|A different sort of green home|
Solar water heating has a very similar rating called the Solar Energy Factor (SEF). Solar equipment is evaluated for thermal performance efficiency by an organization called the Solar Rating & Certification Corporation (SRCC). The federal tax incentives for the installation of solar equipment are contingent on the solar water heating equipment having been evaluated by the SRCC. The SRCC developed the SEF for the purpose of comparing solar water heating systems to a standard 50 gallon residential water heater in an apples-to-apples sort of way. Many natural gas utilities use the industry standard EF rating system to award rebates for high performing water heating equipment. Since SEF is the solar equivalent of EF for a standard water heater, my view is that solar water heating systems that meet EF ratings for high performance equipment should qualify at a minimum for the existing utility rebate programs.
So how do different water heating technologies measure up with respect to energy efficiency?
Tank storage water heaters
|Bradford White 40 gallon gas water heater|
EF rating for gas heated tank water heaters:
- Low efficiency -- < 0.62
- Medium efficiency -- 0.62 to 0.67
- High efficiency -- 0.67 to 0.82
- Low efficiency -- <0.90
- High efficiency -- up to 0.95
Keep in mind that EF rating is only with respect to the energy efficiency of the appliance. The electricity that heats the water in an electric tank model could be coming from sources with varying degrees of efficiency and environmental impact.
|Tankless water heater (top right) in conjunction with a solar water heater|
The EF rating for gas-fired tankless units ranges between 0.82 and 0.96.
Heat Pump Water Heaters
EF ratings of HPWH in hybrid mode is 2.20 and fall off towards 0.93 in electric mode (average ~1.60).
Solar Water Heaters
In solar water heaters (SWH), the heat from the sun's radiation is transferred to drinking water during the day and stored in a tank of water for use throughout the day and night. In homes where a 40 gallon tank water heater is appropriate, you'll find a 60 gallon solar storage tank. Because the amount of energy to move the heat from the solar collectors to the drinking water is minimal and the sun's energy is free, the operating costs of SWHs are the lowest of any water heating technology today. They are also the lowest carbon option when the system offsets fuel from gas appliances or electricity from coal-fired plants. SWHs have the highest upfront cost in the water heating space, but federal and state incentives exist to encourage their adoption since they provide a benefit to both the system owner and society as a whole.
Calculating EF from the SEF rating developed at the SRCC is done with the following equation: SEF x (1 - SF) = EF. SEF and SF (Solar Fraction) are calculations from the OG-300 report on each system found at www.solar-rating.org. EF ratings on SWHs range from 0.93 up to over 5.70 depending on the size of the system, location, and back up fuel option.
The less you pay upfront for a water heater, the more you pay over time. As fuel prices of all sorts rise in the future, an investment in efficiency today will pay increasing dividends over time.