What's the mA output of the iPhone 4s adapter?

I have a car adapter for an old flip-phone that has a built-in USB outlet... the info on the side of it says it's output is 5V DC (or 800 mA). I'd like to use it in the car to charge my iPhone 4s (and Rocketfish headset), but don't want to push too much power into the iPhone. Does anyone from Apple know if the output is compatible with what the iPhone 4s is designed to accept? Thanks.

Mike