Cloud computing is becoming a corporate no-brainer as a sustainable, energy efficient alternative to buying and managing your own servers and software. But is it actually any greener? Greenpeace says not – it points out that the extraordinary growth in the number of data centres means that even significant improvements in efficiency are dwarfed by the increase in total energy demand, and that if the cloud was a country, it would have the fifth largest electricity consumption in the world. But the crucial detail is the source of that electricity. Because data centre owners seek a cheap, constant power supply and have historically favoured locations – Virginia, North Carolina – where electricity is produced from coal, their carbon emissions can be enormous. The good news is that in a fiercely competitive market where all the main providers are building furiously, some have realised the marketing potential of locations like Iceland where the power grid is almost zero-carbon. By shopping carefully, companies can not only reduce their own carbon footprint, but everyone else’s as well. For this piece for the Sunday Telegraph’s Business Technology supplement, I investigated the hidden environmental cost of cloud computing, and the key questions to ask a potential supplier.