Enough unused roof space exists on commercial buildings in the United States to install 145 gigawatts of new solar capacity — nearly double the country’s current total solar capacity, according to a new report by the energy research firm Wood Mackenzie. That is enough to power 28 million homes.

The report, done in collaboration with Station A — a clean-energy marketplace connecting building owners and renewable energy developers — found that just 3.5 percent of commercial buildings in the U.S. have solar panels on their roofs. Another 1 percent are attached to solar projects located off-site. After accounting for buildings too small or that use too little electricity to make solar a worthwhile investment, analysts said 70 percent of commercial buildings in the U.S. — or 600,000 sites — are potential targets for solar projects.

Analysts argue that despite this potential, commercial solar projects come with major logistical and financial challenges for businesses. Utility solar has the benefit of scale to lower costs, and residential solar often has financing opportunities available, but commercial solar typically has neither, according to Greentech Media. Analysts said the Covid-19 pandemic further complicates the picture, with Wood Mackenzie estimating that the U.S. commercial solar industry will grow just 2 gigawatts per year between 2021-2025.