Guardian: Today sees the launch of Realtime Carbon, a neat website that could change the way we think about the environmental impact of electricity. The purpose of the site is to report the carbon footprint of a unit of power as it changes over time. It may sound geeky but it’s hugely significant. Over the course of 24 hours, demand for electricity fluctuates widely. Demand is lowest in the small hours – hence energy tariffs such as Economy 7 – and highest in the daytime and early evening, when …