H

owever daunting it seems to delve into the intricacies of solar panel operation, a basic understanding of the technical characteristics will help make your choice simple. Today, we focus on solar voltage and explain how it affects the power output of your panel system.

Understanding voltage and current

As for any power electronic device, a data sheet for a solar panel includes voltage. To find out what it actually is and why it matters for the efficient work of a PV system, we first need to dabble in a bit of physics.

An electric current is basically the flow of electrically charged particles (positively and negatively) through a conductor, most commonly in the form of a wire. These particles attract each other when charged oppositely and repel each other if they are like-charged. A region where the particles exert electric force on other particles is called an electromagnetic field.

Voltage (also called electric pressure or electric tension) is the difference in electric potential between two points in an electric field, which forces the particles to move around the circuit, or in other words, the amount of work it takes to move a charged particle between these two points. You can think of electric current as water flowing through a pipe, while voltage is the pressure pushing the water.

Voltage is measured in volts, commonly represented by an uppercase V.  The volt was named after the Italian physicist and chemist Alessandro Volta, whose voltaic pile preceded the modern battery. The current intensity is measured in amperes or "amps" for short, sometimes abbreviated to A. Ampere was named after a French physicist and mathematician, André-Marie Ampère - one of the founders of electrodynamics (the science of electric currents). Multiplying volts by amps gives us power output -  the amount of electric energy delivered per unit of time.

What is solar voltage?

Now let’s look at how this all works in solar power generation. Every photovoltaic panel consists of smaller devices called solar cells, which generate electricity by the "photovoltaic effect” – the process of transforming light into electricity. These cells are made of two layers of semiconductor material (most commonly silicon). The p-type layer is doped with an element with one fewer valence electron, while the n-type layer is doped with one extra valence electron. The first doping creates positively charged particles called holes (absence of electrons in valence bands), while the second creates excess negatively charged electrons. When placed in contact, these layers form a p-n junction where some of the free electrons from the n-layer combine with holes from the p-type layer, creating an electrically neutral depletion zone.

When photons hit the surface of a solar panel, they knock loose the electrons in the cell junction, and the electrons can move freely across the depletion zone. As oppositely charged particles attract, the released excess electrons flow to the holes in the electron-deficient p-type layer while the holes flow in the opposite direction. But they cannot cross the depletion zone again to return, creating a buildup of free electrons on one side. This imbalance results in a difference in electric potential between the two sides, similar to a battery. This imbalance is what produces voltage.

VOC - measured without load

If you run through a spec sheet of absolutely any photovoltaic panel on the market, you will notice that it lists two types of voltages. The first one is solar cell open-circuit voltage (VOC) – the electrical tension between the two terminals of a cell without any external load applied. It is the maximum voltage an individual cell can deliver with no current flowing through the external circuit. At a cell temperature of 25 °C, a typical single-junction silicon cell can have a maximum VOC of approximately 0.5 volts to 0.6 volts.

In a typical residential photovoltaic panel, 36 cells are connected in series. Given that a single cell produces 0.6 volts, the open-circuit voltage of the entire panel will be around 21 volts.

VMP - load is attached

The second type specified by manufacturers is the voltage at maximum power (VMP), which occurs when the panel is connected to an external load and operates at its peak power output. VMP is usually about 70 – 80% lower than the VOC of a panel. For example, a standard 36-cell module at its maximum power output will generate a voltage at maximum power of around 17 volts.

The voltage at maximum power is used when determining the total wattage of a panel: if you multiply the VMP of the panel by the current at max power (IMP), you will get Maximum Power Point (Pmax), sometimes also called nameplate capacity – a point where the combination of the volts and amps gives the highest possible power output. The Maximum Power Point is measured under Standard Test Conditions (STC) defined as 1,000 watts per square meter solar irradiance, panel temperature of 25 °C, and air mass equal to 1.5.

Additionally, VMP is used as a reference for the Maximum Power Point Tracking (MPPT) charge controllers, which regulate the charge of battery storage and minimize power losses.

Nominal Voltage – PV panel categorization tool

Unlike the previous two parameters, nominal voltage isn’t an actual measurement but a PV panel classification method. It is not mentioned in the panel datasheet and is used for determining what battery-based solar equipment goes together. For example, a module with a nominal voltage of 12V should be used with a 12V battery bank and a 12V inverter. Its open-circuit voltage, however, is about 22V, and voltage at maximum power reaches about 17V.

Choosing between 12V and 24V solar power system

Most residential solar panels come in 12V and 24V variations, and you should choose between them according to your electricity consumption. If your energy needs do not exceed 1,000 watts, a 12V power system will most likely be enough. Larger setups of over 1,000 watts will require 24V solar modules.

One of the critical advantages of 12V panels is that they cost less than 24V options. Additionally, 12-volt solar power equipment is widely represented on the market, so you will have no problem finding the components for your system, such as inverters, fuses, and batteries.

24V photovoltaic modules are more versatile as they are compatible with both 12V and 24V equipment. Another benefit is excellent heat retention: 24V panels lose less heat, resulting in higher efficiency. The main drawback is that 24V systems cost significantly more than 12V alternatives.

Posted 
Jun 29, 2023
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