30 -45 watts will do you pretty well.
That's about a 2 amp input at peak. On a 6 hour peak day, that's 12amp
hours put in without you doing any work. That may not fill your battery up
every day if you're doing alot with the computer and inverter, those
generally can be pretty power hungry. However for charging phones, evening
lights, and perhaps a bit of computing....
That will likely peak you back up every day.
At the very least for a 2-3 day trip youll be fine as your battery slowly
goes lower.

I've had 30 watts that fit in the luggage rack, and loved it. Its been
plenty for me to run lights, a bit of stereo, charge phones, and a fan all
night.

-Craig

On Sat, Jan 28, 2012 at 4:15 PM, Tom Hargrave <thargrav@hiwaay.net> wrote:

> I suspect you are using it to float the battery & not keep running lights,
> inverter, etc powered?
>
> We lost power here in North Alabama when the tornados came through last
> year and the local O'Riellys was selling about the same size solar panel
> along with a car battery and 300 watt inverter to run small stuff in
> people's houses.
>
> I was in there looking for a deep cycle battery that I used to keep my
> freezer frozen and got in a discussion with the local "expert" peddling
> these setups. He was claiming that the solar cell would keep the battery
> charged and would run anything the 300 watt inverter would support. When I
> started asking "all the wrong" questions about watts, loading, solar cell
> output, etc., he left about 10 potential customers standing around &
> wandered into the back and did not come out until I left with the battery I
> bought from them.
>
> And BTW, I ran the Jeep at 2K RPM 30 minutes twice a day to top off the
> battery then I ran the freezer through a 3000 watt inverter the last 15
> minutes to keep everything frozen. The freezer drew 5 amps and nothing
> inside thawed.
>
> Thanks, Tom Hargrave
> www.stir-plate.com
> www.towercooler.com
> www.kegkits.com
>
>
> -----Original Message-----
> From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM] On Behalf
> Of Chris S.
> Sent: Saturday, January 28, 2012 2:02 PM
> To: vanagon@GERRY.VANAGON.COM
> Subject: Re: solar charger
>
> The infamous and loved Harbor Freight sells different types of solar
> panels. I bought a 12" x 18" (inches not feet, scrutineers) and hooked it
> up via a cig lighter plug.
>
> Chris.
>
> Wys�ane z iPhone'a
>
> Dnia Jan 28, 2012 o godz. 10:32 marc rose <mrose1028@GMAIL.COM>
> napisa�(a):
>
> > Looking for input on a good brand and model of solar battery charger.
> > I know there are alot of different styles.  I just have a weekender
> > poptop, no kitchen.  It will only be needed to keep a charge on the
> > battery for runnig lights, invertor for computer and a few things.
> >
> > Thanks
>
> -----
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