If it what you want is truly a measure of efficiency, then it would be
unit-less. (It's just a ratio)

For example:

The energy of one 680 nm photon is:
E = hc/?
10-19 J

Photosynthesis requires 59 kJ per mol of ATP. Meaning that:
EATP = 59 10 3 J mol-1 / 6.022 1023 mol-1 = 9.80 10-20 J per ATP
molecule.

So, the energy efficiency of photosynthesis is:
= 9.80 10 -20 J /2.92 10-19 J = 0.34 or 34%