Generator Efficiency

Electrial Efficiency

Efficiency as a fraction input mechanical power that is available at the output terminals:

E = Pout / (Pout + Ploss) 
Pout = I^2 * Rl
Ploss = I^2 * Rm
thus
E = I^2 * (Rl / (Rl+Rm))

let Rl = k * Rm then
E = k/(1+k)

for a given desired efficiency keep
Rl >= Rm * E/(1-E)

k = 4 -> 80% 
k = 9 -> 90%