How to read a generator spec sheet
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What running watts, starting watts, THD, run time, outlets, decibels and EPA/CARB labels actually mean on a generator listing.
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
- Running watts against your needs.
- Starting watts against your peak.
- Voltage and outlets for your hookup.
- Run time at the same load.
- Noise if neighbors or campgrounds are a concern.