Power that can be obtained from the photovoltaic panel depends on the conditions in which the panel works and depends primarily on the intensity of solar radiation and temperature. Since comparing panels working in different conditions is not reliable, two standards have been introduced to determine under which conditions measurements should be made:

• conditions for conducting tests: STC (Standard Test Conditions) specifies the type of light source with a spectrum of AM1. 5 and power of 1 000 \( \frac{W}{m_{2}} \), and the temperature of measurement is set at \( 25_{}^{o}\textrm{C} \) [1],
• conditions for PV panel tests: NOCT (Normal Operating Cell Temperature) specifies the type of light source at AM1. 5 with a power of 800 \( \frac{W}{m_{2}} \), measurement temperature at \( 20_{}^{o}\textrm{C} \) and wind speed at 1 m/s.

When designing a photovoltaic system, the parameters determined under STC conditions should be used, while NOCT conditions give a better idea of how the panel will behave under actual operating conditions in our climate.
Measurements of photovoltaic panels are carried out in specialized test chambers. These chambers are characterized by controlled parameters:

• radiation of the so-called artificial sun with AM1.5 spectrum,
• adjustable irradiance (from 30 \( \frac{W}{m_{2}} \) to 1000 \( \frac{W}{m_{2}} \),
• adjustable temperature (from \( -30_{}^{o}\textrm{C} \) to \( 200_{}^{o}\textrm{C} \)).

Fig. 1 shows the environmental simulation chamber [2]. It meets the conditions mentioned above, both STC and NOCT tests can be performed with it. The interior size is 2m*3m and 2m height. The touch screen allows you to input the parameters of the photovoltaic panel measurement conditions: such as temperature, humidity and illumination. The status of the measuring chamber is displayed on the top of the chamber. The controls, indicators, and sensor leads are shown on Fig. 2.

MHG lamps are protected by glass covers that are heated to protect against fogging. Temperature and humidity sensors are used to control the conditions in the measurement chamber. Fan vents introduce prepared air into the chamber to maintain set environmental conditions. At the rear of the chamber there are the humidifier, air heater and evaporator of the refrigeration system. In the picture you can also see the grommets for the measurement cables and the measurement table.
Each photovoltaic panel has a nameplate that shows the characteristic parameters of the panel Fig. 3.

In order to determine them, current-voltage characteristics tests are carried out. Measurements are carried out using specialized equipment, the schematic of which is shown in Fig. 4 [2].

The photovoltaic panel is placed in a chamber to maintain a constant temperature and irradiance. It is then connected to a control voltage source and the voltage and current flowing through the PV panel are measured. The whole system is controlled by a computer used to record the measurement results and environmental conditions.
A typical current-voltage relationship for a photovoltaic panel and the characteristic electrical parameters that describe the photovoltaic panel ( Fig. 5 ) are shown below.

The parameters of the photovoltaic cell obtained at a given operating temperature and a given solar irradiance are:
\( I_{sc} \) – short circuit current, which is the maximum current that the photovoltaic panel can generate,
\( I_{pm} \) – the current at which the maximum power can be extracted from the photovoltaic cell,
\( V_{oc} \) – the open circuit voltage, which is the maximum voltage that can be extracted from the photovoltaic panel,
\( V_{pm} \) – the voltage at which the maximum power can be extracted from the photovoltaic cell.
Based on the data for the graph and the determined quantities given above, the maximum power, efficiency, temperature power index, temperature voltage index \( V_{oc} \), temperature current index \( I_{sc} \) are calculated.
Here is an example of electrical parameters under standard STC conditions for BS-330=6MHB5-EL panel from Bauer Solartechnik GmbH data sheet:

• monocrystalline PV cell / PERC - double,
• PV cell arrangement 12 x 10 (120),
• nominal power at maximum power point \( P_{max} \) = 330 W,
• power production tolerance \( P_{max} \) 0 to \( 3\% \),
• operating voltage \( V_{pm} \) = 33.70 V,
• open circuit voltage \( V_{oc} \) = 40.60 V,
• short-circuit current \( I_{sc} \) = 10.35 A,
• working current \( I_{pm} \) = 9.80 V,
• panel efficiency (efficiency) \( 18.6\% \),
• operating temperature (NOCT) \( 45_{}^{o}\textrm{C} \) ± 2,
• temperature coefficient \( I_{sc} \) \( +0.050\% \)/K,
• temperature coefficient \( V_{oc} \) \( -0.290\% \)/K,
• temperature coefficient Ppmm \( -0.390\% \)/K,
• number of diodes 3x1 bypass diodes.