2% of 280 Whrs = 5.6 Whrs...
Via UHF - data costs about 0.8 Whr / Mbit - so you could get perhaps 7 Mbits of data home via UHF
But of course, with onlly 5.6 Whrs - you've not got enough to warm up some instruments to take data to fill that 7 Mbits
However - to put this into some sort of context..
Shadowing causes losses of up to around 20% per sol