How much fuel am i consuming when using my 12v in car socket continuously at 80W?

Question

I hope the question makes sense, but since i don't understand the sciences involved it may not. I'll elaborate here.

I am a courier, i run my small van daily for around 10 hours per day. I have 2 12v sockets, i'd like to plug an alternator into 1 (a 150W alternator), and continuously run a 25L coolbox from it. The energy consumption of the coolbox is given as 80W. I already have a 12V splitter which i run from the other 12V socket and run my mp3 player / satnav / bluetooth / charge phone etc.

Not sure if it's relevant but i currently get around 550 miles to a tank (costing me £75), roughly equating to 45mpg.

My question is this - If i run this 80W coolbox continuously will my fuel consumption be dramatically increased or will the differences be negligible, rough figures would be good, thanks :)

Answer 1

I really doubt that you will see any difference. Although when your alternator is pulling a heavy electrical load it does slow down the RPM's when at idle. I would think would might only see about 5 miles less on a tank of gas.

Answer 2

It should not affect your fuel consumption at all. The electrical system runs off the battery and alternator and these systems always run, not just under a load (like the A/C does).

Adding increased electrical usage should not affect the fuel consumption, but be careful you are not overloading the electrical usage. Always put in a relay or breaker, just in case. Overloaded wires cause vehicle fires every day.

Answer 3

You do use some fuel, not much. How much is possible to calculate.

80 watts continuously for 10 hours is 800 watt hours.

The energy content of gasoline is about 40 MJ/kg or

1.3 × 108 J/gallon = 130 MJ/gallon

Gasoline engine is about 20% efficient in converting that to electricity, at a guess, so 1 gallon gets you 25 MJ of energy.

800 watt-hours is:

800 watt-hours x 3600 sec/watt = 2.880000e+6 watt-seconds or

2.9 MJoules

so:

2.9 MJ x 1gallon/25MJ = 0.11 gallons of gas, or 0.44 Liters.

So my rough calcs, if I didn't make an arithmetic mistake, says you use about half a liter of gas a day to run your appliances, or £0.5. Actually, I think this number is conservative, and may be higher.

But you say you run the frig continuously. But does that mean it runs continuously? or does the thermostat cycle it off and on, so it really runs a fraction of the time? That would reduce your costs.