Thanks for the explanation.

Too bad the technology is not there yet.

I agree with Rich on pretty much everything in his post except this one
part.   Between Spring and Fall Equinox (which is most folk's camping
season in the northern hemisphere), the further north you are, the more
sun you get!  From my own experience, I've been in northern Scotland
(about 57 degrees north) in late June and the days are *long*.

Other than that nit picking, I agree with Rich's post.

Maybe if you are in the middle of the Sahara Desert.  That is 15 watts
each under ideal conditions.  If you are fortunate you probably get that
for a few minutes each day.

Watts = Current times Voltage.   15 watts is a little over 1 ampere at
12 volts.

Best off to buy extra batteries.  For $200 you can get four 6 volt Golf
Cart Batteries from SAMS.  That will give you 440 Ampere hours at 12
volts (220 not discharging below 50%).  But a lot of weight.

Alternative is to buy a generator.  Preferably a Honda but the Coleman
is really not too bad and a lot cheaper.  Coleman available for about
$300 to $500.

Hi,
Two 15 watt panels will give you approximately 2 amps per hour under
ideal conditions.  Ideal conditions are the panels are facing the sun at
the correct angle and follow the sun as it moves across the sky from
when the sun comes up until it goes down.  You have no shade on the
panels and it is a cloudless sky.

Flat mounted on the roof you might get  .1 - 1 amps per hour if it
sunny with no clouds no shade depending upon the time of day and your
location.  Anything less and you will be getting less.  Figure about 2+
amps per eight hour day maybe. Where you are effects the efficiency too.
The southwest and south are the best areas for solar.  You will get
better results there.  Summer better then winter.

Your furnace will draw more amps out of your battery in one hour then
those two solar panels will put back in one or two days mounted on your
roof.

I have an 120 Watt solar panel on my RV.  That's ideally about 8 amps.
I get from .1 to 4 amps or so depending upon the time of day and time of
year while parked in front of my house.  I use it to keep my 440 amp
battery bank charged in the winter and to replenish some usage.  In the
summer I can sometimes get 20 - 30 amps or so put back in during the day
in ideal conditions and depending where I am.  That's with 4 times as
much wattage as you are talking about.

Those two 15 watt panels are fine for maintaining a few batteries with
little or no draw on them.  If you have little usage then they will put
back a little but a generator will put back more in a half hour then the
solar panels in days.

To find out how many amps you use you need to know the wattage of the
items being used and the length of time using them.  Multiply the
wattage by the number of hours then for 12V items divide by 12.  That
will give you the amps used for that item for that length of time.  Add
a little for line losses.

If you want to use solar get at least a 50 watt panel (3.3 Amp) or two
then you need a charge controller so you don't cook the batteries.  Even
then you have to conserve your battery usage if you are only depending
upon solar.  Rainy cloudy days put little or nothing back but you still
have usage on those days.  Usually more then normal.  

Take care and Happy Campin...

not enough to run my electrical gear, but I use it to keep my battery
charged when Im dry camping.  We have never had a problem with a dead
battery.  BUT we also charge our battery when towing, and if available we
plug in.  Sorry I couldnt have been more help.