How to Read a Generator Spec Sheet: Key Terms and Numbers

Rated watts vs. surge watts

The two most important numbers on any generator spec sheet are rated (continuous) watts and surge (starting) watts. Rated watts is the power the generator can deliver continuously; surge watts is the brief burst it can supply for a few seconds when a motor starts. If you choose a generator based only on the surge number, you may overload it once the motor is running. If you choose based on rated watts only, motor-starting loads can stall the unit or trip its breaker. For a fuller explanation, see running watts vs. starting watts.

To compare models fairly, look at the rated watts figure first, then check the surge figure to ensure it covers your largest motor load. Most spec sheets list both prominently. When a sheet lists only one wattage, assume it is the rated figure. If you are just starting to size a generator, work through how many watts do I need before comparing spec sheets.

Voltage and amperage

Voltage (V) is the electrical pressure the generator produces. Amperage (A) is the current it can deliver at that voltage. Watts equals volts times amps, so a generator rated at 5,000 watts can deliver about 41.7 amps at 120V, or about 20.8 amps at 240V. A spec sheet may list outlets as 120V only or as 120/240V. If your loads include a well pump, electric water heater, or large power tools, you likely need a 120/240V model. See generator voltages: 120V vs. 240V for the full comparison.

For RV use, check for a 30-amp or 50-amp outlet. A 30-amp RV outlet provides 120V at up to 30A; a 50-amp outlet provides two 120V legs for a total of 12,000 watts. If you are powering a home through a transfer switch, the breaker size on the switch must match the generator's output capability; consult the transfer switch documentation for matching requirements.

Power factor and total harmonic distortion

Power factor (PF) is the ratio of real power to apparent power in an AC circuit. For a resistive load, PF is 1.0. For inductive loads like motors, PF is lower, meaning the generator must supply more apparent power than the watts you measure. Most portable generators are rated at a power factor of 1.0, but always read the footnote on the spec sheet. If the sheet lists kVA (kilovolt-amps) instead of kW, multiply kVA by the power factor to get usable watts. See the dedicated article on generator power factor for more detail.

Total harmonic distortion (THD) is a measure of how distorted the output sine wave is compared to a pure sine wave. Conventional generators often list THD in the 5% to 10% range; inverter generators commonly list 3% or less. If you plan to power laptops, phones, or other sensitive electronics, look for a low-THD inverter model. See THD and sensitive electronics for the practical effect.

Noise levels in decibels

Spec sheets list noise in decibels (dBA) measured at a stated distance, usually 7 meters (about 23 feet) or 10 meters. The number is meaningless without the distance, so compare only like-for-like measurements. A 60 dBA generator at 7 meters is roughly the noise of a normal conversation; 70 dBA is noticeably louder. Every 10 dBA increase doubles perceived loudness. If noise is a concern, check the spec sheet for inverter models that often run quieter, and see generator noise levels explained for neighborhood considerations.

Some spec sheets list a sound power level (LWA) rather than a sound pressure level. LWA measures total acoustic energy radiating from the generator in all directions; pressure (dBA at distance) measures what your ears actually receive at a point. You cannot directly compare LWA and dBA-at-distance numbers.

Run time and fuel consumption

Run time is always tied to a load level. A spec sheet that says "12 hours at 50% load" is more useful than one that merely says "12 hours." The rated watts at which run time is measured matters; a 4,000-watt generator at 50% load uses more fuel per hour than a 2,000-watt generator at 50% load. Fuel tank size, measured in gallons or liters, determines how long you can run between refuels. See generator fuel tank size and run time for the relationship.

Fuel type changes both run time and maintenance. Gasoline is common and widely available; propane has a longer shelf life; dual-fuel models accept both. Each fuel has a different energy density, so the same generator will produce less power on propane than on gasoline. The spec sheet should state rated watts by fuel type. For a full comparison, see the article on generator fuel types.

Engine and alternator specifications

Engine displacement, usually listed in cubic centimeters (cc), gives a rough idea of power and fuel appetite. A larger displacement often means more output, but the relationship is not linear, and newer designs can be more efficient. The engine type (OHV vs. side-valve) is also worth noting; OHV engines generally cool better and have a longer service life. For guidance on what to look for, see the explainer on engine types.

The alternator section may list whether it is brushless, a PMG (permanent magnet generator), or an AVR (automatic voltage regulator) design. AVR models maintain stable voltage as load changes. PMG designs offer better motor-starting capability. If the spec sheet mentions THD and voltage regulation, those are the numbers that matter for sensitive loads. See generator alternator: PMG vs. AVR for the difference.

Safety features listed on spec sheets

Modern spec sheets list safety features that are as important as output numbers. A carbon monoxide (CO) sensor automatically shuts the generator off when CO accumulates to dangerous levels in the surrounding area. This is a proven lifesaver; the CPSC notes that CO from a generator is deadly and can kill in minutes. Look for this feature on any portable generator you plan to operate near a home. See what is a CO sensor on a generator for details.

Low-oil shutdown is a standard engine protection feature that stops the engine before it can be damaged. GFCI (ground fault circuit interrupter) outlets protect users from shock in damp conditions. The spec sheet may also list whether the generator is CARB or EPA compliant, which affects where you can buy and use it.

How to compare spec sheets from different models

When comparing two generators, put the spec sheets side by side and compare five numbers: rated watts, surge watts, voltage (120V vs. 120/240V), THD, and run time at a stated load. Everything else, from engine displacement to fuel type, affects these five numbers in practice. A generator that lists a high surge wattage but a low run-time at 50% load may be more suitable for starting heavy loads occasionally than for running continuously.

For home backup, match rated watts to your essential circuits and verify that the voltage (120/240V) and outlet configuration match your transfer switch. For RV use, verify the outlet type (TT-30 or 14-50) matches your camper. For camping, portability and noise level matter more than maximum output. See best portable generators for model-level comparisons.

What to pick for your use

Questions

What is the difference between rated watts and surge watts?

Rated watts (or continuous watts) is the power the generator delivers indefinitely. Surge watts (or starting watts) is the brief boost it supplies for a few seconds to start motor-driven loads. Always size a generator by rated watts for continuous loads, and check surge watts covers your largest motor start.

Is a higher THD number always bad?

Not always. THD of 5% or less is generally fine for most tools and appliances. If you are powering laptops, medical devices, or other sensitive electronics, look for an inverter generator with THD under 3%. Conventional generators with THD above 5% may cause issues with some electronics.

What does dBA at 7 meters mean on a spec sheet?

dBA at 7 meters is the sound pressure level in decibels measured at a distance of 7 meters (about 23 feet). This is the standard measurement for portable generators. Compare only numbers measured at the same distance, and remember that every 10 dBA increase sounds twice as loud.

How do I know if my generator can power my house?

Add up the running watts of the appliances you want to power simultaneously, then add a margin for motor-starting surge. Make sure the generator's rated watts exceed the running total and its surge watts exceeds the largest single motor load. For a full method, see the article on calculating your needs.

Does a higher fuel tank size mean longer run time?

Not necessarily. Run time depends on fuel consumption, which varies with load and fuel type. A larger tank can run longer at the same load, but a more efficient engine or a smaller load will also extend run time. Check the spec sheet run time at a stated load percentage to compare fairly.