Mike,

For the most part I agree with your calculations, they only thing that
puzzles me is how your batteries can be fully charged by noon.

Please define what you mean by fully charged.

What is the battery voltage?

In my system, the charge controller (led lights) will show a fully charged
state in about the same time frame you reference. But in reality the
batteries are only
At a partial charge of around 75% or around 12.3 volts.

As you suggested ... my recommendations are based on fairly conservative
assumptions that would cover lots of solar conditions and locations.

Larry

- - -

Date:    Sun, 17 Jun 2007 18:47:45 -0700
From:    Michael Elliott <camping.elliott@GMAIL.COM>
Subject: Re: Solar panels for your Vanagon..

Using these conservative assumptions, let's analyze my system.

Assume 6 hours of sunlight / day, with panels derated by 15%, as suggested.
My panels are 2x45W, = 90W, derated by 15% = 77W * 6 hours = 450W / day of
power production. Assuming a 12.6 voltage system, call that 35Ah of
available power / day.

Our Norcold draws 2.5 amps when running, so if it ran for 24 hours a day, it
would use 2.5A x 24 = 60Ah in a day. But it doesn't run full-time, it cycles
on every ten minutes and on hot days it might run five minutes per 10 minute
cycle, and on cold nights maybe a minute per cycle. So that's about 1.25 Ah
during the day, and a lot lower at night. So for a 24-hour period, call it
20Ah for the reefer.

We use a laptop about 2 hours a day. Laptops generally pull 3 amps. Say we
run the laptop for two hours -- that's 6Ah.

Our little Philips DVD player draws about 2.5 amps, and a film is usually
about two hours, so that's another 5Ah.

For lighting we have some 8-watt 12-volt dc fluorescent fixtures, and a
couple of 12-volt halogen "reading lights." Each of the various lights draw
an amp or so when running, but we don't do a lot of that - one light at a
time, usually, and no lighting if we're watching a DVD. For general lighting
we use Dietz oil lamps. Electrically, we probably use less than 1Ah for
lighting per night.

All told, we use around 35Ah per 24-hour period. Add 25% as suggested, that
comes to 44Ah.

Based on these numbers, we're clearly using 9Ah more energy per day than I'm
generating from my solar rig. But in real life, our battery is fully charged
by noon every day and until we lose sun in the evening, we have a power
surplus.

Now if my life depended on a solar power system, I would use very
conservative assumptions, at least as pessimistic as the ones that Larry has
proposed. But we are only camping, and in good weather, too. If things get
sucky, we'll move on.

--
Mike "Rocket J Squirrel" Elliott