Physics: How Efficiency Affects Electricity Cost

Energy efficiency sits at the heart of every real-world device, and few comparisons make it as tangible as the humble light bulb. In an incandescent bulb, electrical energy is converted into thermal energy in the filament, which then emits visible light alongside infrared radiation — heat is the dominant output. An LED works differently: electrical energy drives charge carriers across a semiconductor junction, producing light directly through electroluminescence, with far less wasted as heat. This distinction matters because efficiency, the ratio of useful output to total input, determines both running costs and sustainability. A bulb's useful light power follows P(light) = efficiency × P(input), while the remainder is dissipated as heat, P(heat) = P(input) − P(light). Since electricity is billed in kilowatt-hours, E = P × t ÷ 1000, even small power differences compound over thousands of hours of use. Comparing purchase price, lifespan and energy consumption together reveals whether a higher upfront cost is truly justified.