Help with solar for RV

Re: Help with solar for RV

A reasonable rate of charge for a lead acid battery works out to be around 5% to 13% of the battery's 20 Hour (Amp*Hour) rating... below that, not enough charging current to do much good (except for storage/trickle charging). Above that, it usually becomes a waste of money (too many solar panels) and not enough room on your roof to even hold that many panels.

For example, say you have a 100 AH @ 12 volt battery bank... Using the rate of charge rule of thumb, the "typical useful" solar array would be:

• 100 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 94 watt array minimum
• 100 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 188 watt array nominal
• 100 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 245 watt array cost effective maximum

Say you pick the larger array (a pair of 140 Watt solar panels) for 280 watts. The amount of power you may expect (6-9 months of the year, average 4 hours, minimum, of noon time equivalent sun per day) would be:

• 280 watts * 0.62 DC system power eff * 1/12. 5 volts nominal * 4 hours sun per day = 56 AH per day (DC loads)
• 280 watts * 0.52 AC system power eff * 4 hours sun per day = 582 WH per day (AC loads)

Note for summer camping, mounting the panels "flat" to the roof (with some ventilation underneath to keep cool)l, will be OK. The farther north you get and the more towards winter you camp, the better it will be to tilt the panels towards the sun.

The "optimum" MPPT charge controller for a typical 280 watt array/weather conditions would be:

• 280 watts * 0.77 panel+charger eff * 1/14.5 volts charging = 14.9 amp minimum controller (on a 12 volt bank)

So, a couple controllers to look at would be the MorningStar 15 amp MPPT or Rogue 30 amp MPPT charge controllers (both work with 12 or 24 volt battery banks). One advantage with an MPPT charge controller is you can put the two panels in series (for Vmp~35 volts) and use an extension cord to place the panels in sun while the RV is parked in shade (make sure panels are well staked to ground so they don't blow over or walk away).

Otherwise, you could go with a 20 amp or so PWM charge controller (much less money and perfectly OK for most people with fixed panels on top of an RV).

Here is a nice thread with video from Kevin in Calgary Canada that shows designing and installing solar PV in a small RV trailer.

-Bill