Ecotricity's UK Grid LIVE page displays the current carbon intensity of the National Grid, measured in grams of carbon dioxide per kilowatt-hour, with a traffic light showing whether it's a good time to use electricity.
This is a clever idea, but there's a problem: just because current production has a certain carbon intensity, it doesn't mean that marginal production (that is, additional production to meet higher demand) has the same carbon intensity. To see why this is a problem, let's look at the hourly breakdown of electricity generation over the past week, courtesy of my National Grid: Live Status site:
The daily cycles in generation are obvious, and are most striking for gas (CCGT – Combined Cycle Gas Turbine), which varies from below 5GW to over 15GW each working day, accounting for most of the variation in generation.
This means that no matter what the current carbon intensity of the National Grid is, additional power will come largely from burning gas, at a carbon intensity of around 490 grams of carbon dioxide per kilowatt-hour. Counterintuitively, this actually has the effect of lowering the carbon intensity of the National Grid, as it reduces the proportional contribution from burning coal.