Water heating accounts for approximately 15 percent of the average household’s total energy consumption. If your water heater is more than 10 years old it is likely running at less than 50 percent efficiency.
To purchase the most efficient storage water heater for your needs, you need to know two numbers: first-hour rating (FHR) and energy factor (EF).
- FHR represents the amount of hot water your family uses during its busiest hour; the U.S. Department of Energy offers a worksheet to help you determine this amount,
- EF represents how efficiently the appliance operates, with higher values signifying better efficiency. EF ratings typically range from 0.75 to 0.95 for electric water heaters, and 0.50 to 0.70 for natural gas-fired water heaters.
Heaters that maintain a large tank of water at a set temperature are the most common, and are best suited for larger households that use a lot of hot water. They have become more efficient over the years, but some energy is still wasted when the hot water is not being used.
Tankless Water Heaters
Also known as on-demand or instantaneous heaters, these devices use energy only when hot water is needed, resulting in less standby energy loss than storage heaters. They tend to have a higher up-front cost, but are less expensive to operate in the long term due to their higher efficiency. When choosing a tankless water heater, you must first determine your required flow rate, or the total hot water consumption of the appliances you need to run simultaneously.
Solar Water Heaters
By transferring heat from the sun into a conventional storage tank, solar water heaters can supply part or all of your hot water needs. They have higher up-front costs than conventional water heaters but much lower operating costs, and can pay for themselves within 4 to 10 years under favorable conditions.
via Union of Concerned Scientists