Most CO2-emissions are energy-related emissions, meaning that the emissions are caused by the combustion (burning) of primary energy sources such as coal, oil or gas. In cities, energy typically occurs in the form of heat (e.g. for buildings, for manufacturing processes), fuels (e.g. to drive cars), electricity (through grid), batteries (e.g. for electric vehicle) or reservoirs (e.g. geothermal for heat pumps).