When's the best time to use electricity, if you want to avoid adding extra strain to the grid when its coal-fired power stations are already running full tilt?
Until now, there has been no way for the ordinary consumer to know. But today sees the launch of a groundbreaking new website set to change that - realtimecarbon.org.
The website, a collaboration of three organisations at the forefront of trying to measure and manage the carbon intensity of electricity generation, allows anyone to see the carbon intensity of the UK grid at any time of day.
If that intensity, measured in grams of carbon dioxide per kilowatt hour (unit) of electricity, goes above the grid average (around 370 gCO2/kWh), then a red flashing light advises consumers not to run energy intensive appliances - such as washing machines, tumble driers or heaters - at that time.
At the moment, the system is purely for the environmentally conscious - consumers will gain no financial benefit by choosing not to run appliances at certain times. But in the future, electricity customers connected to a smart metering system will be able to save money by changing their energy consumption habits.
The system also works the other way round - it tells consumers when they should be using appliances to take advantage either of low overall demand or periods when the wind is blowing strongly and there are high levels of clean energy on the grid.
A spokesperson for AMEE - one of the organisations involved in the project - said that the website was dseigned to 'catalyse debate' amongst policymakers, as well as to encourage energy 'geeks' to develop new systems to change people's behaviour.
One enterprising team has already created a tea light that changes colour from red to green to advise people on when is the best time to brew a cuppa, depending on the current strain on the grid.