Yep ... but it is not just any battery back-up. It replaces a battery that provides unlimited load and unlimited capacity. The only somewhat practical alternative I can think of is a personal fossil fuel generator as a tertiary back-up. @TheTalkingMule trust me, I thought about ways to cut out the utility entirely using a PHEV as my tertiary back-up but I decided that would be a case of cutting off my nose to spite my face. I'm much happier with the idea of having two EVs instead of one EV and one PHEV; my lack of expertise is not exposed; and weeks like last winter with little sunshine -- albeit unusual -- do not have to be big, big deals. The few cumulative net days a year that I could be burning gasoline in a PHEV instead of coal at the plant was not worth all the costs and risks.