Smaller, Cheaper, Faster: Does Moore's Law Apply to Solar Cells? by Ramez Naam

The way the future looks depends on the rate of decline in the cost of solar power. In this article (my title is a link), Ramez Naam says that solar power is getting cheaper at the rate of 7% per year. Notice how his graph with a logarithmic scale compares to his graph with a regular scale. By the rule of 70, how many years to cut the cost of solar power in half?