YOUR GUIDE TO RESIDENTIAL SOLAR PANELS
HOW DO THEY WORK
The key component in any photovoltaic (PV) system is a solar panel- a structure that can convert the sun's radiated energy directly into electrical energy. It consists of multiple electrically interconnected PV cells mounted together in a support frame.
The reason the panel contains many cells is an individual single-junction PV cell can produce not more than 0.65-0.70 VDC regardless of its size or technology. To generate higher voltages a number of cells have to be connected in series. The interconnected cells are usually mounted together on a frame and covered with transparent glass on the front side to form a flat plate panel (also called module). There are also flexible panels called solar shingles, which use thin-film cells laminated to the shingle's surface. Just like the cells, PV modules can be series/parallel connected into arrays of different sizes to produce higher voltages, currents and power levels. Series connection results in a higher voltage, while parallel connection produces more current. Power, being a product of voltage and current is increased with both schemes.
EFFICIENCY AND OTHER CHARACTERISTICS
Choosing the right panels for your home requires understanding of their characteristics.
In US the PV modules have to be UL-1703 listed and tested according to the requirements of IEC Standards 61215 or 61646.
From a technical standpoint, the electrical characteristics of the PV power modules are basically the same as those of the cells, only scaled up in voltage and/or current according to the number of the connected cells and the connection scheme. Their published specifications typically include peak power, open circuit voltage, short circuit current, maximum power voltage, maximum power current, efficiency and volt-amp curves.
Commercially manufactured solar panels for home use are usually advertised by their peak power (also called rated capacity). Unfortunately, this rating is almost useless or even misleading, because it represents just a potential power the module can generate at certain ideal Standard Test Conditions (STC) adopted by the industry. STC are 1000 watts per square meter solar irradiance at 25 oC cell temperature, air mass equal to 1.5, and a certain standard spectrum. This is basically the level of sunlight at the equator at noon. In other geographical locations you can probably get such irradiance for a short period of time on a bright summer day at noon with the panel directly facing the sun (i.e. tilted according to your place's latitude). Of course, the power output changes as the sun moves across the sky. For a more detailed analysis of this process see solar energy diagram. Actual power generated by a PV device depends on the sun's illumination, temperature, operating point and other conditions.
This diagram provides an example of current vs voltage output of a PV module. On this plot, the vertical axis refers to current and the horizontal axis refers to voltage. The curves vary depending on the sunlight intensity, and the manufacturers often provide I-V curves at various intensities.
I-V characteristics of Mitsubishi 185W module PV-UD185MF5.
Open circuit voltage (Voc) is the voltage a module can generate under no load conditions. This is not a very useful parameter since under such condition no electric power is transfered to the load.
As the load current increases, the output voltage of the device begins decreasing in a nonlinear fashion. At some point a certain maximum output current, called short circuit current (Isc) is reached.
Since power is a product of voltage times current, it equals zero at both Voc (when current is zero) and Isc (when voltage is zero). The maximum output is generated somewhere between these two points, usually at the knee of the I-V curve. In this operating point the solar panel reaches its maximum efficiency in converting sun's radiated energy into electricity (see: efficiencies of various PV technologies).
The voltage and current at the peak power point at the STC are referred to as Maximum power voltage (Vmp) and Maximum power current (Imp) respectively. Today's PV inverters/chargers can perform MPP tracking to extract the maximum watts available from an array.
For characteristics and costs of commercially available PV modules see our review of solar panel cost and ratings.