In 2011, residential solar system installers paid a little over $1.80 per watt for solar panels in both Germany and the United States. In Germany, installers added $1.20 to the cost of the solar panel to complete an installation. But in the U.S., they tacked on $4.36 per watt, more than three times as much.

A report released this month by Lawrence Berkeley National Laboratory explains why.

The most obvious difference between the United States and Germany is the total amount of solar power installed in each country—there’s five times as much installed in Germany.

The study concludes, however, that the learning curve isn’t enough to explain the price disparity—it might account for only half of it. Instead, based on a survey of U.S. and German installers, it seems that there are some fundamental differences in the U.S. and German markets that could keep prices higher in the U.S.—unless something is done to address them.