The Solar Design Book

Solar Battery Systems

Battery systems are more expensive than grid-connected systems because of the batteries. Batteries are expensive to buy and to maintain and can only store so much energy. After they are charged, additional energy generated by the array cannot be stored. Even so, the life-cycle cost of a solar battery system is less expensive than running a generator constantly.

Small battery systems, such as emergency phones along the highway where the battery is infrequently used, can be 100% solar powered. Larger systems, such as a cabin or house, frequently use an alternate source of power, such as a standby generator or wind turbine. The synergy between a generator and the solar system means that the solar system does not have to be as large and the generator does not have to be run very often. This can provide an overall saving in the initial and life-cycle cost of the total system.

Battery banks are sized based on how many days they supply power. Battery lifetimes are shortened by being left discharge, so the time that they supply power is usually short, 3 to 5 days. The time is based on the local climate. If there are long periods of cloudy days, the time is longer. Here is where the standby generator comes into play. It can reduce the days that the batteries have to supply power and thereby reduce the cost of the battery bank. The alternative is to do without power when the batteries are discharged. It is very expensive to provide battery power for the longest possible period of cloudy days.

The array is sized based on the time available to charge the batteries. If there is typically only one day of sun in 5 days of clouds, the array has to charge the battery bank in one day. This makes the array large and expensive. If three days are available, then the array is lower cost. In designing the system, partially sunny days can be counted. Climate data is available on the internet, see the links section. The Solar Design book goes into more detail and provides examples.

Maintenance is an important part of a battery system, see the battery section. Usually flooded cell lead-acid batteries are used which require being topped up with distilled water on a regular basis. So called maintenance-free batteries are available that eliminate this task, but the initial cost is higher. Valve regulated lead-acid batteries are sometimes required by the building department for battery-backup grid-connected systems. Battery terminal corrosion is a frequent cause of failure in a battery system.