The electric A/C compressor is powered by a variable-speed inverter circuit from the NiMH battery. This allows the controlling ECU to vary the compressor's speed, and hence the amount of heat that it can remove from the cabin per minute, independently of the ICE's rpm. For a given outside temperature, this heat removal rate will determine the lowest cabin temperature that can be achieved (the A/C evaporator temperature is always much lower than the cabin temperature). Clearly, one doesn't want to waste energy, and this system can be designed to be more frugal than an ICE-driven A/C compressor. The ECO switch limits the power draw by the compressor, and this will thus limit the lowest cabin temperature attainable at a given outside temperature. If your requested temperature is above this limit, then you will eventually get it; otherwise you won't, unless you switch ECO "off." This means that you can indeed achieve a cabin temperature below 25 degrees C (77 degrees F) with ECO "on," provided the outside temperature is not too high. Switching ECO "off" will get you there sooner, and use more energy in doing so. I would expect that only in the hottest climates is it necessary to switch ECO "off." Certainly, I leave mine "on" all the time.

Since the "hit" that your FE in mpg (or FC in L/100 km) takes from using the A/C depends on your speed (don't forget that you also have the A/C "on" when you're sitting at a stop light), asking for the corresponding decrease in mpg (or increase in L/100 km) is not the right way to assess this. The meaningful measure is the additional fuel usage rate in gal/h (or L/h). I gave some calculations of these numbers in an earlier thread. See posts #14 & 17 of the thread "Effect of Air Conditioning on mileage":
http://www.greenhybrid.com/discuss/f...05/#post175255
http://www.greenhybrid.com/discuss/f...05/#post175421.

Stan