The basic rule of thumb found all over the Internet for solar panel sizing is 80 to 100 watts per 100Ah of battery. Not sure how it got started but a little math will help and here it is for a 100Ah battery.
100Ah(battery size) * 14.5 V(charging based on absorb voltage) * (1/0.77) panel + CC derating factor * 0.05 Rate of charge = 94 Wh or a 100 watt panel.
The figure for panel + CC derating appears to come from losses between panel and CC and CC efficiency.
A 5% rate of charge, roc, is a typical minimum value for most battery manufactures.
to clean it up: 100Ah*14.5V*(1/0.77)*0.05= 94 Wh.
Do an energy audit of your usage to get a better guess at the amount of solar you will need. This will save money in the long run even though it is a pain.
See this post on ours:
http://www.trailmanorowners.com/foru...46&postcount=1
If you use name plate information from each device your guess will be a little high. If you measure everything it will be closer and may save money in the long run for the project cost. There are several spread sheets and other solar calculators on the Internet.
More math:
Watts = Volts*current, w=E*I and another watts = resistance *current squared, w=R*I*I. This one is losses from using to small a wire and why with DC we use large wire sizes.
A way around this is wire panels in series and a MPPT controller.
Down side cost and shade will have a major impact. To size the CC use Voc times the number of panels in series and the Isc of the series string. Panels in series add Voc but the current remains the same as the Isc of one panel.
Panels in parallel will have higher current and therefore higher losses in the wire. Use large wire sizes and keep wire runs short. Wire is expensive.
On the plus side parallel panels are less impacted by partial shading and you can use a cheaper PWM CC. To size the CC use total panel Isc times 1.25.
For my 2 80 watt panels that is: (5.1 + 5.1)*1.25 = 12.75 amps so use a 15 amp CC.
I suggest 10 AWG wire as a minimum size for panel to CC up to about 15 feet. Over that use 8 AWG. The first makes a nice install for a pair of 100 watt panels on the rear shell, the second makes a nice portable set up for a pair of 100 watt panels IMO.
PWM vs MPPT this is more a budget choice then anything but the rule of thumb break even point is around 400 watts of solar. If you want to run a microwave or hair dryer or other high wattage items and have a 225Ah battery bank, you could be looking at needing 400 watts or more of solar.
The break even point for a 225 Ah battery bank is around 400 watts.
The math for going 3 days to reach 50% soc.
225 AH * 12 volt * 0.85 AC inverter eff * 1/3 days storage * 0.50 maximum discharge = 383 WH per day
This is usage per day and equates to solar panel sizing so around 400 watts of solar should cover for those days you can't get 5 to 6 hours of max sun light.
Note the use of 12 volts since we are drawing off the battery.
per rickst29 and he is quite correct one should use 12.4 volts this would be 395 WattH. At 12.0 volts your battery is about totally discharged and a point you
donot want to reach.
For myself I use 12.4 volts and 83% efficiency for the inverter.
A pair of 120 to 150 watt panels on the front shell along with a pair of 100 watt panels on the rear shell will get you a nice solar array for use with MPPT.