Solar installers have many different options when it comes to batteries, but they aren’t one-size-fits-all. Contractors must be careful not to under- or over-size an energy storage system.

According to CS Battery Co.’s senior applications engineer, if the solar battery bank is undersized, its lifespan will be reduced because the daily depth of discharge will be deeper than necessary. If undersized, the battery bank also may not be able to supply enough energy to support the load.

“We recommend limiting DOD (depth of discharge) to 50%, but the lower the DOD, the better the battery life,” The engineer said.

On the other hand, if the battery bank is oversized, the PV panels may not be able to fully recharge the batteries.

“Depending on how the backup charge source (generator if available) is set up, a large battery bank can be deeply discharged after a few days of cloudy conditions,” The engineer said.

If the batteries are deeply discharged, the PV panels will have a large deficit to replenish and may not be able to catch up. An oversized solar battery could also lead to chronic undercharging and poor performance and lifespan.