Britain, a land of cloudy skies and reliable rain, is fast becoming the hottest spot in Europe for many investors in solar energy. Germany is overcrowded with panels. A sudden end to subsidies killed Spanish solar. A sluggish economy is dragging on Italy.

But the U.K. has benefited from a combination of stable subsidies since 2011, public support for solar, amenable planning authorities and creative finance.

In 2010, there were under 100 megawatts of solar capacity in the U.K.—barely enough to power the homes of a modest town. Now, there is between 3.2 and 4 gigawatts. This year, market-research firm Solarbuzz projects that the U.K. will overtake Germany as Europe's largest installer of solar panels, putting in 6% of the world's new solar.