A generator is a device that creates electricity from some other form of energy. In RV terms this usually means a gasoline powered generator, but they can also run on propane or diesel. Residential generators sometimes run on natural gas.
Note: in some areas solar setups and wind turbines are also called “generators”. Portable “power stations” are sometimes, inaccurately, marketed as solar generators.
The most common built-in generator found in RVs is the Onan, now owned by Cummins.
The lower end is dominated by open-frame, non-inverter “contractor” gennies (Champion, most infamously), which are loud but cost much less for the wattage. Recently some inexpensive inverter generators have come to market, including the Harbor Freight Predator line. Lower-end generators may not be intended for repair, and repair parts may not be available.
Inverter generators adjust engine output to meet power demand. This can mean at lower power levels inverter generators can be considerably quieter than older technologies. At maximum rated power the noise difference between inverter and non-inverter gennies may not be as great.
It is common to leave generators quiet in times of darkness, and to not run them when close to other campers.
Generators are not particularly well-suited to charging lead-acid batteries (flooded, sealed, AGM, etc) on their own. They work best in combination with solar; the generator handles Bulk charging and solar handles the long-duration, low-current Absorption and Float stages.
[Alternator charging + solar can do the same thing for about $950 less – Secessus]
If the generator is being used for some other purpose then by all means add on battery charging. But running a generator for hours at 5% capacity only for charging is not optimal.
Generators are well suited to charging lithium-chemistry batteries as that type can take current at all points of charging.
If you are considering a genny for use with an inverter/charger combo, consider this:
Inverter chargers generally prefer conventional generator and not inverter generators. This is because they control the amount of power they pull from the generator just adjusting your island frequency. Inverter generators do not handle an external actor trying to adjust the frequency and will rigidly keep 50 or 60Hz until they trip. – therealrimwarren1)
Generators generally have a minimum amount of wattage they can produce. This means that until you hit that minimum multiple loads can be run for “free”, speaking in terms of fuel consumption.
An example of an efficient use would be:
Runtime means how long the generator will run a single fueling.
Some generators can be paralleled, or combined to make more power in tandem.
Fuel injected generators may offer better performance at altitude extremes.
The physical dimensions of the generator will need to fit in the space you have available. The weight needs to be something you can tolerate if it will not be permanently mounted somewhere.
Eco modes may allow fuel conservation at lower power outputs.
Noise is rated in decibels (dB), traditionally measured at ~23' (7m) when running at full load. Read specs fully to see if a dB claim is made at idle, 1/4 load, or some non-standard distance. Very quiet generators allow normal conversation rather close by, and imperceptible3) outside the immediate campsite. Loud generators might be heard hammering away at a distance of 100yds+.
Generators typically require maintenance after initial break-in and every X hours thereafter. Read the manual for specific instructions, but recommendations are typically for oil changes after the first two hours then every 20 hours thereafter. The frequency of oil changes make make it worthwhile to watch oil-changing videos for each model under consideration. Remember to carry spare oil and a way to transport used oil to a recycle or disposal point.
Depending on design, air filters will require cleaning or replacement. Fuel filters and spark plugs re also replaced as needed or according to maintenance schedules.
Fuel left in an unused generator can leave residue resulting in poor (or no) performance. It is common to run the fuel out of the genny at the end of the camping season to avoid this residue. Fuel stabilization products like Sta-Bil may help reduce the problem.