Separating form from content (xhr based) allows for better caching and overall respond better to changing demands, therefore limit the need for over-capacity. I

Separating form from content (xhr based) allows for better caching and overall respond better to changing demands, therefore limit the need for over-capacity. I think it is a null sum game between the two options. Lowtech magazine investigated energy efficient web experience a few years ago, and the conclusion was much simpler. If we really want to have an impact on the energy, don't use white background in your design and use the default font, dithered images, reduce dynamic content and don't have the server running 24/7.