Although some consumers base their water heater purchases on the size of the storage tank, the peak-hour demand capacity, referred to as the first-hour rating (FHR) on the EnergyGuide label, is actually the more important figure. The FHR is a measure of how much hot water the heater will deliver during a busy hour, and it is required by law to appear on the unit's EnergyGuide label. Before you shop, estimate your household's peak-hour demand and look for a unit with an FHR in the range.