Proponents differ on whether gas or electric stoves and ovens are better for the environment, but ovens that insulate well use less energy. In general, smaller ovens are more efficient than larger ones.
A gas oven burns natural gas and uses it to warm its contents. While natural gas produces carbon monoxide and other chemicals, gas ovens are able to use it efficiently. Electric ovens use electricity efficiently, but their environmental impact varies based on the power plant they use. If powered by a solar, wind or nuclear plant, they only cause the release of trace amounts of greenhouse gases; if powered by a coal or natural gas plant, they release more than a comparable gas oven.
Regardless of which type of stove it used, ovens that are better able to insulate their heat use less energy, making them better for the environment and cheaper to run. Ensuring that a stove is properly sealed after it's installed can result in lower gas or electricity bills.
Gas and electric stoves also differ. One of the advantages of electric stoves is that pots and pans are placed directly on top of them, which prevents heat from escaping. Gas stoves need clearance between the burner and the cooking vessel, which allows a significant amount of heat to go to waste. According to some measures, however, gas stoves are better for the environment than electric stoves powered by a coal plant.