# Does a car use more petrol when the Lights are on ?



## LouisCribben (8 Jun 2009)

I know car lights use energy, and the all the energy generated in a car comes originally from the petrol. 

However, does having the lights on increase petrol consumption ?
Would the energy used to light the bulbs be otherwise be lost as heat or some other form of energy ?


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## chrisboy (8 Jun 2009)

The alternator keeps the 12v battery charged, the battery supplies the lights.. The alernator will keep the battery charged even when the lights are off, so the simple answer is no, having the lights on does NOT use more petrol.


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## mathepac (8 Jun 2009)

chrisboy said:


> ... so the simple answer is no, having the lights on does use more petrol.


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## chrisboy (8 Jun 2009)

mathepac said:


>


 
Ha ha, left out a word!!


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## jhegarty (8 Jun 2009)

chrisboy said:


> The alternator keeps the 12v battery charged, the battery supplies the lights.. The alernator will keep the battery charged even when the lights are off, so the simple answer is no, having the lights on does NOT use more petrol.




Lights that are turned off do not use power, so in theory it will use the petrol.

That been said I would imagine you would light them for quite a while on a cent of petrol.


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## kfk (8 Jun 2009)

The alternator is constantly supplying power to the battery, lights on or off makes no difference to the power coming from the alternator so there should not be any extra petrol used.


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## LouisCribben (9 Jun 2009)

I just googled it today......I also googled it yesterday but I missed this link
http://www.telegraph.co.uk/news/uknews/1578194/Daytime-car-lights-to-be-mandatory.html

Apparently, having the lights on does increase petrol consumption !

It gives a figure of 3% extra for dipped lights---- I'd guess it's probably double that for full beam. 

Must be that the alternator works a bit harder depending on the load put on the battery to power the lights/radio etc. That's my best guess ! Or maybe someone has a better explanation ?


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## mathepac (9 Jun 2009)

LouisCribben said:


> I...
> It gives a figure of 3% extra for dipped lights---- I'd guess it's probably double that for full beam...


Read it again - its gives two figures, one of them "0.3 per cent because they use separate bulbs that are less bright than headlights." for those LED daytime running lights a la Audi / Porche / Halfords accessory counter.

Have you got the lights on?


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## Hoagy (9 Jun 2009)

Surprisingly, from Wikipedia:

_"Headlights, battery charging, active suspension, circulating fans, defrosters, media systems, speakers, and other electronics can also significantly increase fuel consumption, as the energy to power these devices causes increased load on the alternator. Since alternators are commonly only 40-60% efficient, the added load from electronics on the engine can be as high as 3 horsepower (2.2 kW) at any speed including idle. *In the FTP 75 cycle test, a 200 watt load on the alternator reduces fuel efficiency by 1.7 mpg. Headlights, for example, consume 110 watts on low* *and up to 240 watts on high*. These electrical loads can cause much of the discrepancy between real world and EPA tests which only include the electrical loads required to run the engine and basic climate control."_


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## z109 (9 Jun 2009)

For anyone who has travelled around in India, the old Ambassador taxis had a very inefficient alternator that degraded over time. The result was that if you were in a rainstorm at night, you could only use the lights and the wipers by flicking them on and off, so mostly you drove blind on flooded roads! Scary? Only the first few times. After that, a sense of fatalism covered your every move...


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## LouisCribben (9 Jun 2009)

mathepac said:


> Read it again - its gives two figures, one of them "0.3 per cent because they use separate bulbs that are less bright than headlights." for those LED daytime running lights a la Audi / Porche / Halfords accessory counter.
> 
> Have you got the lights on?


 

Thanks Mathepac

On this occasion, for a change, I DO have my lights on ! because the article says 

"The move will anger motoring groups, which claim fuel consumption and carbon dioxide emissions increase by three per cent when drivers use dipped lights."

But you are right too, the 0.3% is the amount the proposed EU regulation daytime lights will consume (less than dipped lights, different bulbs)


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## jhegarty (9 Jun 2009)

3% is ten times .3%


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