Replacing your water heater? Be sure to choose one that is the correct size for your needs. If your water heater is too small, you’ll be constantly running out of hot water. Too large, and you’ll end up paying to heat more hot water than you will ever use. To avoid these extremes, follow best practices for choosing a properly sized water heater:

First, you need to know what type of water heater you will be installing. The most common choices are conventional storage water heaters, tankless water heaters, and solar water heaters.

For storage water heaters, look for the first hour rating. This is the amount of hot water (in gallons) the tank can supply per hour, starting with a full tank of hot water. You can find the first hour rating on the top left of a new water heater’s EnergyGuide label.

Next, you will need to be aware of your peak hour demand. This is the maximum amount of hot water you are likely to consume in an hour. Use this worksheet to calculate this amount (in gallons) for your household.

Look for a water heater whose first hour rating is within 1-2 gallons of your peak hour demand. This type of calculation is also appropriate for heat pump water heaters that use a tank.

For tankless water heaters, you’ll need to determine two numbers: the flow rate (the maximum gallons per minute of hot water you’ll need), and the temperature rise (the difference in temperature between your incoming cold water and the heated water as it exits your unit).

To determine your flow rate, first find out how many gallons per minute each of your fixtures uses. Then, calculate which of these are likely to ever be running at once. Add the gallons per minute together for your flow rate. For example, if you will be running a faucet at 1 gallon per minute and a shower at 2 gallons per minute, you’ll need a tank heater with a flow rate capacity of at least 3 gallons per minute. (You can reduce your flow rate by installing water saving fixtures.)

Calculating your temperature rise is simple. Just subtract your cold water temperature from your desired hot water temperature (usually 120 degrees). Then, choose a tankless water heater rated for the appropriate temperature rise.

Calculating the size of a solar water heater is the easiest of all — because your solar contractor figures it out for you as he designs your system.

As a general rule of thumb, you’ll want your solar water heater to provide 80-100 percent of your summer water heating needs. Your contractor will take into account the many variables in your solar water heating system to achieve this goal. These include climate, sun angles, and panel (collector) siting and sizing, as well as the size of the hot water tank. In Maryland and surrounding states, you’ll typically want 1.5 gallons of hot water storage per square foot of solar thermal collector.

To find out more about sizing a solar water heater to meet your hot water needs, contact us for a free solar analysis.