A conventional water heater stores a large volume of water at a high temperature so it will be available for you to use when you need it. The heater works by pumping out hot water from the top of the storage tank and pumping in unheated water into the bottom of the tank. Most tanks can hold between 20 and 80 gallons. To determine the right size water heater for your household involves taking a close look at your daily hot water usage.

Methodology

A water heater's size is determined by its first-hour rating. The first-hour rating is the amount of hot water the heater can provide in the first hour of use assuming a full tank of water. This measurement takes into account both the amount of hot water the heater can hold in the tank and the rate at which the heater can refill its supply. For example, a tank with a 30 gallon first-hour rating might hold 22 gallons and be able to heat another eight replacement gallons in the first hour. Hot water heater ratings can range from 20 gallons to 120 gallons. To determine the first-hour amount your hot water heater needs look at which hour your household uses the most hot water.

Estimate Hot Water Useage in the Bathroom

For most households, the bathroom is where they use the most hot water. To estimate how much hot water you need, estimate the number of showers, baths, hair washings, shaves and hand washings that take place during the busiest hour. Multiply these numbers by the number of gallons that each activity takes. The U.S. Department of Energy estimates that a shower takes about 12 gallons, a bath about nine, a shave about two, and each hair wash or hand wash about four gallons. For example, if your household took a bath, a shower and shaved in the busiest hour, you would need a heater with a 23 gallon first-hour rating.

Estimate Other Hot Water Useage

Other common drains on your hot water supply include the kitchen and laundry room. Each time you run a load of dishes in an automatic dishwasher you use about 14 gallons, which is about 10 more than doing them by hand according to the Department of Energy. Each laundry load uses between 26 and 32 gallons depending on the type of washer you have. Any additional uses of hot water during the busiest hour need to be added to the bathroom uses to determine the size of your water heater. For example, if you used 23 gallons in the busiest hour of the day in the bathroom and ran one load of laundry that requires 32 gallons of hot water, you would need a hot water heater with a 55 gallon first-hour rating size.

Source:

Energy Savers: Sizing a Water Heater

Energy Savers Tips: How Conventional Water Heaters Work