California, the Golden State, may start requiring solar panels on new homes and low-rise apartment buildings built after 2020, the first such mandate nationwide and the state’s latest step to curb greenhouse gas emissions. The state has positioned itself as the nationwide leader on clean energy, pushing for more electric vehicles on the roads and fewer emissions from residential and commercial buildings. The California Energy Commission, which will vote on the regulations Wednesday, estimates they would add an average $10,500 in construction costs for a single-family home but generate about $16,000 in energy savings. The standards also include requirements around ventilation and indoor air quality. Few industry groups outwardly oppose the plan after working for years with the commission to shape the regulations. However, Republican legislative leaders said Californians can’t afford to pay any more for housing in the state’s already extremely expensive market.