Cal. Fin. Code § 32201

“Alternative energy system” means any device or combination of devices which conserves or produces heat, process heat, space heating, water heating, steam, space cooling, refrigeration, mechanical energy, electricity, or energy in any form convertible to these uses, which does not expend or use conventional energy fuels, except when such conventional energy fuels are used as a back up energy system for such alternative energy system or in conjunction with an alternative energy system.