The Rally defines itself as a forum for solar car developers to communicate instead of simply a race. And don't miss the scoring system: Cars don't get points depending on their speed and how much time it takes for them to make a certain lap, but on how little energy they need.
The Rally also takes in consideration additional measures to produce as little carbon as possible. First of all, the Rally produces all of the hot water it needs using solar energy, and the safety car is a Toyota Prius. Any energy that they need to produce that doesn't come from solar radiation gets offset via the purchase of carbon rights.
[Source: Rallyesolaire (h/t to Imma)]