Generator Efficiency

Electrical Efficiency

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

E = Pout / (Pout + Ploss) 
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Pout = I^2 * Rl and  Ploss = I^2 * Rm thus
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E = I^2 * (Rl / (Rl+Rm))
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let Rl = k * Rm then
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E = k/(1+k)
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for a given desired efficiency keep
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Rl >= Rm * E/(1-E)
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some results:
k = 4 -> 80% 
k = 9 -> 90%

Power Efficiency

For a load that is a multiple of the genetator resistance, a fraction of the maximum output power is obtained. If the load equals the genetator resistance, then the maxium output power is obtained.

Let the power efficiency be:
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Pe = Pout / Pmax
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some math...
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Pe = 4k/(1+k)^2
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Pe of 64% -> k = 4   (80% electrical efficiency)
36% -> k = 9   (90% lectrical efficiency)