How long does it take for an idling car to use as much energy as a car being started?

“more than ten seconds of idling uses more fuel than restarting the engine.”
http://www.consumerenergycenter.org/myths/idling.html

@ragingloli 10 seconds? Damn, I always thought it was much longer.

The answer depends upon whether the engine is cold or hot and how quickly your car starts.
If hot, the energy used by the battery and the resultant fuel to recharge it is about 4x. A one sceond crank will cost you about 4 seconds of idle fuel.
A cold engine will cost about 10x. (The A/F ratio is richer and there is more friction.)
This ignores the cost of starter motor wear. That’s why you will see more idle stop systems on cars fitted with ISG (integrated starter generators).
In other words it is best to turn it off when you are not using it. Same as turning off lights in your home.

Yup, it’s 10 seconds. Very very little. And it’ll only get smaller as we move onto electric cars.

The reason starting your car is “bad” for your car is because it has to rotate the oil, so for a moment you’re running your car with almost none in it. As anyone that’s run an engine on no oil can tell you, that’s not good.

If you are talking about an older model car or truck then it’s probably a minute or two, but with modern fuel injected a ten seconds sounds about right.