The first thing to do is define the load requirements of whatever items you will be running off your batteries. Remember that it’s the batteries that really run the electrical loads while the solar panels are merely a source to help recharge them.
When we run any 12-volt loads such as lighting or water pumps, we are drawing amp-hours from the batteries. Whenever we run 120 volt AC items we will be running these items through an inverter, which converts battery power to AC power. We need to find the wattage of each item.
The next step is to calculate how long items will be operated. The longer time equals more amps they will take from batteries. The intention of a solar system is to run the generator once daily to recharge the batteries to operate various items.
Solar panels only work when the sun hits them. If you’re camping in Arizona you’ll get more output from your solar system. But if you are camping in the north woods during the rainy season the output is reduced.
Solar panels also need to be at a right angle to the sun’s rays for maximum performance. Most RV installations are flat mounted. Even at noon the sun is never directly overhead so there is some loss, and early morning and late afternoon may only give you 25 percent of the performance that you are expecting.
Keep in mind when doing calculations that there are variables to consider. Sunny Arizona versus the rainy Pacific Northwest is one of them. Also, the brand of solar panels makes a big difference. Some panels result in decent output in low light conditions while other brands rapidly fall off when the light isn’t perfect.
A good place to learn more is at AM Solar in Springfield, Oregon.