Private equity and venture capital firms are pouring record investments into rooftop solar, following a model popularized by billionaire Elon Musk’s SolarCity Corp. (SCTY) — sell power, not panels. They’re on pace to supply $5 billion this year for residential and commercial solar projects, up from $3.3 billion in 2013, according to the researcher Mercom Capital Group LLC. The funds are going to companies such as Sungevity Inc. and Sunrun Inc.