What is the best method for measuring efficiency?

Eff = Output / Input. How is this accomplished. I would
think some calculation must come into play. If we know
our power in, how can we measure our power out?

Pout =  .5 * Cs * (Vout^2) * BPS.
Pin = .5 * Cp * (Vcap^2) * BPS.

We have equations, but this all appears a bit difficult
to measure. Pout / Pin is meaningless without a true
measure of Vout. I can't see the arc length as a good
measure of power out either since arc characteristics
are far more than simply the length. What about the
diameter and intensity?

I've seen posts on efficiency's hitting 80% on good
designs, but how do you guys measure efficiency?