Each tree planted will absorb approximately 1 tonne (1000kg) of CO2 over 100 years. (The 'US Department of Energy Report on Greenhouse Gasses 1998' determined that a tree can absorb 1121kg CO2 in 100 years). 1 tonne of CO2 equates to approximately 1900kWh of electricity (0.53kg per kWh). Divide your total carbon footprint (kg CO2) for a year by 1000 to provide the total number of trees required that year. Alternatively, divide your energy use in kWhs by 1900.