A report at DayTrading.com breaks down what makes a cryptocurrency "green".
"A ... Nano transaction, on the other hand, uses less electricity than a Google search (estimates for a single Google search generally range from ~0.0003 to 0.0009 kWh)."
"Nano (XNO) is almost in a league of its own. Its block-lattice design means transactions confirm without heavy validation, clocking in at around 0.0001 kWh. This is less than the energy it takes to blink your router lights."