Solar panel usage on motor homes has become quite common place in the last few years. There are lots of sites on the internet which allow one to estimate battery loads. That part is easy.
The hard part is how to estimate panel output. Everyone has a different response. Have a look at the table below which I have prepared using some basic assumptions and you will see why. Winter (21 Dec) calculations give huge differences based on latitude, while summer (21 June) output is fairly consistent no matter how far north you are parked. The longer day more than makes up for the lower sun angle the further north you travel in the summer. In the winter, both the effect of a shorter day and really low sun angles decreases output dramatically the further north one is located. Tilting the panel in winter would make real sense as it also helps keep snow off, although I am not sure how many RV's boondock that far north.
We have always preferred horizontally mounted panels for their simplicity. We started Rving with only one 50 watt panel and this was sufficient for our travels in North USA and Canada in the summer only. On reaching retirement 15 years ago, our travels were further afield to Florida, Texas and Arizona. The first few years were explore years. Lots of short stays and short jumps to new and better boondocking spots. The 50 watt panel was enough since the jumps recharged the battery and the panel was acting like a battery extender rather than as a recharger. Now we spend longer in one spot, sometimes with shade, and a lot more winter boondocking. We now have five panels totalling 360w, and are marginal only after several days of rain in the winter in Florida or Texas.
Our experience with different panel manufacturers has led us to the following rule of thumb. Max panel amp from the all maufacturers falls off a bit for the first year as they age. Divide the panel rating by 20 to approximate output in amps of the best panels after one year. For the cheaper Chinese panels divide by 25, and flexible panels or film panles divide rating in watts by 30.
I have experimented with tilted panels and do not think they are worth the
extra hassle. I also experimented with reflectors to increase the output
of a horizontally mounted panel and am really impressed. I got improvements
using a car window heat shield of up to 25% over PANEL RATING in winter time .
Ie a panel rated at 4 amps would put out 5 amps. This panel was horizontally
mounted and output was only say 3 amps with no reflector.
Amazing increase,
but watch out for overheating.
In Summer the flat panels are sure easier to look after, remain safely bolted in place when you want to move, and there are no wiring flexibles to worry about. Tilted panels also lose some daylight hours in the longer days of summer when the sun gets behind them. This may then require rotation and/or more operator care when parking to maximize peak at noon. You must be parked facing true west , or true east for side tilting panels to be effective.
Estimated Amp-hour Output for 100 watt solar array, horizontally mounted,
clear day, clean panels.
Latitude 21 Dec. 21 March 21 June 21 Sept
Deg.N. amphours amphours amphours amphours
25 (S. Florida) 27.0 40.7 49.1 40.7
30 (Tex,N.Fla.) 23.1 38.9 50.6 38.9
35 (S.Cal, Az.) 19.2 36.7 52.0 36.7
40 (Mid. USA ) 15.3 34.4 53.1 34.4
45 (W.USA,E.Can,Me.) 11.6 31.7 54.0 31.7
50 (W.Can,Nfld) 8.2 28.9 54.5 28.9
55 (N.BC, Lab.) 5.1 25.8 54.6 25.8
60 (Yukon,AK,NWT) 2.5 22.5 54.1 22.5
What does all this mean. How do you size panels your own installation is an often asked question.
My answer is now this:, if you buy good quality panels.
For a truck camper or class B ...100 watts.
For a Class C or smaller Trailer...200 watts
For a Class A or larger Trailer .....300 watts
If only used in summer months divide the above in half.
If Fulltiming with all the extras, (satellite TV, Broadband internet etc) double the above.
I wanted to make estimates of output while boondocking at Bear Island
in the Everglades last winter. The above table was the result and seems to agree
quite closely with other sources. Little
technical advice was available, so I simplified the calculations as follows.
Knowing that the output of a horizontal solar panel is proportional to
the angle of the sun above the horizontal, I measured the output over several
days. It was found to be a sine wave where the peak output
is at local noon, and sunrise and sunset are the zero crossing.
The amp hour output is then the area under this sine wave
with base equal to hours of daylight and peak at noon.
Next, my power squadron navigation knowledge came in handy. The peak value
is sin( elevation of sun) where elevation of sun at noon = 90-lat+dec.
Lat is the latitude , and dec is latitude of the sun on a particular day.
There were only 4 days a year I knew the dec of the sun. This was when
the sun reaches the tropics or crosses the equator, so I used them.
21 December = -23.45 deg.
21 March = 0.00 deg.
21 June = +23.45 deg.
21 Sept = 0.00 deg.
Then I calculated the length of daylight on those 4 days for each of the
latiudes using a sun program called geoclock I happened to have handy.
and found it to be (by comparison) approximately 12* sin(90-lat).
And the area under a sine wave is 0.637 times the height times base.
This gave me a formula I could use
Amp hours = 0.637 * 12 * sin(90-lat) * sin(90-lat+dec) * panel Amps
Amp hours = area factor * daylight hrs * peak sun factor * Amps
Which amazingly agrees quite closely with the results at this site.
More accurate Solar calculator
Throw out the GPS
A byproduct of all this thinking led me to the realization that an output plot of a horizontal panel clearly shows local noon. For few days we checked this theory and found that we could regularily get a longitude position within 10 miles, using a clock calibrated to WWV. Amazing. Just needs a clear day, and you could get lat too.
73 de VE3LGS ... Return to Boondock