Although trivial, the question that titles this article reveals some facts that deserve attention. What is important in the photovoltaic module: more power or more efficiency?
The answer is: both things are important and are closely related. Photovoltaic modules are increasingly powerful due to increased efficiency. One thing leads to another, naturally, the most powerful modules are also the most efficient.
The power of the photovoltaic module
Known as peak power, the maximum power of the module is that stated in the data sheet. The unit of peak power is the peak watt (Wp).
The watt-peak is just a variation of the watt unit that is useful to highlight that we are referring to the maximum power that a photovoltaic module can provide.
During operation, the module's power varies depending on the temperature and mainly the irradiance (W/m2) from sunlight.
Irradiance is the unit we use to quantify the intensity of sunlight – it is a measure of light power per area.
The following figure shows the current and voltage curves (curve IV) and power and voltage (PV) of a photovoltaic module and illustrates its behavior depending on solar irradiance.
The efficiency of the photovoltaic module
Efficiency is the ratio between the energy of sunlight falling on the surface of the module and the electrical energy produced by it. If all solar energy could be transformed into electricity the efficiency would be 100%. Since this is not possible, efficiency is always a number less than 100%.
The best commercial photovoltaic modules currently have efficiencies around 20%, which may be slightly below or above that. In other words, around 20% of the energy received from the Sun will be transformed into electrical energy.
The efficiency of the module varies according to its operating conditions: irradiance, temperature, light composition, angle of incidence of light.
The photovoltaic module catalog informs the efficiency in the STC condition (standard test conditions), which has the following characteristics: irradiance of 1000 W/m2, AM1.5 solar spectrum and temperature of 25 oC (from cells).
The AM1.5 spectrum corresponds to an internationally standardized light composition, which is always used in the characterization of photovoltaic modules.
How are power and efficiency measured?
Photovoltaic modules are subjected to a flash light test when they leave the production line. The flash of light reproduces the composition of sunlight with a short-duration light pulse, with a maximum intensity of around 1000 W/m2.
In factories, the modules are tested one by one and are classified according to their power and efficiency.
The test result will determine the power, efficiency, maximum voltage (open circuit) and maximum current (short circuit) characteristics shown on the product label.
In the case shown in Figure 4, the IV and PV curves allowed extracting the following characteristics of the photovoltaic module in STC.
More power or more efficiency?
Efficiency and power are closely related. The increase in the efficiency of photovoltaic cells has allowed the production of increasingly powerful modules. So, as a rule, a high efficiency module will also be a high power module.
Module efficiency may vary depending on model or manufacturer. Modules of the same power with different efficiencies will have a difference in their physical size. Both will produce the same energy, but the more efficient module has a smaller area.
In the example below, we observe the characteristics of two photovoltaic modules of the same power chosen randomly from the market, from different manufacturers.
The first has an efficiency of 20,4% and its area is 2,209 m2. The second, with an efficiency of 20,7%, has slightly reduced dimensions, with a total area of 2,173 m2.
In summary, we can conclude that efficiency and power go together. When we talk about modules with the same power, although both are capable of generating the same energy, the module with greater efficiency (due to its reduced dimensions) can be advantageous in large projects or even in small projects with large area restrictions.
