How does a solar cell its relative thermal coefficient "alpha" expressed in 1/K relate to %/K?
You need to know the actual value being converted to convert between systems.
That is the 1/K and %/K values do NOT have a constant ratio - it depends on the actual value under consideration.   eg 


1A change in 100A = 1%.  
1A change in 40A = 2.5%  
1A change in 10A = 10%




For variable "N" where N may be watts or amps or volts:

N/K means absolute change in N per degree K (or per degree C) 

%N/K means percentage change in N per degree K change.

The conversion depends on the current (or voltage or power)
For eg I_25C = current at 25 degrees C.


%N/K value  = (100 / I_25C) x N/K value
N/K value  = %N/K value  x (I_25C / 100)


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So eg "40A - 0.2A/K" means current would decrease by 0.2A for every degree K (or C) that temperature rose. So eg 50K rise = 50 x 0.2 = 10A so current would decrease from 40A to 30A.

"40A - 0.5%/K" would be the same as the above as 40 x 0.5/100  = 0.2A.

The same applies to figures for Voltage or Power where applicable.