So, you’ve thunk it over and decided you want a generator but you don’t know what size. The size you need is — IT DEPENDS (I’m getting to love that phrase).
It depends on what you want to run in the RV, or what you want to run in your house, or both.
It depends on what kind of fuel you want to carry or have in the RV.
It depends on how handy you are with tools and/or electricity (or how much money you have to hire ‘experts’). NOTE: An “expert” is defined by the two syllables in the word – an EX is, of course, a has been, and a SPURT is nothing more than a drip under pressure. You pays your money and you takes your chances.
If you pick up any manufacturers advertising brochures, you will find listings of appliances and their typical wattage draw. You can drive yourself to distraction with all the math involved. A fairly simple way to find out what size generator you need is to look at the size of your shore cord.
If it is a 30 amp service, you will not need anything bigger than a 4000 watt generator (33.3 amps).
If you have a 50 amp shore cord, you have an upper limit of 12,000 watts capability.
You 50 amp people are going to have to run through the coach and decide what you want to run SIMUTANEOUSLY. Twelve kW generators are heavy, heavy, heavy. Most 50 amp campers have an upper limit of a 7000 – 8000 watt generator. That is the upper side of the installation.
On the lower side of the generator sizing is battery charging. You need a generator big enough to run your built in battery charger/converter. Rummage through your paperwork or look at the data tag on the converter and see what it needs for AMPERAGE. Multiply the amps times 120 to get the wattage, then add 15% more for starting or overload conditions. You need the batteries charged because the refrigerator and water heater use battery power for their controls and maybe ignition as well as the furnace and air conditioner thermostat. Usually the majority of lights in the coach are battery powered and most coaches have inverters to change battery power to ac power to run a TV or home entertainment center. For this reason, you should size the generator for “worst case” conditions. This means; dead batteries, night coming on, it’s hot and muggy and all of you need a shower and supper. An 1800 watt generator is not going to hack it under these conditions. You need to run enough lights to see what you’re doing, charge the batteries at maximum rate, run the air conditioner and the water heater and water pump.
Sizing a generator is a personal decision that all in the camping party must agree to. It will involve constraint and discipline to keep everything in the coach from being turned at the same time. If you go boondocking to ‘get away from it all’ why turn it all back on? If you are full-timing and want ‘it all’, get a larger generator.