But that's less interesting than what just happened in the U.S. 

To appreciate what's going on there, you need to understand the capacity factor. That's the percentage of a power plant's maximum potential that's actually achieved over time.

Consider a solar project. The sun doesn't shine at night and, even during the day, varies in brightness with the weather and the seasons. So a project that can crank out 100 megawatt hours of electricity during the sunniest part of the day might produce just 20 percent of that when averaged out over a year. That gives it a 20 percent capacity factor.

One of the major strengths of fossil fuel power plants is that they can command very high and predictable capacity factors. The average U.S. natural gas plant, for example, might produce about 70 percent of its potential (falling short of 100 percent because of seasonal demand and maintenance). But that's what's changing, and it's a big deal. 

For the first time, widespread adoption of renewables is effectively lowering the capacity factor for fossil fuels. That's because once a solar or wind project is built, the marginal cost of the electricity it produces is pretty much zero—free electricity—while coal and gas plants require more fuel for every new watt produced. If you're a power company with a choice, you choose the free stuff every time. 

It's a self-reinforcing cycle. As more renewables are installed, coal and natural gas plants are used less. As coal and gas are used less, the cost of using them to generate electricity goes up. As the cost of coal and gas power rises, more renewables will be installed. 

The virtuous cycle has begun.