One alternative to a solar battery is a natural gas, diesel, or gasoline whole-home backup generator. Unlike batteries, however, Project Solar does not currently offer generators.
We also cannot directly facilitate the tie-in of a generator to your system--this would need to be done through a third party. Generators will need to be compatible with Enphase equipment, as we exclusively use Enphase microinverters for our systems.
A list of Enphase-approved standby generators can be found here: Generator Integration with the Enphase Energy System
To connect a generator to your system, you'll also need a System Controller 2 (the same component used to connect a battery to your system). Project Solar systems do not include this equipment by default, so please be sure to request it specifically.
For more information on Enphase's options for generators, check out this page from their website: Enphase - Generator Support