In terms of CO2 emissions, a large fraction of the emissions saved is dependent on the degree to which gas or electricity is used to supplement the sun. Using the Eco-indicator 99 points system as a yardstick (i.e. the yearly environmental load of an average European inhabitant) in Greece, a purely gas-driven system may have fewer emissions than a solar system. This calculation assumes that the solar system produces about half of the hot water requirements of a household. But because methane (CH4) emissions from the natural gas fuel cycle dwarf the greenhouse impact of CO2, the net greenhouse emissions (CO2e) from gas-driven systems are vastly greater than for solar heaters, especially if supplemental electricity is also from carbon-free generation.