Catalog Pg. 4

Introduction To Solar Electricity

A PV system does not run on a scheduled daily basis. The sun may shine for two days and produce 300 watt-hours and then not produce any electricity on a third cloudy day. The load can also be distributed unequally. No energy might be used for two days, then three days of charging might all be used on the third day. A PV system produces an average amount of electricity dependent on the average amount of sunshine. When a PV module produces an average of 100 watt-hours per day for ten days, it stores 1000 watt-hours in the battery. In this same ten day period no more than 1000 watt-hours should be used to power the loads.

A remote site home system consists of a PV array, a charge controller, a battery bank, a DC fuse box, an inverter, and an AC fuse box. (See Figure 2, right) Functionally, the PV array and charge controller together are no more than a simple battery charger that uses sunlight as its energy source.

The PV array produces electricity when the sun shines. The charge controller regulates the flow from the array to the battery bank. When the battery bank is low the charge controller feeds all of the electricity from the array to the batteries. As the batteries approach a state of full charge, the charge controller tapers the supply of electricity to prevent overcharging of the battery. At night it prevents a reverse flow of current from the batteries to the array. The battery bank stores the electricity as low voltage DC, normally at 12V or 24V. The electricity is distributed through a DC fuse box to power low voltage DC appliances. The batteries supply electricity to a device called an inverter which changes the low voltage DC to 120V alternating current and then sends it to an AC circuit breaker box.

The charge controller and inverter are purchased in a specific voltage and a specific size based on the proposed Battery Box

performance. The batteries and PV modules, however, are modular. A PV array may consist of one PV module at 12V, or 10 modules producing 10 times the current, but still at 12 volts. Likewise, a battery bank may be enlarged by increasing the number of batteries to allow more storage at the same 12V. A small system can be enlarged at any time by simply adding more modules and more batteries.

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