Solar Module Specifications

• Solar modules have a variety of important specifications that you need to be aware of before purchasing your module. Solar energy image by lefebvre_jonathan from Fotolia.com

  Solar module systems are made-up of individual photovoltaic cells. The performance of the module is dependent on the performance of these cells, and that performance is summed-up on the manufacturer's specification data sheets. At first glance these specifications can seem daunting, describing Rated Power Tolerance, Maximum Power Voltage, Peak Power and Open Circuit Voltage. But understanding manufacturer specifications is the key to picking the right solar power system.

Types of Cells

• A photovoltaic module consists of many PV cells wired in parallel to increase current, or in series to produce higher voltages. Module specifications will indicate the type of silicon used, which is based on the manufacturing process. These are typically monocrystalline, multicrystalline, ribbon, or amorphous thin film silicon. Monocrystalline cells are the most common and produce the highest efficiencies of up to 12 percent, but they are also the most expensive. Though multicrystalline and ribbon silicon cells are slightly less expensive, these cells are also less efficient than monocrystalline cells. Polycrystalline or multicrystalline cells average about 10 percent efficiency, while ribbon module efficiency averages around 8 percent. Amorphous or thin film cells have the lowest cost but they average only 5 to 7 percent efficiency.

Power Output

• Peak power is the total number of watts that is available, expressed as either peak power or average power produced during one day. Rating power is done at the 77 degrees F standard temperature conditions in order that all modules are compared and rated under equal conditions. The STC is actually the cell temperature, not the surrounding air temperature. Because dark-colored PV cells absorb more radiant energy, their temperature will be higher than the temperature of the ambient air.

Module Efficiency

• Module efficiency is the ratio of output power to the input of energy from the photons in sunlight used to generate electricity. If 1,000 watts of sunlight hits a module and that module produces 100 watts of power, it will have a 10-percent efficiency rating. Rated power tolerance specifies the range within which a module will deliver its rated power at the STC of 77 degrees F. A 100-watt module with a minus 9 percent power tolerance will have a 91-watt output. The output will decrease even more at higher temperatures.

Voltage & Current

• Modules are designed to produce direct-current electricity at 12 volts, the industry standard, or 24 volts. The amount of current produced depends on how much sunlight strikes the module. Maximum power current is the total amount of current, measured in amps, produced when the module is powering a load. Maximum power voltage is the total electric power measured in volts produced when electricity is being used. High temperatures will cause voltage to decrease while lower temperatures cause a increase in voltage. Open-circuit voltage is the total voltage the system can generate when there is nothing connected to it. All the solar power system components, including the modules, inverter and charge controller, are designed to handle this maximum open-circuit voltage.