I've been contributing to the "Model S" thread over at The Gear Page - Powered by vBulletin (excellent forum, btw, for musicians). A question came up on how much impact using the AC has on range, and I don't know the answer. Would someone please educate me? Thank you! Here's a direct link to the thread: TESLA MODEL S - The Gear Page
EVTripPlanner does a great job at predicting battery usage for verious conditions. Put in your route and push the temps up and down to see how much range is affected.
It's not noticeable. There will be larger natural variations due to weather and wind that will obscure any noticeable effect from the air conditioner's energy consumption.
Turning off the AC allowed me to hit 280 wh/mile on my daily commute. With it on, I was getting 310 wh/mile.
The compressor is variable speed, so it doesn't always use the same amount of energy. Mostly it's fairly minor 10-30 Wh/mile but a lot depends on how you use it, how hot the weather is, whether you can pre-cool from shore power, etc. I never hesitate to use the a/c and my average Wh/mile is reasonably low.