Solar panels performance is tested by their energy output. When the photovoltaic cell on the panel absorbs the sunlight it generates DC power and the wattage produced is the based on the formula of current x voltage. Now, when there is maximum sunlight exposure then the generation depends on the resistance of the electrical load. The resistance is measured in Ohms according to Ohm's law. Therefore to measure the voltage output and performance of the solar panels you need to install resistors of different values. Each resistor is connected to the solar panels and the output is measured through a multimeter.

To measure the performance of the solar panels it is essential to find the positive and 

negative wires from the panels. These locations are normally imprinted with signs as in batteries. You can clip the multimeter to both wires of the panel and record the reading. The output is shown in voltages and that is the rated efficiency of the panel. Solar panels are available in 250 watts and 185 watts. But, the output is not found to be same as claimed by the manufacture. 

This is due to transmission loss attributed to several factors.

To get better performance you need to install quality solar panels. The installed solar panels should generate the required amount of electricity for the purpose. The installation should be conveniently made to the source of sunlight for maximum exposure. If necessary you may add few more tools for getting the advantage of sunlight. The panels should be durable and its longevity should be around thirty years. 

The solar panels should be tested for its electrical performance. Proper cleaning is necessary at regular intervals for better output. All the information is available in the internet and a thorough research should be made before buying the solar panels.

The installation of the solar panels plays an important role in its output. The difference in out put can be distinguished by installing the panels above sea level and with a tilt towards the equator. Therefore, the appropriate location should be chosen for maximum result. When you buy a solar panel it becomes a hefty capital investment, thus, adequate guarantee and warranty should be given by the manufacturer. To get the best output ensure that the materials used in the solar panel and its proper configuration with the ambient conditions. When you compare between two manufacturers consider the minimum warranted power rating.