Clean-tech innovators like biofuel and solar energy companies have growing footholds in San Diego. But the sector also faces difficulties that could sap momentum.

Given the realities of intermittency and the need to back up renewables with on-demand systems, natural gas presents one of the cleanest ways to fill in the gaps.

The mayoral candidate wants to power all city and school buildings with solar energy in five years. For it to work, details matter. And, for now, Filner doesn’t have them.