Calculating photosynthetic efficiency
We can actually use either depending on the specified convention, but a more convenient way to characterise the photosynthetic efficiency is:-  


Considering the total incident solar radiation instead of that part which is absorbed (so that the calculated efficiency takes into account the total available solar energy on earth)
Using Net Primary Productivity instead of Gross PP.(so that the efficiency is characterised by the useful output (product) of photosynthesis by removing the non-useful respiratory losses of plant.)   


I should also mention that using GPP instead of NPP was the practice followed in a large number of texts I read. But I think using NPP is not only the modern view, but also scientifically more convenient.

Here is the wikipedia page on photosynthetic efficiency and here is the book which houses the technicalities of these definitions along with several values of the various efficiencies.