Light stopped being a luxury and became almost free

For most of history, light after dark was a real expense. You paid for it in tallow candles, in whale oil, in the hours spent gathering fuel, and you rationed it. The number here is the cost of a fixed amount of light, a million lumen-hours, in Britain across seven centuries, held in constant money so the eras compare.

Then came gas, then the incandescent bulb, then fluorescent tubes and LEDs, each throwing far more light per unit of energy than the last. The cost fell so far that lighting a room is now a rounding error on a household bill. The series is for the UK alone, the one country with a record reaching this far back, and it ticks up a little in recent years as energy prices rose, but the arc across the centuries only points one way.