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12.4 Efficiency Efficiency • Efficiency refers to how well a load (electric device) changes electrical energy to useful energy • Consumers can determine which appliances are most energy efficient by reading the appliance’s EnerGuide Label Efficiency • By comparing the efficiency of different devices, we can judge both their energy cost and their environmental impact. • A mini refrigerator from the 1970s uses the same energy as a modern full-size refrigerator as a result of improvements in insulation and efficiency of the compressor motor. Energy Star • The Energy Star symbol is used to indicate that a product meets specific criteria of energy consumption set by the EPA (Environmental Protection Agency). • The Energy Star label was established to reduce greenhouse gas emissions from inefficient use of energy and to help consumers purchase energy efficient products, which help lower energy bills. Percent Efficiency • The efficiency of a device is the ratio of the useful energy that comes out of the device to the total energy that went in. • The more input energy that a device converts into usable output energy, the more efficient the device is. • Efficiency is usually calculated as a percentage: Percent Efficiency • An incandescent light produces a lot of heat. In fact, only about 5% of the energy that goes into the bulb is used as light, useful energy. • A compact fluorescent bulb in comparison is about 20% efficient. Sample Problem Determine the percent efficiency of a 60-W fluorescent light bulb that uses 2000 J of electrical energy to produce 400 J of light energy. Given: Total electrical energy input = 2000 J Useful energy output = 400 J Required: Percent efficiency Analysis: Useful energy output Percent efficiency = Total electrical energy input x 100 % Sample Problem, continued Solution: Percent efficiency = Percent efficiency = 400 J 2000 J x 100 % 20 % Paraphrase: The percent efficiency of a 60-W fluorescent light bulb is 20%. Homework • “Power and Efficiency” Worksheet