In general, how much power (HP) to the components on the the engine belt actually use? I'm trying to come up with an estimate of how much horsepower an engine wastes on things like the AC compressor, alternator, water pump, pullies, supercharger, etc.

Edit: I realize it's impossible to say specifically since all vehicles are different and put different components on their engines drive belt. Prius and Highlander Hybrid have electric AC, for example, removing the compressor from the drive altogether.

What about some general concept: So a typical small-to-mid-size car with:

- Tensioner
- AC
- Alternator
- Water pump
- Power Steering pump

HP required for each? In a medium scenario? In a large scenario?

Answer thanks to @Scott and more research done:

  • Small cars (e.g. Corolla, Civic, Focus): 15 HP - 25 HP. (typically have ~80 amp alternators, smaller water pumps, many with elec steering)
  • Medium cars (e.g. Camry V6, Fusion V6): 25 HP - 40 HP.
  • Larger cars (e.g. Traverse, Explorer, Mustang V8): 30 HP - 50 HP.
  • 7
    "Wastes" is an interesting term for powering those components. :)
    – Ricket
    Jul 28, 2011 at 16:19
  • 1
    It's going to be a fairly large range. Will depend on the size/capacity of the components, not just the quality. Also, it most likely increases with RPM, so you'll have to decide if you're looking for idle, cruise, or worst case numbers. Jul 28, 2011 at 16:28
  • Agreed that an actual number is impossible to give for an answer, will be different for each vehicle, not even model, but each individual vehicle could have fluctuations. Perhaps modify the question to ask about ways to lessen the power drain of the components (aside from removal)?
    – ManiacZX
    Jul 28, 2011 at 16:58
  • Thanks for the feedback. What about a small / medium / large estimates? See question, updated.
    – Joshua
    Jul 28, 2011 at 18:16

2 Answers 2


Firstly, the comments made by others are correct. The power used by each of the components listed will vary on a component by component basis and even on an installation by installation basis. The power used by each component will also vary depending on the speed that it is running at. Also the number and type of components will vary from car to car.
With that out of the way, here are some numbers that could be used for rough approximations.


The power used by the tensioner will be minimal and probably negligible. It will depend on how good the bearings are, but if it was using much energy at all, all that energy would be being converted to heat (or possibly noise). So tensioner can be pretty much ignored I think.


According to Wikipedia's Airconditioner Article

In an automobile, the A/C system will use around 5 horsepower (4 kW)

It is marked as "citation needed' though so take it with a grain of salt, but it would be approximately correct for when the aircon is activated. Remember that aircon is electronically clutched so that it's not always on and therefore not always using that much power.


According to Zena Incorporated

... a 150 Amp alternator, operating at full output, the expected diesel engine load would be about 7-8 hp

Like all the others this number should only be used as an approximation and note that a 150 Amp Alternator is a pretty large alternator.
Also note that this figure is when 'operating at full output' it's pretty rare that an alternator is ever running at full output.

Water Pump

According to Davies, Craig FAQ Question No 19

Davies, Craig performed a number of tests which concluded that a normal water pump uses up to 10kW of power to operate at its high speeds.

Be aware that Davies, Craig sell replacement water pumps that require less power to run so assume the numbers are only approximate.
It's also worth reading through Question 2 on the same page where they describe how the power pulled by a mechanical pump that runs from the drive belt will increase as the cube of the operating speed. So if power draw at 1000rpm is 0.1kW at 2000RPM it will be 0.8kW and at 4000rpm it will be 6.4kW.

Power Steering

According to an EE Times article by Dave Wilson

...replacing your three- to five-horsepower steering pump and its associated bulky hydraulics with an electric motor is one enhancement that actually improves performance

Be aware that he's selling the virtues of replacing your power steering pump with an electric one, so assume the numbers are only approximate.

So to summarise

  1. Tensioner: ~0 kW
  2. AC: 4kW (5hp)
  3. Alternator: 5-6kW (7-8hp)
  4. Water Pump: 10kW (13hp)
  5. Power Steering: 2-4kW (3-5hp)

Bear in mind that all these values are approximate to start with and they are for max draw, an AC will draw close to nothing when it's not active, as will an alternator. Traditional water pumps will draw more power the faster they are spinning (so the higher the engine is revving). I'm not entirely sure about Power Steering pumps but I believe that they would only draw max power when being fully utilised.

The references I've used here are by no means 'definitive' so if anyone finds any references more appropriate then by all means edit this answer to improve it.

  • Thanks. Added some updates to my question based on your answer.
    – Joshua
    Jul 29, 2011 at 14:22
  • 1
    5-8kW for a 150A alternator? 150A @ 14V = 2100W electrical power. Does that mean the alternator is only about 30% efficient, the remaining 70% or so of the mechanical input going to heat?
    – Anthony X
    Jun 29, 2016 at 3:16
  • How accurate is the above answer? There seems to be a lot of discrepancies.
    – Narcotixs
    Feb 14, 2017 at 3:07
  • Had to downvote due to claiming alternator to be only 30% efficient at full load and due to having only full load figures. The part-load figures would be way smaller, and most of the time the components are operating at part load.
    – juhist
    Apr 28, 2018 at 10:47
  • @juhist Why the hate? Let be clear about a few things, I never stated anywhere that an alternator is only 30% efficient, what I did was cite an article where they give those sort of numbers. What I didn’t do was provide a set numbers with no source for where those numbers were obtained. Furthermore, I clearly stated that the “number should only be used as an approximation” and I then go on to say that “all these values are approximate to start with and they are for max draw, an AC will draw close to nothing when it’s not active, as will an alternator”, couldn’t be much clearer.
    – Scott
    May 11, 2018 at 0:40

10kW for water pump?!! That sounds way too high! Water pump draws maybe 100 watts, also more advanced cars uses electronic thermostat that bypasses water so hydraulic resistace is even less.

Also power steering pump draws less than 1kW at peak while you steer when car is stationary. When you are moving there are minimum resistance in hydraulic fluid since valve is open. Same is with AC when disengaged or alternator is at partial load. Normal electrical load at today car is round 300W when using daytime LED lights and efficient engine and other electronics. Alternator has cos fi of round 0.85 so it takes 15% to generate magnetic field in a rotor.

I don't know about AC but 4kW sound way to much. If this is right than car AC piston compressor is very inefficient.

You have forgot that car has also an oil pump, air compressor for power brakes (only diesel), high pressure fuel pump (driven by a belt or sprocket), and that most of the power (10% of engine nominal power) is lost in the exhaust system due to flow resistance (silencers and catalist converters). Also don't forget that rubber belt is very inefficient method of transporting power and that it doesn't matter how good ball bearings you use, resistance is between wheel and rubber belt.

So if all the consumers together takes 10kW engine would lose 15kW due to resistance in a belt. If you don't believe touch the belt when you'll return from the drive and see how warm it is.

  • AC does draw a LOT, and I wouldn't be surprised if it typically draws 10kW in some cars. I have noticed it will drop MPG's by as much as 5-8%, so it's definitely sucking power.
    – Joshua
    Oct 16, 2015 at 6:19

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .