Re: [VAC] Solar Power

[Jim Dunmyer <jdunmyer@xxxxxxxxxxx>]

Roy,
 I know you've been around here for a long time and know this, but for
the newer readers:

If you're considering solar power and want to have a successful
operation, it is imperative that you do some simple measurements and
calculations in order to get your input and output to coincide. Both
numbers are measured in "Amp-Hours" or "AH", which is one amp over a
one-hour period of time. Almost any Digital Volt Meter (DVM) has a
10-amp scale, which is sufficient for our needs.

Measure the loads that you use while in the trailer and estimate the
hours of usage each day. With only a 10-amp meter, you'll need to do
this one load at a time. Add these numbers; let's use mine as an
example:

Flourescent lights @ 1.7 amps: 1 for 3 hours, 2 others for maybe an
hour/each. Total = 5 hours X 1.7 = 8.5AH.
Reading light @ 2 amps: 1 for 3 hours. Total = 6AH
Five inch color TV @ 1 amp. Time = 3 hours, total = 3AH
Water pump @ 7 amps. Maybe 20 minutes of operation, total = 2 AH

Grand total usage = 19.5 AH per day

My solar panel is rated by the manufacturer at 75 watts, arrived at by
multiplying the short-circuit output corrent of 3.8 amps times the
open-circuit voltage of approximately 19. A better number would be 50
watts, which is 3.8 amps at 13 volts. A general rule of thumb is that a
panel will give its maximum output for about 4 hours per day, and half
output for another 4 hours. That gives us 3.8 X 4 plus 1.9 X 4 or 22.8
AH per (sunny) day. As the overall efficiency of a solar system is on
the order of 80%, I should expect to put 18.4 AH of usable energy into
my system per day. 

You can see that as long as the days are sunny, we can expect to operate
almost entirely by solar power without running our battery down.

Now, look at the usage figures above, and make some substitutions: 

Leave your old incandescent lights in place; mine drew about 5 amps
each, so we'd use 20 AH instead of 8.5 AH for lighting. Use a 9" color
TV and expect to draw 5 amps, for a total of 15 AH per day. Add in the
reading light and water pump and we're at 43 AH/day, more than twice
what I use now. This would require 2 solar panels like I have. At the
retail price of about $500.00/panel, you can see that there's a
significant savings by simply replacing the light fixtures and using a
smaller TV. Plus, we can easily operate for a cloudy day or 2 and not
run our single battery flat, where you'd be pushing things at the higher
usage number. (our common Group 27 "RV/Marine" battery is rated at 105
AH, but is more likely capable of only about 80 AH, and it's a good idea
to not use much more than half the capacity of the battery on a regular
basis) That means having 2 batteries in addition to the 2 solar panels.

As you can see, the first thing that you must do with a solar system is
to get your USAGE as low as possible. It's much easier and cheaper than
simply buying more and larger solar panels.

That said, solar power is definitely nice, and nearly mandatory for our
particular camping style at the moment. (we're often camping at
fairgrounds and other places w/o hookups) I can't imagine hauling a
generator along and annoying ourselves and the neighbors with the
racket. Count me as "A Believer".

                                  <<Jim>>