Solar Module Power Characteristics
The current and power output of photovoltaic modules are approximately proportional to sunlight intensity. At a given intensity, a module’s output current and operating voltage are determined by the characteristics of the load. If that load is a battery, the battery’s internal resistance will dictate the module’s operating voltage. A module, which is rated at 17 volts, will put out less than its rated power when used in a battery system. This is because the working voltage will be between 12 and 15 volts. As wattage (power) is the product of volts times amps, the module output will be reduced. For example: a 50 watt module working at 13.0 volts will produce 39.0 watts (13.0 volts x 3.0 amps = 39.0 watts)
This is important to remember when sizing a PV system. The I-V curves as illustrated here show all of a typical possible operating points (voltage/current combinations) at a given cell temperature and
light intensity. Increases in cell temperature increase current but decrease voltage. Maximum power is derived at the knee of the curve.
Mixing Sizes and Brand of Modules
In most cases mixing dissimilar modules in the same array is not a problem. When paralleling units of different amperage ratings, however, the output of the array will simply be the sum of the combined voltages. The lower voltage units will simply begin to taper off sooner as high battery voltage is reached. If the solar modules are used for array direct power, the array voltage will be the approximate average module voltage. When series-connecting strings of dissimilar modules, however, the amperage will be approximately that of the
weakest module in the string. It pays then, to pay attention to matching the modules connected in series.