The CO2Stats calculation includes the contribution of client electricity use. In order to more accurately measure the time clients spend on a given CO2Stats-powered page, it's necessary to have the client infrequently check in.
Thanks for your comment! In fact, every time our customers' footprints have gone over their monthly cost, we've paid for the full corresponding number of RECs. That clause is there to protect against gross abuse. For example, if a site consistently receives 20M hits per month (a custom tier) and signs up for a 5M hit per month package, we reserve the right to manage only the first 5M.
Great question! We measure quite a variety of signals, including but not limited to client locations (e.g., is traffic coming from coal-burning areas?), visit lengths, server locations, transfer sizes, transfer times, and window sizes. These signals are fed into a "hand-built" database that allows us to compute aggregate carbon footprints.
Automatically purchases renewable energy to offset CO2 emissions due to your clients, network, and servers, independently of your hosting provider, so you can use it with Slicehost.