Solar irradiance measures

The surface of the Sun, called photosphere, is at a temperature of about 6000K and its behavior approximates that of a "black body". With this consideration in mind, it is possible to calculate the total power emitted by the sun: it is approximately equal to 9.5 x 10 ^ 25 W.
The total power emitted by the sun is not composed of a single wavelength, but is composed of many wavelengths.

Moreover, not all the radiation emitted by the Sun arrives on Earth.
The average energy radiated by the Sun per unit time on a normal surface outside of the Earth's atmosphere, is called solar constant and its average value is about Ics = 1353 W / m ^ 2.
However, the presence of the atmosphere has several effects on the radiation that reaches Earth's surface.

The radiation undergoes a power reduction due to absorption, scattering and reflection in the atmosphere. The spectral content of solar radiation also changes due to increased absorption or scattering suffered by the long wavelengths. Finally, there are components of diffuse and indirect radiation.
The result of all this is a value lower than the total radiation reaching the Earth's surface, next to 1000W / m ^ 2, and a different spectral distribution.
The electic production of a solar cell depends on several factors.
First of all, a solar cell does not respond consistently to all frequencies of the incident radiation. The efficiency of a silicon cell is maximum in the range of frequencies of visible light.
Secondly, the electic production of a solar cell, and consequently of a PV system depends on the radiation incident on its surface.

There is another effect that influences the performance of a photovoltaic system: the temperature. Like all the other tools in semiconductor solar cells are sensitive to temperature. An increase in temperature reduces the "band gap" of a semiconductor, thus affecting most of the parameters of semiconductors. High values of the temperature causes a reduction of electric production of a photovoltaic system.
Since the energy (and economic) production of a photovoltaic system is a function expected of these factors, a decline in energy production, that's not expected out of this function must be interpreted as a synonym for failure or breakdown, for which measures must be taken.

Then, to know at a certain time how much energy a photovoltaic system should produce, you need to know how much energy is reaching the surface of the photovoltaic modules at that moment. Rather, it would be useful to know how much solar radiation (the range of the spectrum to which the silicon cells are sensitive to: 300 nm - 1100nm) arrives on the photovoltaic modules, so we know how much energy a PV plant should produce at any time of the day.

The solar radiation sensors are able to detect how much solar radiation reaches the site where they are installed.
There are essentially 2 types:

SOLARMETER

A solarmeter is an instrument used for measuring the solar radiation. It uses the photovoltaic effect to measure the amount of solar radiation reaching a given surface.

A solarmeter using the photovoltaic effect has the same behavior of a photovoltaic system: it produces an electrical signal as a function of incident light, especially responds to visible light and its response depends on the temperature of the cell.
More specifically solarmeter with a silicon cell is able to capture light waves with a spectrum range of approximately 330nm to 1100nm.
In order to obtain a measure not influenced by the temperature, the values measured by a solarmeter using the photovoltaic effect must be corrected according to the temperature of the photovoltaic cell. This measurement can be done thanks to a thermocouple, and the fix factor should have a level of precision is not easy to achieve.

PYRANOMETER

The pyranometers are instruments used to measure the global radiation on a surface (direct and diffuse radiation). The operating principle is generally based on the measurement of the difference in temperature between a clear surface and a dark one. A dark surface can absorb most of the solar radiation, while a clear surface tends to reflect, and so it absorbs less heat. This temperature difference is measured using a thermopile. The potential difference that is generated in the thermopile due to the temperature gradient between the two surfaces, allows to measure the value of global solar radiation incident.

A thermopile is usually composed of thermocouples connected in series. A thermocouple is a junction between two different metals used to measure the temperature difference between two points. A thermocouple produces a potential that depends on the temperature gradient.
The response of a pyranometer of this type may cover the entire range of wavelengths of the solar spectrum: 300 nm to approximately 2800nm.
Note that the spectral range detectable by a pyranometer is wider than what can be measured by a solarmeter with a silicon cell. For this characteristic using a pyranometer to test the proper operation and performance of a PV system could lead to believe that the plant is not working properly. With the use of a silicon-cell solarmeter instead, the values given are synchronized to the plant, since the portion of useful spectrum for the operation of a photovoltaic system is the same as that measured with a solarmeter. In addition, the response of a pyranometer is much slower (on the order of tens of seconds as you can see in the image at left) than the response of a silicon-cell psolarmeter (from fractions of a second per second).