Experiment: The cost of Headlights

[brucey]

I know it's been said to keep the headlights off for MPG, and I'm sure it does make a difference. But I had never seen the difference quantified. I had a few extra minutes today so I did a test with my car, using the scangauge to get numbers.

Temperature was in the mid 70's (I'm in florida, clear day) at 2:15 PM on 11-15-10. (in case anyone wants to look up the weather) I did 2 runs in each direction on a long flat stretch. Roughly 1.1 mile apart from each marker. Cruise set at 55 mph and given plenty of time to stabilize the speed before markers and then resumed for each run. You know, standard testing here.

Test 1: No headlights.

A: 35.0
B: 36.2
A: 35.7
B: 35.1
Average: 35.5, Testing Range is 1.2


Test 2: Headlights on. Low Beams only
A: 34.8
B: 34.7
A: 34.4
B: 34.6
Average: 34.6, Testing Range 0.4

Test 3: Headlights Back off
A: 36.4
B: 35.5
A: 35.2
B: 35.1
Average: 35.6, Testing Range 1.3

So without Headlights, I'm seeing 35.55 MPG Average.
With the headlights I'm seeing 34.6 MPG Average.

So 0.95 MPG, or around 2.5%. I'm sure with the high beams on the difference would be larger.



Another quantified reason why winter mileage seems like an uphill battle.

[euromodder]

Thanks for taking the time and effort to test this, but the variation in the results is at least equal or even bigger than the observed difference between the 2 conditions.

These small improvements simply can not be tested and quantified on the open road with a few drives up and down.

[jiggersplat]

what is the theory behind reducing electrical load to increase mileage? it's not like your alternator is on a clutch and only kicks in when there is more electrical load than your battery can supply. it runs at the same speed all the time (or at least the same speed in proportion to engine rpm).

[Daox]

Increasing the amount of electric power you use increase the load that the alternator puts on the engine.

[NeilBlanchard]

The battery only supplies power when the engine isn't running. While the engine is running, the battery is actually a load, and it is the alternator that powers everything. If you increase the electrical load, then the alternator has to work harder, and so it saps more torque from the engine.

The alternator doesn't put out "extra" power either -- if only all the hydrogen generator proponents knew this...

[basjoos]

The higher your mileage, the greater the mpg hit from the headlights or any other electrical load. On the aerocivic, using the headlights knocks my mileage down about 2-3 mpg, but the mileage hit is almost unnoticable against the background noise on my F150.

[PaleMelanesian]

From the theoretical side, with some VERY quick-and-dirty math:

My car uses
110w = 2x 55w headlight
75w = 3x 25w taillight, including high-mounted
185w extra load

Alternator: 60% efficient (Alternator - Wikipedia, the free encyclopedia)

Lights on = 308w extra load on the engine
308w/ 745 watts in hp = 0.41 hp extra load

Using the calculator, my car needs ~10hp to roll at 55 mph.

0.41 hp is a 4.1% loss of efficiency at that speed.
11% loss at 35 mph
2% loss at 70 mph

[RobertSmalls]

Surprisingly, high beams are typically 60W, versus 55W for low beams. It's not so much that there's more light, it's mostly that it's cast further down the road. Also into oncoming drivers' eyes.

Good work, Brucey (and Pale).

Btw, what speed did you set the cruise to?

[Weather Spotter]

Thanks for the data!

I added it to the electrical loads page in our wiki:
Electrical Loads - EcoModder

[Frank Lee]

I would tend to give more weight to the derived value than the road tested one because as noted road testing is so loaded with variables. The general agreement with the results is good though!