Efficiency measures how much useful output you get from a given input. An incandescent bulb converts about 5 percent of electricity into light. The rest becomes heat. An LED converts 40 to 50 percent. Same brightness, far less power. That gap is why efficiency improvements are often called the cheapest energy resource.

Efficiency applies across the economy. A car engine loses most of its fuel energy to heat and friction. A power plant loses roughly half its energy as waste heat. A data center spends a large share of its electricity on cooling. Each loss is a chance to save.

Efficiency is not the same as conservation. Conservation means using less. Efficiency means getting the same result with less input. Both reduce demand, but efficiency usually comes from better technology, while conservation comes from behavior. The two work best together. A more efficient car driven less saves the most fuel.

Common measures

• Energy efficiency ratings
• Heat rate in power plants
• Miles per gallon
• COP for heat pumps
• Power usage effectiveness in data centers