A fuel cell may provide new or replace existing supplies of electricity. Together with a source of hydrogen (such as natural gas or propane) and air, a fuel cell will be able to generate electricity, with water and carbon dioxide as by-products. Typically, fuel cells have a 40% to 60% efficiency (compared with 25% for cars, 25% for PV generators and 30% for coal generation stations), which can climb to 85% if the exhaust heat is fully utilized.