Water evaporation is a ubiquitous natural process(1) that harvests thermal energy from the ambient environment. It has previously been utilized in a number of applications(2,3) including the synthesis of nanostructures(4) and the creation of energy-harvesting devices(5,6). Here, we show that water evaporation from the surface of a variety of nanostructured carbon materials can be used to generate electricity. We find that evaporation from centimetre-sized carbon black sheets can reliably generate sustained voltages of up to 1 V under ambient conditions. The interaction between the water molecules and the carbon layers and moreover evaporation-induced water flow within the porous carbon sheets are thought to be key to the voltage generation. This approach to electricity generation is related to the traditional streaming potential(7), which relies on driving ionic solutions through narrow gaps, and the recently reported method of moving ionic solutions across graphene surfaces(8,9), but as it exploits the natural process of evaporation and uses cheap carbon black it could offer advantages in the development of practical devices.