I have seen a few postings that some have installed a few solar panels on your bus. In the larger picture the dollar spent might not be a wise investment.
That is an astute observation. Or maybe an understatement. It's hard to make definitive statements about the cost-effectiveness of solar, because it depends on so many factors that vary greatly from installation to installation, such as the amount of incident sunlight, installation angle of the panels, ability (or lack thereof) to track the sun, plus the variability in the lifetime of the panels, which will be impacted by environmental considerations (snow, heat, lack of cool airflow underneath, sleet, etc.) as well as mechanical stresses from the installation, such as on a moving bus.
That being said, on average
for motor coach installations, solar is not
a cost-effective way to make electricity. So you really have to want to do it for other reasons, such as being able to leave the coach parked someplace while you are away for a few days, weeks, or months without having to worry about the batteries, or you want to be able to park in some national park campgrounds where generators are prohibited or severely restricted, or you just prefer to keep your generator operation to a minimum for the peace and quiet.
But if you want to keep all your batteries topped off and your fridge maintained as a luxury>> what are your real world experiences?
We have "330 watts" of panels, along with an MPPT charge controller to maximize the available output. For us, if we are parked where the panels get good light for most of the day, that's enough to run the fridge (high-efficiency Novakool 24-volt model, 7.5 cubic feet) pretty much indefinitely, and the air compressor as well if we are not actually on board using air, plus perhaps a Fantastic or two on thermostats. So we think nothing of taking off for a week or two with a fridge full of food, but we do turn off all unnecessary and parasitic loads, and we leave the genny on auto-start as a precaution (it has not had to run).
Real life, of course, is different, and using things like the coffee maker, microwave, computers, and occasionally the TV, not to mention using air by flushing the toilet and opening and closing the plug door means we need to run the genny every few days for a few hours to charge back up; the solar extends that time by a little bit, but the natural variability of daily life makes the amount of extra time we get very difficult to calculate. (We average one hour of genset run time per day when boondocking in temperate weather, starting on day three -- the first two days are provided courtesy of the 50DN alternator.)
Die-hard desert rats have arranged their rigs and their lives to use a bare minimum of electricity, and those folks can get by on solar alone indefinitely, but it is a spartan lifestyle and requires alternative technologies (chiefly propane) for such things as refrigeration.
I put the wattage in quotes, because that is the "nominal" wattage of the panels under ideal conditions. Ideal conditions are never encountered most places, and especially on a bus. A 110-watt panel might generate 90% of that figure on a crystal clear day in the Arizona desert while aimed directly at the sun at high noon. When you allow for other latitudes, lower incident angles (more atmosphere to traverse) throughout most of the day, inability to keep the panels directly aimed at the sun, etc., the number drops to 60% or less of that figure. If there are times when it is not sunny out, even less. If there is a shadow of any kind on even a small fraction of a panel, that panel's output will drop almost to zero. The list goes on and on. So in real world conditions, you can expect to see perhaps 20%-40% of ideal "rated" output across the daylight hours.
What I use for the purpose of "napkin" calculations is to use 50% of rated output times 8 hours in the summer, 6 in winter, to get an approximation of available output (in sunlight). So my "330 watts" of panels might
provide a single kilowatt-hour of electricity on a good day. That's about 40 amp-hours on my 24-volt plant. For comparison, we use 125-175 amp-hours per day on average, if we are in temperate weather where we need neither air conditioning nor heat.
That's all the panels that would fit on our roof, BTW, given that we have three roof airs, three Fantastics, a MotoSat dish, a roof hatch, and a 7'x7' deck up there. If you have a clear roof, with today's technology you could squeeze perhaps 3.5 kW (nominal) of panels onto a 40' coach, ten times what we have. But bear in mind, at 50% average performance, that's still less than a single household 15-amp circuit can produce. Still, we can (and have) run our whole coach indefinitely on a single house circuit, so it can be done. Mind you, that much solar will cost north of 20 grand for the panels alone, before figuring mounting hardware, cables, charge controllers, and other miscellaneous bits -- figure a quarter c-note for a complete installation. (At today's prices, BTW, that would buy you 10,000 gallons of diesel, which would run the average 6.5kW genset for about 20,000 hours -- 833 days straight, or two years, three months, and 12 days.)
Hope that gives you some useful perspective.