I'm wondering about alternators used in automobiles and the mechanical load that they present to the engine. My question is: what is the relationship between the mechanical load presented by the alternator and the electrical load presented by the car's electrical system? Similarly, if I were able to reduce car's need for electrical power or provide a supplemental means for energy harvesting, what potential increase in performance or fuel economy might I see?

  • 1
    \$\begingroup\$ 1 horsepower = 746 watts. 50 amp alternator @ 12 volts = 600 watts. Enough said? \$\endgroup\$
    – Dave Tweed
    Jul 17, 2014 at 14:47
  • \$\begingroup\$ Surely there is some loss in the conversion. \$\endgroup\$
    – kjgregory
    Jul 17, 2014 at 14:53
  • 2
    \$\begingroup\$ Worth pointing out that the alternator will only be consuming the power required by the electrical load presented to it, so a 50A alternator will only put out 50A if you demand 50A of the car's electrical system. \$\endgroup\$
    – John U
    Jul 17, 2014 at 15:57
  • \$\begingroup\$ cdn.base.parameter1.com/files/base/acbm/ooh/document/2015/03/… For me, this the best "article" about the subject that I discovered so far \$\endgroup\$ Jun 29, 2021 at 16:31

2 Answers 2


An alternators load on the engine will be however many watts of power it is producing, plus any inefficiency of the alternator itself.

This question: https://mechanics.stackexchange.com/questions/1536/how-much-power-hp-do-the-components-on-the-engine-belt-generally-use has an estimate of the power consumption of a 150 amp alternator at full load, being 7-8 horsepower. A 150 amp alternator is pretty big, and it's pretty rare that the alternator will be running at full blast.

You won't see very significant changes in fuel economy replacing the alternator, it generally doesn't draw enough power to be worthwhile to replace. Once your battery is recharged after starting up, the majority of your alternators power is probably going to be powering your car stereo or charging your phone, as the ignition and engine control systems only require a few handful of watts.

One thing that is interesting though, is some automobile manufacturers are considering eliminating the serpentine belt from their vehicles, and powering all auxiliary systems (AC, power steering, cooling pump, etc) by electric motors powered by the alternator. Apparently this can yield improved performance, or dramatically increase maintainability.

edit: With all that said, these guys Remy International, a GM subsidiary, claim the opposite of my conclusion.

  • \$\begingroup\$ Interesting. I've noticed with my BMW that some of AT models include a belt-driven radiator fan while my MT includes an electrical fan. It seems to be a common aftermarket modification among those with the belt-driven fan to replace it with the electrical one. \$\endgroup\$
    – kjgregory
    Jul 17, 2014 at 15:01
  • \$\begingroup\$ I suspect the electrically-driven ancillaries also aid "packaging", AKA how much stuff they can cram under the bonnet. Being able to divorce big lumps from the belt and put them anywhere a cable can go does make life easier. \$\endgroup\$
    – John U
    Jul 17, 2014 at 16:02
  • \$\begingroup\$ @KGregory There's electric steering in my car as well as an electric water pump and electric-driven hydraulics for the top. Air conditioning compressor, vacuum pump (brakes) and alternator are all that get belt driven. John is right about the cramming- there was a day when you could easily see the ground from above the engine on both sides- now everything is packed in like sardines. \$\endgroup\$ Jul 17, 2014 at 16:50
  • \$\begingroup\$ That Remy document is exactly what I was looking for. It seems to me that even if you were to reduce power consumption or generate supplementary power, the voltage regulator in the alternator would gobble up the surplus that the alternator generates (see figure 28). \$\endgroup\$
    – kjgregory
    Jul 17, 2014 at 17:49
  • \$\begingroup\$ The regulator is a small circuit that controls the field current. It does not "gobble up" any power. \$\endgroup\$
    – user28910
    Jul 17, 2014 at 18:51

A further refinement:

  • The sperpentine belt it self has inefficiencies (98%)
  • there is windage drag from the alternator spinning even when not loaded
  • 1 Hp = 748 Watts
  • 150 A alternator putting out 13.8 V = 2.77 Hp at 100% efficiency.

From the website Inteckopen.com you can see that 50% is typcial.

enter image description here

so 6 - 8 Hp is not surprising.

  • \$\begingroup\$ Im suprised. I thought car alternators have much better efficiency. I was thinking about using car alternator in amateur wind turbine. Looks like thats bad idea. \$\endgroup\$
    – Kamil
    Jul 17, 2014 at 19:31
  • \$\begingroup\$ That is just one alternator. The take away lesson is to pick your alternator wisely. \$\endgroup\$ Jul 17, 2014 at 20:27
  • \$\begingroup\$ Can you add better image source link? I can't find article, link points directly to image source. \$\endgroup\$
    – Kamil
    Jul 17, 2014 at 20:33
  • \$\begingroup\$ What's not shown here, which is really the heart of my questions, is what is the efficiency as a function of electrical load. I think that answer is buried in the Remy International report that was linked in the other answer. \$\endgroup\$
    – kjgregory
    Jul 18, 2014 at 3:02
  • \$\begingroup\$ @KGregory Sure it is, but you need to know: the curve for your car, your electric load and since the efficiency changes with RPM, what RPM you will be running at. Or the cumulative efficiency can be calculated by tracking the RPM. \$\endgroup\$ Jul 18, 2014 at 3:22

Not the answer you're looking for? Browse other questions tagged or ask your own question.