hmm your close.. try this..
I measure 12.8V, take 65% eff and my amp have two 30 amp fuse:
30A * 2 = 60A
60A + 1/3 = 80A (current point to blast fuse)(sinwave input-----squarewave output)
Now we need to calculate the RMS output value ...we will use 60A value
60A * 12.8V = 768VA
768VA * 0.65 = 499 ...say 500Watt RMS output
it is a simple maths formula which is average power = work done (unit of work is the the Joule) divided by time taken (unit of time is the second)
the unit of power = Joule/second (J/s)
Si unit of power is watt (w) = 1 J/s
so what we are looking for her is work done = force x distance to give us the Joule.
then we can work out how many watts it putts out which is power = work done/ second will give you how many watts anything is putting out
Hmm. English? Lol my brain just pooped out.