When contemplating the use of a solar electric photovoltaic system, it is important to assess how much energy in theory the system can produce according to location, orientation and plant conversion efficiency. Employing a performance monitoring system is a must for being able to account for the amount of energy produced by a system in real time, and to ensure the forecast conversion efficiency will remain intact over its service life. We are all familiar with our residential electric meter used by the utility company to record and bill us monthly the kilowatt-hours consumed. Over the course of a year, these bills can be compared to determine monthly consumption. While this scenario illustrates usage consumption, it is different for monitoring production with PV systems. A meter is also used to measure the energy produced, but instead of a monthly basis, we are interested in the amount of energy produced during short time intervals - perhaps every hour or every five minutes. The recording frequency requires more sophisticated data loggers than the meters installed for residential systems. Data loggers feed data into a database that can be archived for use at a later time. They also have communication interfaces through Ethernet or serial ports that allow computers to access the data.