My aircon costs me 20% fuel consumption in the height of summer, which makes a big difference on a very frugal car. I get 6.2 litres per 100km in the hottest months in city driving only, using no overhead or high speed roads, compared to around 5.3 litres per 100km at this time. In the winter it will improve slightly further.
On the highway I can achieve a maximum 3.2 litres per 100km stuck at 86kph with the aircon off. It makes only little difference with aircon on at that speed however.
With climate control fitted to the Prius I notice a considerable ramp up in fan speeds and engine starting whenever I enter sunny areas during the hottest months. Sticking to the shade will make a considerable difference. I feel therefore that a louvre system could potentially make quite a difference when under the sun’s direct burn.