When sizing your system its important to have a good understanding of your usage.Energy In -->[Battery]--> Energy Out. Our most important tools for this, is our Voltmeter and the Equation; Volts x Amps = Watts.
WATTS (W) = IN THE MOMENT ENERGY USEAGE
If someone were to measure their continuous energy usage (WATTS) at idle, aka going about a normal day, then one can determine what would be considered the minimum Solar Array or "Photovoltaic System" input is needed.
Ex. 1)I have (3) 100 Watt Solar Panels coming in at 300 Watts collectively and I'm using(creating a "LOAD" or "DRAW") of 200 Watts, then my battery(s) will stay "FULL".
Ex. 2) I have (3) 100 Watt Solar Panels coming in at 300 Watts and I'm using (creating a "LOAD" or "DRAW") 400 Watts my battery(s) will slowly "DRAIN".
WATT HOURS (Wh) = ENERGY OVER TIME
If someone were to measure their energy over time or "WATT HOURS" (Wh) though out a 24 hour period, then one can determine what would be considered an adequate amount of "Sun Hours", aka hours of direct sun, needed per day to fill a battery to its max "CAPASITY" also measured inWh.
Ex. 1) I have a battery that measures 12 Volts and 100 Amp Hours giving the battey a known "Capacity" of 1200 Watt Hours.
EX. 2)I have a 1200 Wh battery and I want to charge it off my (2) 100 W Solar Panels. With some simple math we can see that 1200Wh / 200W = 6 Hours of required "Sun Hours". I can run this example backwards to determine my outgoing energy.
Below is an example of a common Wiring Diagram of small scale, off grid, solar systems to use as a base reference.
**Remember, most of the time the best Solar Panel(s) for the job is whatever you have or can get.""