blank

This is going to be a bit engineerish, my apologies.  Allow me to lay out some facts that we can work with:

• That 25W rating is for perfect conditions -- in other words, a sunny day at noon with the thing pointed perfectly at the sun.
• If you have a 64 ounce airplane, it'll use about 45 Watt-hours (W-h, about 135kJ) of energy.
• The battery itself is only about 88% energy efficient in charge/discharge.  So you need to put about 51W-h into the battery
• Your battery charger is probably about 85% efficient, so you need to put about 60W-h into the charger

So if you're just going to haul that panel to the field and charge a battery with it, it'll take about 2 1/2 hours, at noon on a cloudless summer day.  Basically, I think that it's woefully inadequate, unless you could have it on the roof of a shed charging batteries all week so that you could go fly on the weekends.

I would search on "off grid solar power" and do some studying.  It looks like Home Depot has stuff, so I suspect that Lowes and other such places do, too.  They'll know all the stuff that I don't about choosing the right charge management controller for the deep-discharge batteries, and how many batteries and panels to get.  They'd also be much more likely to sell you something that'll actually last.

I think that the purpose of having one or some deep cycle batteries is to get around the standing and waiting.  Hopefully the solar panel will keep the storage batteries charged (except during use)and then you have those watts to use for your charger and batteries with inefficiencies included.  The number of deep cells to be determined by use.  In general, you would still charge at home and then recharge or top off at the field.  You have already seen it in action and know that it works, so it comes down to price, and adjusting to number of users.

I don't have experience with the system, but your link shows most of the pieces needed.  A generator is a good alternative as noted.

For your own use, you can charge and top off using old 6 cell packs to run the charger.  Using 3 or 4 packs in parallel of 6s keeps the draw rate low, and the packs may go another hundred cycles of soft use after retired from flying.

My problem with the 'linked to' panel is the poor efficiency of the thing - 25w out of 525 sq ins! A regular PV panel is about 2525 sq ins and produces 275-300w (0.3KwP roughly) so the little one is only producing about a 1/3rd the power if pro rata basis! The full size panels aren't that spendy these days - $150 + a 12v battery to even out the feed to the charger? Yes, its a pain to lug around but it is 'free energy'! You could always buy a Tesla Roadster and just plug in to the ciggy lighter socket LOL!