[Al Skierkiewicz]

Avianna,
Right now typical solar panels are in the range of 12 watts per square foot. (give or take a couple of watts.) Say you could find (and afford) a 15 watt/sq.ft. panel, you would need 20 sq. ft. of cells to generate 300 watts. The panels are available in several different output voltages which you can then wire in series/parallel to get the right output voltage. What you are looking for (at 28 volts) is a panel that can generate close to 11 amps total.
If you do not have a height specification, it is possible to mount two panels, one behind the other in the 24x48 space depending of course on what latitude you are designing this to be used at. The front one will shadow the rear at higher latitudes.