same way.

Emissions labels

EPA and California Air Resources Board (CARB) certifications concern engine emissions compliance and where the generator can be sold. CARB-compliant models are required for sale in California.

CO safety features

Many newer models advertise a CO sensor that shuts the engine off when carbon monoxide builds up nearby. It is a useful backup, not a substitute for correct placement; see carbon monoxide safety.

Parallel capability

Some inverter generators can be linked to a second unit to combine output. This matters mostly for RV use.

What to compare first

1. Running watts against your needs.
2. Starting watts against your peak.
3. Voltage and outlets for your hookup.
4. Run time at the same load.
5. Noise if neighbors or campgrounds are a concern.

A sample spec sheet, decoded

Here's how a typical mid-size dual-fuel portable generator might be listed, and what each line means:

• Starting watts: 9,500 (gas) / 8,500 (propane). The peak output the generator can supply for a few seconds to start motors.
• Running watts: 7,500 (gas) / 6,750 (propane). The output it can sustain. The propane figure is lower because propane holds less energy per gallon than gasoline. EIA lists about 91,452 Btu per gallon for propane versus about 120,214 for gasoline.
• Fuel tank: 6.6 gallons.
• Run time: 10 hours at 50% load (gas). Use this to estimate gallons per hour: about 0.66.
• Outlets: 2 × 120 V 20 A duplex (5-20R), 1 × 120/240 V 30 A locking (L14-30R). The L14-30R is what you'd use with a transfer switch or interlock inlet.
• Noise: 74 dB(A) at 23 feet. Note the distance and load. Manufacturers often measure at 25% load, which flatters the result.
• THD: under 23%, typical of a conventional (non-inverter) generator.
• Engine: 420 cc OHV.
• CO shutoff: yes.

Ratings to treat with caution

• "Peak" or "max" watts in big print. Some marketing emphasizes the starting number. Always size on running watts.
• Run time at 25% load. Real-world household loads are often higher. Use the 50% figure if both are published.
• Noise at a quarter load. Compare noise ratings only at the same distance and load.
• "Up to" claims on runtime or fuel efficiency.

Inverter-specific terms

Inverter generators often list:

• Eco or economy mode, which lowers engine speed at light load to save fuel and reduce noise. It can slow response when a large motor starts, so you may need to turn it off for refrigerators or AC units.
• Parallel capability, which lets two compatible units be linked for more output with a parallel kit.
• Clean power / low THD, typically under 3%. That's preferred for sensitive electronics.

Safety and compliance labels

• CO safety shutoff. Many generators now include a sensor that shuts the engine down if carbon monoxide builds up around the unit. It's a valuable backup, but not a substitute for operating outdoors. Ready.gov's 20-foot guidance still applies.
• Emissions certification. Look for EPA certification, and CARB compliance if you live in California or plan to use the generator there.
• Listing marks from a recognized testing laboratory.

Comparing two generators side by side

When you shortlist models, put these in a table:

1. Running watts on the fuel you'll use.
2. Starting watts.
3. Run time at 50% load and tank size.
4. Outlet types: does it have the 240-volt locking outlet you need?
5. Noise at the same distance and load.
6. THD, if you'll power electronics.
7. Weight and wheels.
8. Warranty length and service network.

Running watts and outlet type usually settle the decision. The rest helps you choose between finalists. Use our calculator to total your loads first, then shop for a generator whose running watts on your chosen fuel exceed that total with headroom.

Keep the manual

Once you buy, keep the owner's manual with the generator, or save the PDF on your phone. It has the real numbers you'll need later: oil capacity and grade, spark plug type, maintenance intervals, altitude derating, and the wiring diagram for each outlet.