APPLICATIONS

When planning an off-grid solar electric power system (one that isn’t connected to a larger power grid), the output must be matched closely with the load in order to provide sufficient power without considerable waste. Although the price of photovoltaics has dropped by almost fivefold in my lifetime, it is still quite expensive.^[1] Good planning and system design ensure that you can provide sufficient power without having to buy more PV than necessary.

The power output from photovoltaics is directly related to the insolation level. Although seasonal and annual average insolation levels for most major U.S. cities are available on the Internet, cloud cover and other weather effects can be extremely localized depending on the topography. Thus, the data for large cities isn’t always the same as the data for the smaller towns in its vicinity. In addition, this data is not available for every part of the world. While accurate predictions of power output are important for grid-tied solar electric systems (in which the grid is used as ‘storage’ for any excess electricity produced and drawn upon for any electricity shortfall), this mainly impacts the return on investment expectations rather than the system sizing.

PV power systems are the only devices currently available for generating electricity without any moving parts. This makes them brilliantly simple and easy to care for, which is a real benefit for homeowners who do not want to spend their weekends greasing bearings and performing regular system checks. But without any maintenance needs or means for visual inspection, it is easy for system faults to go undetected for quite some time.

Large commercial PV installations typically have sophisticated sensors and monitoring software that can detect system faults and activate an alarm when maintenance is needed. These features are usually too costly for residential sized PV systems. The central component of these monitoring systems is an insolation sensor whose output is compared with system power production. When the power production strays significantly from what would be expected given the insolation, an alarm is triggered and maintenance checks can be performed. Although you may not need a fully automated, integrated monitoring system, some means for determining your PV system’s efficiency enables you to perform maintenance only when it’s necessary and to have peace of mind at other times that everything is functioning properly.