Solar panels: 500 times cheaper

An interactive explorer in one HTML page. Switch the chart between a log and a linear scale, drag the two sliders to pick any span of years, and the headline numbers, the sentence under them and the highlighted stretch of the line all recompute. Nothing is precomputed: the page carries fifty prices and does the arithmetic as you move.

Price explorer

Why a log scale

On a linear axis the story ends in 1985: the price has already fallen from $132 to $17 and everything after is a flat line on the floor. On a log axis each step of the grid is a tenfold change, so a steady percentage decline is a straight line — and the solar line is close to straight for fifty years. That steady rate is the pattern known as Swanson's law: module prices tend to drop by about a fifth each time the world's cumulative shipments double.

Data

How it's built

• One state, one render. The scale and the two years live in one small object; every control writes to it and calls the same function that redraws the chart and rewrites the numbers, so nothing can drift out of sync.
• The controls are native: two <input type="range"> tinted with accent-color, and a pair of buttons whose aria-pressed doubles as the style hook for the selected one.
• The sentence is generated: the halving time is ln 2 / −ln(1 + rate), written out as words, so a reader who never looks at the axis still gets the answer.

Source: Nemet (2009), Farmer & Lafond (2016) and IRENA Renewable Power Generation Costs (2025), as compiled by Our World in Data.