As a heads up i hear solar power chargers can be very rf noisy.
Field day solar panel.
Thus the output for each solar panel in your array would.
5 hours x 290 watts an example wattage of a premium solar panel 1 450 watts hours or roughly 1 5 kilowatt hours kwh.
That said there is a simple equation to calculate the amount of kilowatt hours kwh your solar panel system will produce.
There are plenty of solar calculators and the brand of solar system you choose probably offers one.
Perhaps a more economical mobile phone could be used.
This is exactly what my club did.
A 10 ah lifepo4 battery was charged in the days prior to the contest using a solar panel a 10 watt panel will take approximately 18 hours of full sunlight to fully.
I see nothing in the rules about you must start field day with a discharged battery so i would plan starting with a fully charged battery having a fully charged battery in reserve and having a solar panel charging station off site to recharge the first depleted battery if you think your power usage would require more than one.
Solar field day 2017.
For the sake of example if you are getting 5 hours of direct sunlight per day in a sunny state like california you can calculate your solar panel output this way.
This is called the nameplate rating and solar panel wattage varies based on the size and efficiency of your panel.
So a fresh deep cycle marine battery with 100w solar panel about 5a charge if the sun is good will run a kx2 at 10w all day with ease.