Heating water is one of the most energy-consumptive activities in a household, accounting for about 49 percent of California’s residential natural gas consumption. Data collected during a pilot field study in California indicate that significant amounts of water and energy are wasted while waiting for hot water to be delivered to the point of end use. We calculate the water and energy wasted during shower events from data collected using a wireless sensor network that monitored water flows and temperatures in three single-family residences. The total calculated water waste for a typical shower event was 30 percent. Forty-one percent of the hot water energy for the same event was wasted. This relatively low efficiency highlights the importance of examining further the energy and water waste of residential hot water distribution systems in order to improve them.