engine power consumed

[sfnz]

ok this is probably in the realms of WTF but i'll ask anyways, would a 30 amp altenator producing say 10 amps consume anymore or less crankshaft power than say an 80 amp altenator producing 10 amps, engine is a 2QM20 yanmar in an H28

[leftbrainstuff]

The simple answer is no. You can use the simple formula P = V*I for dc loads. Strictly speaking the alternator is producing rectified ac but for load calcs that's not a first order effect.

So P = 10 * 12 = 120 Watts

Nor including friction, bearings and belts or electrical inefficiencies. Just add 20% as a rough approximation.

So say 120 + 24 = 144 Watts

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[Reefmagnet]

Originally Posted by leftbrainstuff

The simple answer is no. You can use the simple formula P = V*I for dc loads. Strictly speaking the alternator is producing rectified ac but for load calcs that's not a first order effect.

So P = 10 * 12 = 120 Watts

Nor including friction, bearings and belts or electrical inefficiencies. Just add 20% as a rough approximation.

So say 120 + 24 = 144 Watts

Sent from my SM-N900T using Cruisers Sailing Forum mobile app

That's the basics.

Alternators are around 60% efficient and will be generating at around 14 volts with some rectification and regulation losses as well.

This puts the output as 10 * 14 / 0.6 = approx 230 watts or around 1/3 hp.

In theory, the larger alternator will require more power if for no other reason than the fact it will have more rotating mass so differences would be marginal. Of course, different regulator and alternator designs may have differing efficiency ratings which makes it only applicable to same design comparison.