FACTBOX-How big is Bitcoin's carbon footprint? Reuters 2 hrs ago May 13 (Reuters) - Tesla boss Elon Musk's sudden u-turn over accepting bitcoin to buy his electric vehicles has thrust the cryptocurrency's energy usage into the headlights. Some Tesla investors, along with environmentalists, have been increasingly critical about the way bitcoin is "mined" using vast amounts of electricity generated with fossil fuels. Musk said on Wednesday he backed that concern, especially the use of "coal, which has the worst emissions of any fuel". So how dirty is the virtual currency? POWER HUNGRY Unlike mainstream traditional currencies, bitcoin is virtual and not made from paper or plastic, or even metal. Bitcoin is virtual but power-hungry as it is created using high-powered computers around the globe.