I’d like know how much I’m reducing my fuel efficiency by using my seat heaters? I don’t notice a huge difference, if any at all, but a friend of mine believes otherwise. I drive a Volvo, he drives a VW. I’d love to learn others think about this.
Not enough measure – same as having your heater blower motor on. The only accessory which may significantly affect fuel mileage is AC as it consumes about 5 HP.
Twotone
Zero to very minimal effect. The heaters in the seats draw a few amps of current which comes from the battery. To keep the battery charged the alternator has to put a bit more current into the battery which may cause a tinsy bit more resistance to turn the alternator.
So many other variables have much more impact on mpg that you will never notice a difference.
It’s a matter of Watts…How many Watts do the seat heaters consume?? 50? 100? 150?? How warm do your buns get?? The bigger the fire, the more fuel it burns…But like the first two posters have said, it’s not something that threatens the well being of the planet…
It’s very minor. Enjoy the warm seat and let your friend suffer.
Thank you - I can’t to show him the responses.
It might also depend on the engine output as well. A low power 4cyl will work harder to make those extra amps than a v6 will.
But, the effect will probably amount to a teaspoon more fuel over the course of your tank regardless.
To save fuel, maybe you should leave the radio off too! Or the lights!
I can get a fairly accurate blip at about 3% I have never seen it from the seat heater. I only have only looked at it when I only had one on.
The rear window defroster takes a lot more power than the seat heater. Your friend is paranoid, since he has no idea how much power is actually cosumed by this device.
Even French cars (very energy concious) such as the Peugeot, have electric seat warmers.
Or the ENGINE! 
Probably seat heaters are less significant than daytime running lights, which are on winter and summer.
100% of all the energy used by your car comes from the burning fuel. Some directly, some indirectly via the electrical system.
However, to simply illustrate how small the impact of the seat heater is, let’s say the seat heats up 20 degrees over a 10"x10" area. The amount of gas it takes to provide that amount of energy is too small to be measured. 1 tiny spray of an injector when burned can provide far more energy than that. Even if you add up all the losses inherant in converting the burning gas to the piston’s motion and that to electricity, the total amount of energy needed to heat the seats is almost immeasurable. I could probably warm my fanny up with the energy in a few burning matches.
I’ll leave it to the younger posters here to do the math. How many BTUs in a seat heater? How much gas would it take to provide that many BTUs? You may ignore the losses.
True but miniscule.
Anything electrical needs power from the alternator.
You can hear an alternator draw down under stress with heavy loads. Hood up, listen as someone turns on eveything elecrtical at once. ( radio, wipers, read defog, htr fan on high, headlights, work the seats, windows etc. )
Any thing that produces heat electrically takes more power.
A more instantly obvious load is these big bumping stereos.
To compensate the engine rmps raise to keep the alternator producing. This is a slight but true increase if fuel consumption, similar to when the a/c compressor kicks in.
( At home watch the bathroom lights as you turn the hair dyer on high, turn on the garage electric heater, or even the food dispose-all. They dim for an instant until compensated for. Go watch the wheel on your electric meter speed up to see the compensation. )
It’s not enough to measure or worry about. If your buddy thinks it is wasting energy it is safe to say there are other things in life that consume more but we do not notice or reason away.
In the midwest here it is about 10 degrees, if I had them I would use them and not feel guilty about it. Be glad you have them, right now my heater does not work. ):
Seat heaters are about 50 watts each.
100 per pair.
3.4 BTU per watt-hour =340 BTU/h
assume 30% efficiency overall in conversion of chemical to mechanic to electrical energy.
340/.30=1,133 BTU/h required in gasoline
1 gal gasoline = 125,000 BTU
So, 0.0091 gallons per hour are required.
At 60mph, assuming 25MPG without heated seats…
1hr=60 miles=2.4 gallons
add .0091 gallons for the heated seats… =2.4091gallons
60miles/2.4091gallons=24.9 MPG, a reduction of 0.37%
At 30MPH and 25MPG,
1hr=30mi=1.2 gal+.0091gal ->1.2091gal 30/1.2091=23.2MPG, a reduction of 0.75%
At 60 MPH and 40 MPG,
1hr=60mi=1.5 gal+.0091gal ->1.5091gal 60/1.5091=39.8MPG, a reduction of .60%
At 30 MPH and 40 MPG,
1hr=30mi=.75 gal+.0091gal ->.7591gal 30/.7591=39.5MPG, a reduction of 1.2%
EDIT: Fixed extra zero. Worst case is a percent or two… much less than the variability of the volumetric energy content of gasoline from source to source.
-Thanks circuitsmith
And BTW, the reason I chose 30% eff overall is because the belt drag, bearing drag, and other losses asscociated with the system are already there when the seats are not on. I was just considering the efficiency of additional generation for the added load.
3.4 BTU per watt-hour X 100 watts = 340 BTU/h
Just chemical to mechanical by itself is only 25-30% eff.
Mechanical to electrical is only ~50% due partly to the drive belt.
I’ll go with 15% overall efficiency
That works out to 0.018 gallons per hour.
At 30 MPH and 40 MPG, a reduction of 2.2%
I think you’re overcomplicating it. The amount of fuel used to operate the seat heaters will not vary with the vehicle speed. The seats do not get hotter as the car goes faster. It’s simply a question of how much gas/hr does it take to generate 3400 BTU/h.
Assuming 15% and one heater is used, that would mean .091 gal/hr.
So, if the car is going 60 mph and getting 30 mpg, it would use 2.091 gallons instead of 2.0 gallons. If it’s going 30 mph it’ll use 1.091 gallons rather than 1.0 gallons.
The amount of gas used is too small to really measure using the method of measuring mileage by dividing gallons driven into gas used. There are too many other more consequential variables. Whether the actual efficiency is 15%, 30%, or 2%, the extra gas used will still be too small to measure.
It’s 340, not 3400 BTU/h
Agree. However my point is no less valid.
Truth is, we don’t really know efficiencies, the wattages, or any other real details, but the amount it would take to even operate a 100 watt seat heater won’t be visable in the OP’s mileage monitoring.
In truth, his friend is technically correct. Running the seat heater will use more gas. But he’d be hard-pressed to try to measure how much.