People with rooftop solar systems almost always ask about using batteries to store their energy for nighttime use. Tesla recently announced an off-grid battery storage system, SolarCity announced a home backup power system, and a dozen other companies already have comparable products on the market. These announcements have re-energized (grin) the hype and excitement in this new product category. But are these systems practical and will they save you money?

For the vast majority of homeowners, the answer is “no” to both questions. Only if you have a completely off-grid application -- such as a cabin in the woods -- can the new generation of lithium ion battery backup systems combined with solar be more practical and cost effective than traditional lead acid batteries. If you are currently on the grid, energy arbitrage makes no sense at all: with current electric rates it is simply not cost effective to generate your own energy with solar during the day and use it at night (or buy cheap electricity at night and sell it back during the day). In a nutshell, the life cycle cost of the batteries outweigh any potential savings. If you are looking for backup power during a blackout, a generator is still the cheapest way to power your house for a few hours or a few days.

Nevertheless, there is no doubt that batteries will continue to improve, and control electronics will get cheaper and more efficient. Besides the current off-grid and backup power applications of home energy storage, there will come a time in the future when home energy arbitrage (buy low and sell high) and grid support services will make economic sense. If you’re curious about the practicality of home battery storage systems, please Listen Up to this week’s Solar Energy Show.