We love boondocking and/or dry camping since we both like to get away from the big RV parks and do hiking and exploring in the less crowded areas so I started thinking how we could improve the experience. We have a 40′ DP which has a 7.5 kVA diesel generator so we had lots of power but it is noisy and running it twice a day for 45 min or so was something we really wanted to avoid if possible, hence the solar.
I estimated how much energy we typically used per day and from that I did some very rough calculations and figured that somewhere between 400W and 600W worth of panels should be able to meet our requirements on a typical winter day in the south. Our biggest power consumers are the propane furnace (somewhat surprisingly the fan draws a lot of current), the satellite receiver, the TV and the sound system. The lights, water pump, charging portable equipment etc. are relatively minor draws and we don’t use the microwave on the inverter or any other heavy draw appliances when we are out in the boonies.
I decided to install 400W worth of solar initially but designed the system to be readily expandable to 600W by simply adding two more panels. I bought four 100W panels instead of fewer large ones for two reasons: 1) they are a lot easier to handle if you do the installation by yourself as I did and 2) they have much less surface area so aren’t so susceptible to damage during high winds if you tilt them up.
I also did some calculations to see what the benefit of tilting them would be in southern AZ or CA during the winter months and discovered that I could gain about 50% more energy by tilting the panels at the optimum angle so I made brackets that I can tilt to about 45degrees which seems to be just about optimum in January in the south. If you are interested here is a link to a java applet that you can use to see what effect tilting your panels will have at different locations during different times of the year.
Another consideration is what type of charge controller to buy. There are two basic types, Pulse Width Modulation (PWM) and Maximum Power Point (MPPT).
The PWM controllers are a lot cheaper and provide smart charging of your battery bank but you must wire your panels in parallel and not use the high voltage panels. Because your panels are in parallel you have relatively high current on the wires from the panel to the controller and if you don’t have sufficiently heavy wires you will suffer significant losses in this wire. Also the “12V” panels, which typically have an open circuit voltage of about 21V, will be operating well below their optimum voltage which is typically around 17- 18V. So this type of controller is less efficient and requires heavier wiring but is much cheaper.
The MPPT controllers use a DC to DC converter to step down the voltage from the panels to the optimum charging voltage for the battery bank. This gains some efficiency and allows you to wire the panels in series. By wiring the panels in series the voltage is much higher but the current carried by the wiring from the panels to the controller is correspondingly less. My coach had 8ga wire already in place but I still went with a 40A MPPT controller to get a bit more efficiency out of the panels and minimize the losses in the wire.
Edit: There is a downside to running solar panels in series that I didn’t anticipate. I discovered after using them for some time that if one of the panels is shaded by a tree or other object the entire string will be effectively putting out no power even if the other panel(s) are in direct sunlight. This makes sense because the shaded panel is effectively an open circuit so even if the other panels are up to voltage, no current will flow through the shaded panel. I think if I had it to do over I would use 24V panels wired in parallel instead of the 12V panels wired in series.
Alfa designed a battery system where two 6V batteries were wired to power
the DC systems in the coach while the remaining four 6V batteries were wired
to power the inverter thus powering the AC systems in the coach. There was
also a large relay (commonly called “bigboy”) which connected these two battery banks together based on the voltage levels of the battery banks.
While this system looks good on paper it has some shortcomings. In particular
some of the batteries may be discharged to a lower voltage more often and the banks will
not be charged at the same time/rate meaning that some batteries may be overcharged while the others are undercharged.
Tying all six 6V batteries into a single bank ensures that the entire capacity of
the bank is available for both DC and AC (inverter) and they are all charged at
the exact same rate.
Having a single bank also makes the addition of a solar array and charge
controller much more straight forward as there is a single point of connection
required which will charge all the batteries.
NOTE: The above diagram applies to Alfas where the left two batteries are wired to the House and the remainder are wired to the inverter. This was the arrangement in my ’04 SeeYa but I have been informed that not all Alfas were configured the same way. If you have a different arrangement you will need to modify the connector accordingly.
I mounted the controller as close to the battery bank as feasible and used 4ga wire on that short run. I also installed a remote monitor on the dash so I can see what the battery charge state is and how much the panels are putting out.
I bought the panels from Renogy which seemed to have reasonable prices and were very efficient and good to deal with. The installation was pretty straight forward although I should say I am a retired electrical engineer so I am very comfortable working with most things electrical.
Last winter we boondocked and dry camped in various locations so it was a pretty good test for the system. I discovered that during sunny conditions (which was most of the time) the battery bank would be fully charged well before noon. With high overcast it took a couple of hours longer and even with fairly heavy overcast we would be charged before the sun went down. I should mention that I have 6x6V batteries for the house. I never had to start the generator during the entire time boon-docking.
I decided that, given our lifestyle, 400W is sufficient to keep our batteries charged so I don’t plan to add the extra two panels.
I should also mention that in preparation for going solar I swapped out some of the most power hungry incandescent light bulbs in the coach. I just went around and felt the bulbs we use most and any that were too hot to touch got swapped out for LEDs. I also modified a couple of the fluorescent fixtures in the kitchen replacing the tubes and ballast with LED strip lighting. This last mod only saved about 1A per fixture so maybe not worth the effort,
those fluorescent are already pretty efficient.
I summary we are very pleased with the installation. It meets our power requirements very well and is much less obtrusive than that diesel generator.