A gas water heater burns natural gas (methane, CH4). Each gram of natural gas burned yields approximately 13,000 calories of energy. A typical water heater takes in water at about 20oC (68oF) and raises its temperature to 60oC (140oF). A typical “low-flow” shower-head might use around 10L of water per minute (which we can estimate 5L of hot water and 5L of cold water mixed), and a typical shower might last 8 minutes. A liter of water has a mass of 1000g. So, how much natural gas does a typical shower require?.