Ok since the loss of MPG through the heavy use of the Prius A/C system has become such a big subject I thought it would be nice to have a thread that focuses on mitigating or reducing those losses. I have posted some basic ways in which you can reduce the losses but I am hoping those smarter and more creative than I can help me expand the list so that other Prius owners are able to benefit from our knowledgebase here at PC. Problem: Heavy A/C usage in hot climates and significantly reduce your average MPG. Why? The A/C system is electric and therefor requires energy. The energy essentially comes from your HV battery which then requires recharging. The energy for recharging is supplied by your ICE which runs on gasoline. A/C systems inherently run less efficiently in hot temperatures. The hotter the ambient temperature the less efficient the system is and the harder it has to work to reduce temperature inside your car (or house). The temperature inside your car is often much warmer than the outside ambient temperature by 30 degrees or more depending on many factors like interior color, number of windows and their construction, etc.. The A/C system has to do a lot of work to reduce those temperatures to your desired temperature. Imagine trying to reduce the temperature inside your car from 130 degrees to 70 degrees. That is a lot of energy! Where does that energy come from? Ohh yeah, gasoline! The lower your temperature setting, the higher the energy required. I have measured an 1800watt continuous draw for over 10min while the A/C tried to cool the car! Even after 30min the draw was over 600w. This was measured by Torque app in my GenIII. The video below is an example of how much of a MPG hit you can take when initially starting your A/C system and trying to reduce cabin temperature. This effect can stay active for 30 miles or more depending on the cabin temperature and your desired A/C temperature! Methods for controlling heat buildup in your car: Park in a shaded area. Under a tree may help but a better tactic is to park on the eastern side of a large building. As the sun moves to the west in the later portion of the day, the building will block the sun from hitting your car. With your windows cracked, the heat from the earlier part of the day will dissipate faster than if you parked your car in the full sun the entire day. Crack your windows a few inches to allow warmer interior air to vent outside the vehicle. Assumes you are parked in a safe area Tint your windows Use a high quality windshield sun shade. Priuschat shop has a very nice one! If you cannot park on the east side of a building try parking your car facing the west and use a quality windshield sun shade. This will reduce the number of windows directly facing the sun and thus not allowing as much of the suns radiation to enter the car. Drive the car for the first few minutes with all the windows down to circulate the hot air and direct it outside. Best Practices for A/C usage and maintenance: Use the A/C sparingly or use the crossflow venting technique (see below) Crossflow Venting - Roll the driver side window down approx. 3" then do the same for the passenger side rear window. This will create a nice cross breeze and will not adversely affect aerodynamics in a noticeable way. In the event this is still not enough, roll the driver's window completely down and stick your arm outside the window. This allows wind to flow through the arm hole in my shirt and flow out through the other arm hole and the bottom of my shirt. This can dramatically add to the the cooling effect and is still more efficient than running the A/C , even on the freeway at 60mph. Use a setting that is high enough for maximum efficiency but makes you comfortable. 78 degrees seems to be sufficient for most drivers Ensure your A/C system is charged to spec levels and operating efficiently. i.e. condenser is clear of obstructions, bugs, etc. Use Eco Mode if your car is equipped with the feature These tips will not only help reduce the fuel economy hit of the A/C system but it may also extend the life of your HV battery by reducing interior temperatures while the car is sitting in the parking lot. If your interior is 130F then your battery may be too! When outside temps are over 100F it is recommended to run your A/C on longer trips to try and cool down the HV battery. I've measured a 15F difference between ambient temps and battery temps when the A/C was running. After turning off the A/C the battery quickly returns to near ambient temperatures. Fuel economy is important but your bodily well being and HV battery life are more important.