Energy Efficiency & Conservation
- Distinguish energy conservation from energy efficiency
- Calculate the energy used and cost of running an electrical appliance
- Explain how efficiency measures reduce energy demand and emissions
Conservation vs. efficiency
Two related strategies cut energy use. Energy conservation means using less energy by changing behavior — turning off lights, driving less, lowering the thermostat. Energy efficiency means getting the same service from less energy through better technology — an LED bulb that produces the same light as an incandescent using a fraction of the electricity. Both reduce demand, which lowers fuel use, emissions, and cost. Because reducing demand avoids the need to generate power in the first place, efficiency is often the cheapest and cleanest "energy source" of all.
Where energy is lost
No conversion is perfect — the second law of thermodynamics guarantees energy is lost as waste heat at every step. A typical incandescent bulb converts only about 5% of its electricity into light and wastes 95% as heat, while an LED converts far more into light. A gasoline car engine turns only ~20% of the fuel’s energy into motion. Cogeneration (combined heat and power) captures the "waste" heat from electricity generation and uses it to heat buildings, raising overall efficiency from ~35% to as high as 80%.
Measuring and paying for electricity
Electrical energy is billed in kilowatt-hours (kWh) — the energy used by a 1-kilowatt (1,000-watt) device running for one hour. To find energy used, multiply an appliance’s power (in kilowatts) by the time it runs (in hours); to find cost, multiply that energy by the price per kWh. This simple calculation reveals why high-wattage devices used for long periods — space heaters, air conditioners, water heaters — dominate a household’s electricity bill, and why switching them to efficient models saves the most.
A 1,500-watt space heater runs 4 hours per day for 30 days. Electricity costs $0.12 per kilowatt-hour. What does it cost to run the heater for the month?
- 1.Convert power to kilowatts: 1,500 W ÷ 1,000 = 1.5 kW.
- 2.Energy per day: 1.5 kW × 4 h = 6 kWh.
- 3.Energy for the month: 6 kWh/day × 30 days = 180 kWh.
- 4.Cost: 180 kWh × $0.12/kWh = $21.60.
A 100-watt appliance runs for 10 hours. Electricity costs $0.15 per kWh. How much does it cost to run?
Distinguish the two strategies: replacing an old bulb with an LED that gives the same light for less power is efficiency (better technology); simply turning the bulb off when you leave is conservation (using less). Exam questions often ask you to classify an action as one or the other.
A cogeneration (combined heat and power) system captures the waste heat from generating electricity and uses it to warm nearby buildings. Why does this improve overall efficiency?
For energy-cost problems, the killer step is unit conversion: watts → kilowatts by dividing by 1,000. Then kWh = kW × hours, and cost = kWh × price. Lay the units out and cancel them; if your answer is off by a factor of 1,000, you skipped the W-to-kW conversion.
Answer the 2 checkpoints as you read.
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