FCs work on the principle of the reverse electrolysis of water. This involves a chemical reaction between hydrogen and oxygen that can be harnessed to produce electricity. Since the only by-product is water, the process is completely non-polluting. The efficiency of power generation is also high because the process converts the chemical energy in the fuel (hydrogen) directly into electricity. The heat generated by the reaction can also be used to heat water. This combination raises total thermal efficiency above 80%. In addition to cutting fuel bills, the CO2 emissions of an FC are 30-40% lower compared with the combination of gas and electric power.