Business Green: The mayor of San Francisco, Gavin Newsom, has issued a challenge to the city’s largest 1,500 businesses to install solar energy technologies, as he continues his drive to position the city as one of the greenest in the US. The mayor last week announced the creation of the Solar Founders’ Circle, a club of solar-friendly businesses that will receive free energy audits and solar assessments. The largest 1,500 companies in the city will be invited to join the group in the next few …