Yeah, the lack of standardization - or at least the lack of adherence to said standards - is apparently a big issue in solar cells and panels.
A while back, I picked up some "5 Watt" panels from Harbor Freight, for what amounted to 50% off (big coupon, on top of an existing discount). One of those sitting inside the workroom window (with double panes, a window screen, and some minor filtering) will float the gel cells on the test beacon, apparently indefinitely. I had it running for around 3 weeks solid into a dummy load, at a slightly higher duty cycle, and they stayed right up there near the full-charge voltage the entire time. In an open-air mounting situation, one should be able to float a beacon plus a PIC- or AVR-based keyer without any problems, probably even with a few bells and whistles added on.