In the past these two terms were considered synonyms. Nowadays, due to the development of technological equipment and measuring instruments, metrology has differentiate their meanings.
Accuracy: indicates how a given measurement is close to the real value.
Precision: refers to the closeness between independent measurements.
Here’s a very useful image for easily understand the meaning of that terms. Imagine a target with many arrows stuck in the same area near the center (upper right target). The accurate measurements are represented by the arrows stuck near the center in a scattered way (see the concept of measurement “dispersion”).
The accurate and precise measurements are represented by the arrows stuck close together in the middle of the target (lower right target).
Now we are going to highlight the importance of accuracy and precision of solar radiation sensors.
The accuracy concept applied to solar radiation measuring devices
Physicists and those involved in solar energy business know that the Sun irradiates the Earth’s surface with a broad spectrum of rays, from ultraviolet to far infrared.
The pyranometer is the instrument that measures with the highest accuracy the amount of energy penetrated within Earth’s atmosphere. It can measure from 250nm to 2400nm (some go up to 2800nm). When the pyranometer has a good quality, we have also a precise tool.
That is however a relative concept.
In fact the meteorology looks for the energy generated from the entire solar spectrum and received at a given place of the earth’s surface, while silicon photovoltaic cells don’t need the broad spectrum. Photovoltaic cells are able to convert only part of the solar spectrum into electric energy: this part goes from 330nm to 1100nm.
In figure 2 you can see the spectrum area convertible into electrical energy. As you can note from the spectral energy analysis diagrams, the efficiency of the photovoltaic conversion varies as a function of the spectrum wavelengths read by the solar cell.
Figure 3 represents the only portion of the spectrum convertible into electricity by a silicon cell.
Figure 4 shows how the pyranometer measures the entire solar spectrum; this is obviously not an accurate value of the amount of solar energy effectively convertible into electrical energy.
Trial measurements ( at Lat.45 -46 ° N) made with a “secondary standard” pyranometer and silicon cell solarimeters have demonstrated that the solar spectrum actually read is effectively narrower compared to the one represented in the figure above. In fact, except for sunrise and sunset, the values given by the two instruments didn’t differ much.
From our frequent observations we can nonetheless affirm that the solarimeter is the most accurate instrument for measuring the amount of solar energy convertible into photovoltaic energy.
Solarimeter is the reference tool for measuring the amount of energy that can be produced by a photovoltaic system.
Making a parallel with the concept of benchmark existing in the financial world, we can say that a solarimeter is the proper benchmark for the investment fund called photovoltaic system.