Interesting question!Let's start with the definition of AFUE straight from the Energy Star website.
- The Annual Fuel Utilization Efficiency (AFUE) measures the amount of fuel converted to space heat in proportion to the amount of fuel entering the furnace. This is commonly expressed as a percentage.
AFUE is a measurement of efficiency
In simple terms, the AFUE rating of a heating appliance defines its efficiency.
- A 90% AFUE appliance will convert 90% of the fuel provided to the burner into heat.
- Similarly, an 80% AFUE appliance will only convert 80% of the fuel into heat while the other 20% escapes out the chimney or exhaust vent (hopefully).
- Thus the higher the percentage, the greater the efficiency.
- For all-electric furnace systems there is no fuel loss through the chimney; rather, the efficiency is based on the heat loss from the appliance.
Measuring a solar water heater's efficiency
While some water heating appliances are measured using the AFUE rating, the efficiency of a solar water heater is established using different metrics. The Department of Energy (DOE) on their EnergySavers website recommends using
- Solar Energy Factor (SEF) and
- Solar Fraction (SF)
The definition of these two metrics as provided by DOE:
- Solar Energy Factor is defined as the energy delivered by the system divided by the electrical or gas energy put into the system. The higher the number, the more energy efficient. Solar energy factors range from 1.0 to 11. Systems with solar energy factors of 2 or 3 are the most common.
- Solar Fraction is the portion of the total conventional hot water heating load (delivered energy and tank standby losses). The higher the solar fraction, the greater the solar contribution to water heating, which reduces the energy required by the backup water heater. The solar fraction varies from 0 to 1.0. Typical solar fractions are 0.5-0.75.
Look for the SRCC label
To make an informed comparative decision I recommend choosing equipment that has a Solar Rating & Certification Corporation (SRCC) label.
- The federal tax incentives for installing a solar water-heating system require that you install equipment with the SRCC label.
- For more information on the current incentives for installing a solar heating system, have a look at the Energy Star's Frequently Asked Questions regarding this tax credit.
Good luck on your project!
For more information:
Read Paul Rosen's Q&A "Can you tell me how well solar hot-water heaters work?"
Also, read Cameron Habel's Q&A "I've heard solar water heaters are expensive and easily damaged by winter freezes. Is this true?"