The question is how much power output is needed? Is 25 watts output enough?. Not sure how long this will extend the operating time at 50 watts RF output. Since it is just a math game - 50 watts out uses 7 amps. Duty cycle with CW is 33%. So 7/3. Receive current drain is 300 mA or 0.3 amp - so let's sy 3 amp continuous draw. Discharge to 50% energy from 24-amp hour battery. The math says the operating time with 1.5 amp maximum input is 14h. Let's just say with real life conditions - input is 1 amp or 66% rated max. So we go from 4 hours at 50 watts to 6 hours with just a 25 watt panel. This folding panel weighs 1.75 lbs and easily folds into a backpack. Dimensions are the size of a piece of paper 8.5 x 11 in folded form. It is also available from Amazon at $135. Is this a worthwhile investment for a 2 hour extension of operating time? Does it actually work in practice?
So to answer the above questions I have done a little research:
Type of Panel: Powerfilm or Crystalline?
This is a very important question to answer. Crystalline (either poly or mono crystalline panels) are highly affected by shading such that even partial shade in a small part of the panel drops output by closel to 90%. There is a video of this at the following link.
Powerfilm vs. Crystalline Panel
Goal Zero Versus PowerFilm
So in reality with a powerfilm panel - effectively 70% of the panel rating is retained and the requirement is for a much larger crystalline panel to keep up under real world conditions.
Run Time (or run time extension via a solar panel)
Cost as measured in dollars per hour of run time
Weight - as in lbs per hour of run time
QSO per session (before power runs out)
Assumptions: Drawdown to 50% battery capacity, 33% duty cycle for CW, 100 QSO's per hour
The above are my self imposed parameters to optimize on because I have to buy what I carry, and carry what I buy.
The baseline is a 24 amp-hour Lithium Iron Phosphate battery which weighs 2.6 lbs and which cost $147 dollars which allows me to run for 4 hours at 50 watts on the beach. 50 watts is a good power level because my ERP with a vertical near the ocean is 200 watts according to EZNEC models. So my baseline is $36.75 per run hour and 1.54 hours per pound. The baseline set-up allows me 400 QSO's assuming a 100 QSO/hr run rate.
Let's put a Powerfilm solar panel in the mix. First a 20 watt panel ($214). The 20 watt panel reduces my current draw by 1.33 amps so I can effectively operate for 6 hours. This translates to $63 per run hour and 1.82 hours per pound, and 600 QSO's.
Now a 30 watt panel. (Price $283-$350) The 30 watt panel enables a current draw of 1 amp or an operating time of 12 hours. That is - $63 cents per run hour and 1.89 hours per pound and 720 QSO's.
The crystalline panels are going to be much heavier in general and about the same price since you have to buy more wattage to get the same output. Air St. Pierre charges 3.3 Euros per kilo of extra baggage. So a 30 pound panel will cost me $100 extra just to transport plus I have to carry it on my back - a non starter. DXpeditions to Sable Island, for example, carry a weight limit of 1400 lbs for people and cargo for a one way trip costing $6000.
The above are best case scenarios assuming there is 100% sun.
Let's examine the case of an extra battery. It is identical to the first (baseline) case except, now you have 800 QSO's which can be had with or without the sun. So the overall winner is: Extra Battery
Best Solar Option
Either 30 watt or 20 watt Powerfilm are equivalent on a $/hr or hrs/lb basis. The edge goes to to 30 watt Powerfilm on the basis of 120 more QSO's than 20 watt panel which is, after all, what you are paying for. The extra battery gets an extra 400 QSO's over the baseline which can be had with or without the sun. But If I only had $350 or so to spare, I would get the extra battery and the 20 watt panel. That combination gets you 1000 QSO's using the above mentioned assumptions. For an extra $100 get 1120 QSOs with the extra battery and a 30 watt panel.