Energy Storage

Rechargeable batteries can be used to store electricity from a photovoltaic system. This type of storage system consists of a photovoltaic power source connected to a battery bank via a charge controller and inverter. Lead acid batteries are the most common type of battery associated with photovoltaic systems because of their relatively low upfront costs and high availability. Lead acid batteries have an energy density of 110-140 kJ/kg, a charge/discharge efficiency of 65-90 percent and cost $150-200 per kWh ($45 to $55 per MJ). Lead batteries used in off-grid applications should be sized for three to five days of capacity and should limit depth of discharge to 50 percent to minimize cycling and prolong battery life. Newer batteries can be deep discharged for over 25,000 cycles.

Excess electricity from photovoltaic systems also can be sent to the transmission grid where it can be used to meet existing demand or temporarily stored for later use. Grid-tied electrical system policies often give photovoltaic system owners a credit for the electricity they deliver to the grid. This credit is used to offset electricity provided from the grid when the photovoltaic system cannot meet demand. Where there is net metering, the credit is equivalent to or greater than the cost of electricity to the consumer.