From the information I've gathered, it seems that the general consensus is to dose excel right before light goes on for its maximum effectiveness, which doesn't seem to make much sense to me.
I know that photosynthesis is comprised of light reaction and dark reaction, and the dark reaction can go on without the light reaction, assuming that there are enough energy saved during the light reaction in the form of NADPH and ATP.
So basically, the light reaction goes:
Light + H2O + ADP + NADP -> O2 + ATP + NADPH
...which ATP and NADPH are the energy molecules produced.
Then, the energy generated in the form of ATP and NADPH from the light reaction are used during the dark reaction for plant growth:
CO2 (or other carbon source, such as excel) + ATP + NADPH -> glucose + ADP + NADP
...where energy stored in ATP and NADPH are used up to form glucose, and you get ADP and NADP again.
So, in all reality, the light reaction begins when the light turns on, but it will take a while for the plants to produce enough ATP and NADPH.
The ATP and NADPH, in turn, are used to produce glucose in the dark reaction along with CO2 (or alternative form of carbon source).
Based on the information above, wouldn't it make more sense to dose excel once the light has turned on for a while? (Say, 1 hour or so) By then, the light reaction would have produced enough ATP and NADPH, which will prime the dark reaction. Then, as soon as you add excel it would be immediately used up, rendering it more effective.
Just my 2 cents!