Will my car consume more if I plug in my laptop?

I drive a Volvo v40 with a 110hp diesel engine. It usually use 4.6L/100km.

If I plug my laptop which is 85W, will this change the fuel need? If so, by how much?

$$110 \text{ hp} = 82 \text{ kW}$$

This is 1000x what the laptop draws. You won't notice.

To put that change in perspective, you would see a similar increase in the power (85 W) used by the car if your speed changed from 65.00 mph to 65.02 mph, since $P \propto v^3$ (at high speed, the power goes as the velocity cubed) as per this answer, so $\frac{dP}{P} = 3 \frac{dv}{v}$ (the fractional change in the power is three times the fractional change in the speed)

We can check the power consumption another way. You gave your typical fuel consumption, which we can equate to an average power. To do so, we need to know how much energy is released in burning gas, your fuel efficiency, and we'll take a guess at the average speed your car travels at. We obtain:

$$P = \left( 10^5 \text{ BTU/gal} \right) \left( 7.1 \text{ L}/ 100 \text{ km} \right) \left( 30 \text{ mph} \right) \sim 20 \text{ kW}$$ Where I got the gasoline energy equivalent from wikipedia, I used your fuel economy number, and I estimated that on average over all of the time that you're car is on, it's going about 30 mph. This last number is the source of the greatest uncertainty, and maybe wrong by a factor of 2 or so, but the estimate we get for the power consumed by your car is within a factor of 4 of the cited horsepower. It isn't too surprising that it comes in lower, as your car is rarely going at full tilt. But, still, an 85 W laptop constitutes less than a half a percent of this power, which, at high speeds corresponds to about a tenth of a percent change in your speed (65.0 mph to 65.1 mph). You could improve your gas mileage a lot more by just slowing down 5 mph while on the highway (by about 10%).

• To be precise you need to take account of the conversion efficiency of the laptop charger - it gets warm which tells me that it draws more than 85 W from the 12 V of the car. And then there is the inefficiency of the alternator, the battery's charge/discharge cycle (possibly) and the other circuitry as well. But even if all those double the power draw, the impact on the fuel mileage will still be negligible. – Floris Aug 12 '14 at 23:35
• @Floris Are you sure this is not accounted for in the 85W? – Bernhard Aug 13 '14 at 6:05
• @Bernhard - as sure as one can be about these things. – Floris Aug 13 '14 at 6:13
• Good edit - your actual power used is rarely even close to the power the engine is capable of. Just look at the instant fuel consumption when you floor it at a traffic light - you get maybe 2 mpg (sorry, US cars) where you might get 25-30 mpg for cruising. So yes, you are probably running at about 20 hp when coasting along on the highway. Small but significant tweak. – Floris Aug 13 '14 at 17:38

This is really just a footnote to alemi's answer.

The electricity for your car is supplied by the alternator, and the torque required to turn the alternator depends on the current it's supplying. As you draw more current more torque is required to rotate the alternator and the car has to use more power to do it. So yes, plugging in your laptop will increase the fuel consumption, but as alemi says the change will be negligable.

Math already done by others, but my first impression would be yes and no. Yes, because any load requires energy from somewhere; No, because electrical loads less than a major sound system are so insignificant you will never notice outside of a lab. Driving up one decent hill will consume more gasoline than the added load of your laptop over the entire tank, coasting down the other side will ruin the measurements even more.