Photovoltaic systems are very reliable, and batteryless systems are nearly maintenance-free. Adding batteries increases maintenance costs and responsibilities. Flooded lead-acid batteries—still the most common energy-storage medium—require checking electrolyte levels periodically to see if distilled water must be added. Sealed batteries require no electrolyte checks, but still have connections and terminal posts that must be periodically inspected and cleaned of corrosion, as is the case with any battery.
Batteries also come with environmental issues that you may want to consider—mining, chemical manufacturing, and spills. The owner’s responsibility continues beyond the batteries’ life cycle, since this potentially hazardous element must be disposed of properly or recycled into new batteries. There are battery recyclers that reuse or recycle all parts of a battery: lead plates, electrolyte, and plastic case.
A consequence of adding batteries to a grid-tied system is a drop in overall system efficiency. A batteryless grid-tied inverter will be from 90% to 97% efficient at turning the available PV input power into grid-quality output—the bulk of most tested inverters hover around 95% efficient. Introducing batteries drops the inverter efficiency to about 92%.
In addition, there are battery inefficiencies. A lead-acid battery is about 80% efficient, with 20% of the energy wasted as heat during the battery’s chemical reactions. Better efficiency can be had by charging and discharging a battery slowly; quick charging and discharging means lower efficiency. But once the battery is full, almost all of the energy from the inverter is directed to the grid or loads, although a little energy will be used to keep the battery at float level (full).
With the inverter sending energy to the grid, the main efficiency difference between battery-based and batteryless systems is how well the inverter processes energy from a renewable source and delivers it to a load. When the grid is down, the battery efficiency comes into play during the cycle of charging and discharging the battery to power your backed-up loads.
Even sealed batteries require maintenance and have a shorter life-span than most
other components in a system, which come at an efficiency and economic cost.