Sizing the solar generator is essential to ensure there is sufficient power to charge the battery and supply the loads. The solar generator must be configured in accordance with the voltage and current limits of the solar controller. In this FAQ, we use the variotrack (vt) as an example, with a maximum open-circuit voltage of 145 Vdc (note that this voltage differs for the VT65-175 and VT80-175 models. Please always check the maximum open-circuit voltage corresponding to your device, this voltage may be different than the one we use in this FAQ).
To achieve the optimum solar voltage, the photovoltaic modules must be connected in series to form a string with the desired voltage. The optimal operating voltage of the string (Vmpp) must always be higher than the battery voltage. The maximum operating voltage must always be less than 145 Vdc (because we use in this FAQ a variotrack with maximum open-circuit voltage of 145 Vdc), regardless of temperature and sunlight conditions. This value must be calculated based on the open-circuit voltage of the modules, taking into account the temperature correction corresponding to the lowest temperature expected for the installation.
For example, if we are working with a 60 cells PV module with the following characteristics:
- Open circuit voltage, Voc = 39,1Vdc (at STC, 25°C)
- Thermal coefficient for Voc, βVoc = -0.33 %/°C
For this module the open circuit voltage at -10°C (lowest temperature of your installation) will increase by 11.55% up to 43.61dc. Therefore, it is possible to install up to 3 of these modules in series for this installation, in order to respect the maximum of 145Vdc of the vt80-145.
Thus, we have a maximum of 3 standard modules (60 cells) per string in our typical configuration for 24 and 48V batteries.
Possible arrangement in series for common PV panels according to the number of cells and type
The voltage calculation is critical, as a potential voltage above 145V will damage the device and may cause fatal defects in the installation.
In terms of power, several strings are usually connected in parallel. Each string connected in parallel should be composed of the same number of modules of the same type. The number of strings connected in parallel will determine the solar charging current. The vt has a current limit: 65 Amps for vt65 and 80 Amps for vt80. This will be the maximum current that the vt is able to take from the solar array towards the battery. If the number of strings in parallel is increased, the vt will reach its maximum charging current quicker and even when solar irradiation is lower. This will also mean that there would be a bigger potential of energy that will not be used when the solar irradiation is high.
As soon as the voltage limit is respected, the solar power to be installed depends on the dimensioning of the installation. Thus, it is possible to oversize the solar power to maximize the solar production, even if this means an increase of potential solar energy not used.
In terms of irradiance this could be relevant, for example in European countries, during most of the year the solar production is lower than the nominal power because of low irradiation (winter, clouds, etc.). If we take the example of our factory in Switzerland, we could have a 10% over dimensioning in solar power, which will represent a very little excess of solar (solar not used) during summer, but a 10% increase in the production all over the rest of the year:
In addition, there are other weather and context conditions that could drastically influence your solar production and that you should consider (such as dust, air particles, pollution, snow, shadows, etc.). For example, in countries where there is snow during the winter, the dimensioning should consider the reflexion (albedo) that can be quite high and the irradiance on the solar modules that can be higher than the standard test value of 1000W/m2, influencing the Voc and Isc values of the modules in winter.