# How many horsepower does an alternator take from an engine at different speeds? [closed]

I want to know how much horsepower the engine deliver to the alternator at idle rpm,normal rpm and maximum rpm

## closed as off-topic by laptop2d, brhans, pipe, RoyC, Dwayne ReidNov 16 '18 at 20:02

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The efficiency of the alternator is terrible under most circumstances.

The Alternator itself is only usually about 50-55% efficient. For example a 100A alternator will provide an output of about 1.5kW at 15V, but will dissipate about 750W internally. That's why they have such a big fan on the front of the alternator.

If you then consider the mechanical power required to spin and cool the alternator your overall efficiency drops to about 20-25% including the engine losses.

If you like to deal in HP, then 1.5kW is about 2HP. At an overall efficiency of 20% this means you will need about 10HP from an engine to provide this.

You could read this report from Remy which lays out some of the losses. You will notice that the losses are dominated by the engine and mechanical drive system. From the report this graph may address the alternator losses alone:

The amount of power at idle, normal and maximum is alternator dependent. Most alternators need to spin at about 6000 rpm or more to provide maximum output, and in most vehicles you will see max alternator rpm around 8-10000 rpm.

The efficiency (and power output) of an alternator can be improved markedly by moving to synchronous rectification and raising the output voltage using load dump configurations. For example a 48V 100A output may improve to almost 75% or better.

• Don't forget that the large fan needed is because the ambient temperature under the bonnet is around 75 to 85 Deg C, so the alternator is trying to cool itself with warm air... Also is your max rpm for the alternator or the engine? I do know of some engines designed for cars that get 10k but there are not many... As to why an alternator is not efficient - what do you think the car manufacturer really wants to spend money on? The best alternator in the world or a fancy paint job? which is likely to sell the car... – Solar Mike Nov 15 '18 at 20:01
• So that fancy alternator with synchronous rectification and dump load is going to cost how much? and what about extra weight? Bet they are not used on anything except for the most specialized radio trucks for the military or somesuch... – Solar Mike Nov 15 '18 at 20:10
• I was about to write up a similar answer, but you should also show figure 28. That would answer the OP's question, determine electrical load, divide by efficiency, and convert into HP. It would, of course, be highly dependent on the particular model of alternator. – user71659 Nov 15 '18 at 20:12
• @SolarMike I'd suggest the engine bay temperatures will exceed 100 DegC and in many cases will approach 120 DegC. I was talking alternator rpm. – Jack Creasey Nov 15 '18 at 20:16
• @SolarMike Synchronous rectification is just FETs and an MCU, easily cost justified. The alternator is exactly the same....identical ….any 12V car alternator will easily produce 40-70V output using a load dump configuration. – Jack Creasey Nov 15 '18 at 20:19