Solar Power on a Budget, Cont.

The next step is to choose the battery for this application. Batteries are rated in ampere-hours, NOT watt-hours, so divide the watt-hour capacity by the voltage - in this case, 12 - to get the ampere-hour capacity.

The closest battery size available is 10 ampere-hours. Bear in mind that this battery is only large enough for one day's operation. If there are a couple of cloudy days, the battery would be exhausted after the first day and there would be no more power until the sun came back out. For this reason, it is wise to figure in a couple of days' reserve capacity.

To obtain the required 30 ampere-hours capacity, one could buy ONE 30 ampere-hour battery, or connect TWO 15 ampere-hour batteries in parallel, or THREE 10 ampere-hour batteries in parallel.

After choosing which battery configuration is best, one must chose an appropriate solar panel, or panels. To do this, refer to the daily load of 109.6 watt-hours mentioned above. Solar panels are rated in terms of their working voltage and their wattage. Just as the loads USED electricity at a certain watt-hour rate, the solar array will SUPPLY electricity at a certain rate. How much battery charging is accomplished will depend upon the wattage rating of the panel VS. the usable daylight hours that they will be operating. Depending on the time of year and how far North your location is, the usable daylight hours could range from 2 to 8 hours. The shorter the usable daylight hours, the larger the solar array needed to maintain the battery.

For the 109.6 watt-hour system, the user based his calculations on an average 5 hour day, given the published climate data for his location. To calculate the solar panel capacity needed for a given installation, divide the system watt-hours by the number of hours of daylight available. Given 5 hours of daylight, we have

A 25 watt panel would be adequate for this job, and would allow for slight inefficiencies in the battery and the solar array. Note that TWO 12 watt panels could be wired in parallel to do the same job.

ONE WORD OF CAUTION: Gel batteries need to be charged at a 15 - 20 hour charge rate, based on their capacity. Stated another way: the charge rate should be set so that a completely dead battery will be fully charged in 15 - 20 hours. For this example, the ideal charge rate for one 10 ampere-hour battery would be

A 25 watt panel will easily charge this battery at nearly 2 amperes, quickly overheating and destroying it! This is the OTHER reason for using an oversize battery, besides the need for reserve capacity.

A 3 day reserve capacity necessitated a 30 ampere-hour battery, as calculated earlier. Consider the ideal charge rate for this battery:

The 30 ampere-hour gel battery is thus well suited for this system. The solar panel will charge it at a safe rate, even in full sun, and will keep up with the estimated daily power consumption. It will also supply up to 3 days of reserve power during cloudy weather.

ONE MORE WORD OF CAUTION: If you intend to leave a solar panel and a battery unattended for longer than it will take to fully recharge the battery, you MUST install a CHARGE CONTROLLER device to prevent damage to the battery from overcharging. The charge controller senses the battery voltage and reduces current flow from the solar panel to a safe 'trickle' as the battery becomes fully charged.

Watch for my next article on solar power systems, and a future article on choosing and caring for batteries.