Since 2009, the price of solar energy has come down by 90 percent. That’s no accident. It’s the result of policy interventions from the US to Germany to China. As policy analyst Gregory Nemet puts it, “No one country is responsible. It was a relay race rather than a competition.” The global flow of knowledge, people, technology, and policy helped bring down the price per watt from more than $100 in 1976 to less than 50 cents today, according to this analysis from the folks at Our World in Data. If we can learn the right lessons from solar’s success,…