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Why does climate control disproportionately impact city vs highway range?

I'm using the range tool at http://www.teslamotors.com/goelectric#range .

There is a much larger mileage cost to using climate control when doing city driving as opposed to highway driving.

Can someone explain why this is?

85kwh battery
Highway speed = 55 MPH
Climate control = on

90 degrees:
Highway -> 284 miles vs 308 miles
City -> 237 miles vs 299 miles

32 degrees:
Highway -> 249 miles vs 287 miles
City -> 211 miles vs 291 miles

Climate control is a fixed amount of energy per time. If you're going 55 it's only going to take 5 hours to go 275 mile, so you're talking 5 hours of using the heater/AC. With city driving you're talking about 11 hours of driving to go the same distance, so that's 11 hours of heater/AC use.

The HVAC uses approximately the same amount of energy regardless of what speed you are going. Because of this, it is a much larger % of the total energy being used when you are going slower (using less energy to actually drive the car) while driving in the city.

Ok, that makes perfect sense.

Does the city mileage assume stop-and-go driving or is it at a constant (lower) speed?

The slower the worst it gets. All things being equal, the Model S is very efficient in stop and go traffic also in a constant bumper to bumper. Climate control, headlights, wippers, radio, heated seats is a heavy penalty on range when you are not moving. At about 283.3Watts/mi the car travels 300 miles. Anything more reduces your range. At 400Watts/mi like I drive my range is closer to 212.

Just trying to get a sense for the stop-and-go penalty due to (regenerative) braking loss without considering other electrical drains.

What would you say the percentage mileage penalty is for stop-and-go versus constant speed at city driving speeds?

The city mileage definitely assumes stop and go driving. A constant lower speed is the ideal driving condition, and you can get 400 miles driving a constant 30 mph. Normal city mileage is slightly less than highway mileage even without climate control because of the stop and go.

Let say you don't have anything else on because it is a perfect spring afternoon, same. Providing you are not burning rubber each time you step on the accelerator. I drive through NYC during rush hour every evening. Use very little juice to cross the city because of the slow moving traffic which is sometimes stop and go.