Physics: Why 20% Efficiency Still Powers Homes

Efficiency is the ratio of useful output energy to total input energy, expressed as a percentage: efficiency = (useful output / total input) × 100. No energy transfer is ever perfectly efficient — some is always dissipated, often as heat — so this ratio captures how much of what goes in actually does the job we want. This matters because real systems, from rooftop solar panels to power stations, must be judged on what they deliver, not just what they receive. A panel rated at 20% converts only one-fifth of incident sunlight into electricity, and since that incident energy varies with weather and season, output is intermittent while demand stays continuous. Storage and grid backup therefore become essential. Evaluating such systems also raises ethical questions: manufacturing and disposal carry environmental costs, yet over a long lifespan the avoided emissions can outweigh them.