It's not one big thing as a thousand thousand little things that individual engineers have worked on.

"Simply" making the turbines bigger helps quite a lot, but that requires engineering the blades and support structures. There's also a continuous improvement of the generators, and the inverters to connect them to the grid. There's also clever fluid modelling software to reduce the extent to which one turbine takes energy out of the airstream of another, and so on.

For solar cells, the onward march of silicon processing helps a lot, but this time we don't even have to reduce the feature size! One paper that sticks in my memory is the development of "reduced kerf diamond wire saws": since a "boule" of silicon is more or less a fixed cost, if you can make the cuts between wafers thinner you reduce the amount of waste material and increase the number of wafers produced.

There was a great episode of the Planet Money podcast they recently re-ran that got into at least one of the factors of the drastically cheaper solar availability: https://www.npr.org/2019/08/14/751234092/episode-616-how-sol...

As I recall -- there was a boom of manufacturing plants for the solar cells in China as they were forecasting a drastically growing demand. The plants came online and boosted the supply greatly before the demand was high enough, so the prices sank.

There's also apparently been a number of improvements to tooling in regards to the labor required to install solar panels that has generally lowered labor costs.

Mass production of both along with greater effieciencies in manufacturing and installation. There are huge factories that churn out solar panels, and the installation systems have become simpler and faster. On the domestic side it can now take just a couple of hours to fit the panels once the brackets are in place, it used to take a day or more.

Basically both technologies have been matruring for a number of years, getting cheaper and better at the same time, so the cost per kwh drops.

On the wind side of things, the turbines have also got larger, which increases output significantly (power is proportional to the square of blade length, roughly).

We also seem to have hit a price point where there is ample capital to invest in projects of all scales. Add in the flexibility of gas to deal with varying demand and the you have a better market to invest.

As with any mass manufacturing technology, we climbed S-shaped learning curves. There is a substantial literature; search for "learning {curve|rate} {wind power|solar pv|renewable energy}"[1]. A simple explanation to the concept [2]. For solar, it's mostly silicon manufacturing techniques, a sort of distant cousin of the techniques used to manufacture your laptop's CPU. For wind it's that but also design (you want VERY tall towers geared for maximum efficiency at somewhat below median winds IIRC) and especially construction. The techniques for getting giant blades to site are not trivial; the techniques for building off-shore turbines are embodies in some of the world's more sophisticated engineering companies.

[1] https://scholar.google.com/scholar?q=learning+curve+analysis... [2] https://www.energytoday.net/economics-policy/credit-renewabl...

You have to realize that 15 years ago solar was really exotic and 20 years ago close to nonexistent. Wind energy existed, but not in its modern form.

The prices you payed 15 years ago was the prices of an exotic technology produced in small quantities. The price you pay today is a mass product.

No expert, but I would imagine that decreased cost to manufacture (as factories are getting built and established), improvements in its total output (and generally the technology behind them), and longevity/durability.