Faris Alhamdany,

there is SI unit system, to control and ensure the consistency and comparability across different region in the world.

similarly to ensure consistency and comparability across different PV modules in different labs around the globe we need a standard system to compare with each other.

So, we can say, these are the SI unit system for Solar energy engineers.

good luck

The STC's (AM1.5G, 1000 W/m2, 25°C cell) is only used as a set of conditions making it easier to compare solar cells of different designs, materials, or variations in processing. This way, you can make new solar cells, and compare them to already established technologies, or optimize your existing production of solar cells in a factory.

For instance, in any silicon solar cell manufacturing plant, the cells are quickly tested under these conditions to "bin" them by how well they perform. This way, when integrated into photovoltaic modules, the best cells are interconnected from one bin, and lower performing from another bin. This way you make sure you don't "waste" good performing cell. In principle, you try to fabricate them all identically, but on an atomic level, small variations (not necessarily possible to control) can influence the performance of your solar cell.

But why then AM1.5G (Air Mass 1.5 Global)? It's the most representative average spectrum considering certain latitudes where the STC's were defined. You don't often get AM1.0G, and in many countries, most of the year you don't even get it every day.

Why then 1000 W/m2? That's the integrated power density of the AM1.5G - you have to specify both explicitly, as the spectral distribution (how many blue photons, how many green, etc.) is not given by the power density. In principle you could achieve 1000 W/m2 using a red laser, but then the solar cells using photoabsorbers with a larger bandgap wouldn't respond at all, yet they would actually respons outside. Therefore, it's critical to make sure you have both the irradiance and spectrum defined.

Why then 25°C cell temperature? This you could argue should in principle be higher, because the NOCT (Nominal Operating Cell Temperature - the actual temperature of the solar cell under normal operating conditions) is much higher, easily 45-50°C, and solar cells perform worse under higher temperatures. But, using a higher-than-room-temperature test condition makes it more cumbersome for laboratories to test, and also for the factories, as you all of a sudden have to heat up your solar cell and wait for the temperature to be stable. Furthermore, the NOCT is not the same everywhere in the world. That's why solar cells in Spain may be subject to a lot of irradiance, but their efficiencies are lower, whereas the efficiencies of solar cells in Trondheim (Norway) are notably higher, but there's not as much Sun.

In summary, it makes it easier to compare performance of different types of solar cells, and detect small variations in manufacturing from which you should bin your cells differently.