Different type of monitoring technology used in Rooftop solar system

Either reference cells or pyranometers are used for solar irradiation measurements. Under irradiation, reference cells generate specific voltages at specific irradiation levels and temperature. A data logger records the voltages that are calibrated under the reference spectrum at STC according to IEC 60904-1.

Referance cell: These encapsulated small solar cells with integrated thermocouples should be made of the same photovoltaic materials as the cells incorporated into solar PV modules used for SHS or PPS technologies. Additionally, encapsulant, backsheet and glass should be very similar to the ones used for the installed solar panels. Furthermore, the reference cells have to be mounted at the same orientation and inclination as solar PV modules. If this is the case, the reference cells measure the irradiance that is available to the solar module for conversion into electricity.

Pyranometers consist of two glass domes and a collection of thermocouples mounted on a black carbon disc (thermopile), which generates a specific voltage at a specific temperature. When sunlight strikes the pyranometer, the thermopile heats up, with its output voltage being a linear function of solar radiation intensity. Due to the glass dome, sunlight impinges on the thermopile from all angles. The use of double glass domes increases the thermal inertia. For detecting diffuse irradiation, a ring is used to block the direct radiation.
Reference cells do not measure the whole solar spectrum like pyranometers in meteorological stations. Therefore reference cell measurements are less accurate when used for efficiency calculations. Performance ratio (PR) or performance index (PI) calculations are up to several percentage points more accurate using pyranometers, but due to high cost it is inappropriate for small solar house system

Data logger
Data loggers for solar PV applications record solar irradiation as well as the solar modules’ output voltages and currents, and store these together with dates and times. The data logger either is equipped with a USB port, or more sophisticated devices collect the data from the inverters and the reference cell to act as an Internet gateway to send the data in near real time for archiving at the inverter manufacturer’s servers, where they are accessible via a web browser. Some manufacturers also provide equipment for local monitoring.