Solar Panels Query

[CharlesHarris]

Photovoltaics produce DC electricity from sunlight and are ideal for maintaining battery banks against self-discharge during extended periods of non-use. Properly designed solar arrays can provide sufficient current to run communications equipment directly or to maintain battery banks at operating capacity during periods of heavy load.

A photovoltaic panel producing a current of from 1% to 1-1/2% of battery capacity is recommended to compensate for self-discharge in storage batteries and can maintain them on "float" indefinitely. No charge controller is required because maximum current doesn't reach the threshold to evaporate electrolyte after full charge is reached. A Shotky diode is wired in series with the panel to prevent battery discharge at night when panel voltage drops below battery voltage. This method of float charging is recommended for storing vehicles or motorized equipment which are not used at least monthly.

Additional panels may be added to the system to replace energy consumed daily by the loads and to compensate for energy used by charge controllers, etc. To avoid overcharging batteries during periods of non-use, a charge controller is required to disconnect the panels from the batteries when they reach a fully charged state.

While voltage is nearly constant, the charging current produced is dependent upon incident light in watts per square meter under operating conditions. Under ideal conditions (full sun perpendicular to the panel in clear sky at 20 degs. C) this figure is about 800w/m2.. A 50w panel produce 1,500+watt /hours of energy weekly under full sun or more under ideal laboratory conditions, but only a fourth as much during a cloudy mid-Atlantic winter.

To properly size a photovoltaic system adequate to operate continuously, total all loads in watts, multiply by the average daily use in hours, plus a 30% allowance for DC line losses. Subtract the average daily energy produced by back-up generators and divide the total by the product of the module power rating times the “Area Factor” which compensates for local sun conditions (4 - in the US mid-Atlantic states) to yield the number of panels required.