`` If the '' `` no upfront cost '' '' business model led to huge growth for rooftop solar -LRB- now $ 2b per year -RRB- and large energy efficiency projects -LRB- $ 6b per year -RRB- , which other opportunities are ripe for the same idea ? ''
What is the future of solar energy ? Will solar 's costs decrease ? If so , by how much ? Will its efficiency increase ? If so , by how much ?