L38. Energy Efficiency and Conservation

Energy, Technology, and Society

R-report

L38. Energy Efficiency and Conservation

How can two lamps that look equally bright use very different amounts of electricity and make the room feel different?

What energy efficiency means in everyday devices

Energy efficiency compares the useful job a device does to the energy you put into it. For a light bulb, the useful job is producing visible light. For a heater, the useful job is producing warmth. Efficiency is written as a fraction or percent: useful energy out divided by total energy in. Importantly, conservation of energy still holds — energy is not destroyed. If a device is inefficient, more of the input energy becomes waste (usually heat, sound, or vibration) instead of the useful form we want. Try a simple observation: two desk lamps that look similar can behave differently. A 60-watt incandescent bulb and a 10-watt LED that give similar brightness use different power. The incandescent wastes a lot of input energy as heat, so the lamp and nearby air feel warm. The LED converts a larger share of input energy into light, so it stays much cooler. The total energy used and the part that becomes useful light are what efficiency measures.

How to calculate and compare efficiency

You can calculate efficiency using energy or power numbers you often find on labels. If a bulb uses 60 watts and produces 6 watts worth of visible light, its efficiency (in light output) is 6 ÷ 60 = 0.10, or 10%. An LED that gives the same light might use 10 watts and produce 5 watts of visible light: 5 ÷ 10 = 0.50, or 50% efficient. Comparing those percents shows which device turns more input into the useful output you want. To compare quickly, follow these steps:

• Find input power (watts) on the device label. Estimate useful output in the same units (watts of light or heat), or use reported luminous efficacy for lamps. Divide useful output by input and multiply by 100 for percent.

Where energy typically goes when it’s not useful

Knowing common ways energy is lost helps pick better choices. In many machines and appliances, losses show up as heat, friction, sound, or wasted motion. A gas-powered lawn mower loses fuel energy to exhaust heat and mechanical friction. An old refrigerator leaks cold air through worn seals and uses extra electricity to keep the interior cool. Even well-designed devices have some unavoidable losses; the goal is to reduce unnecessary ones. Common loss pathways (short list):

• Heat from resistance (wires, filaments) or combustion. Friction in moving parts (bearings, gears). Unwanted light or sound that isn’t part of the task. Leaks of energy carriers (air leaks in buildings, heat flow through windows).

A classroom investigation and simple actions to conserve

Spend a few minutes comparing two light sources or two small heaters. Without special tools you can use label numbers: multiply watts × hours used to get watt-hours, then divide by 1000 to get kilowatt-hours (kWh). Compare cost by multiplying kWh by a local electricity rate. This quick math shows how much energy and money different choices use. Simple conservation steps that come from understanding efficiency: - Choose appliances with higher efficiency ratings (LED lights, ENERGY STAR appliances). These give the same service with less input energy. - Reduce waste by fixing leaks and insulating: sealing drafts around doors and windows cuts heat loss in winter. - Match the device to the task: use a heater to warm a room, not a hairdryer for long heating; use a small, efficient tool rather than a larger one when possible. Remember: some efficient devices change how we behave (for example, cheaper running costs can lead to using them more). Good conservation combines efficient technology with mindful habits — turn off lights when not needed, set thermostats wisely, and unplug chargers that draw small amounts of power when idle.

Big idea and classroom next steps

Energy efficiency measures how well a device turns input energy into the useful output you want, while conservation combines efficient choices with behavior that reduces total energy use. Conservation relies on the physics idea that energy is conserved: energy is moved or changed in form, not destroyed. Efficiency tells you the fraction of input energy that becomes the desired form.

Try a short class activity: pick two devices (like an LED and an incandescent bulb), record their watt ratings, estimate use time, compute kWh and cost for one week. Discuss how technology choice and simple habits together change energy use and costs.