Solar PV panels generate power anytime there is daylight—even on cloudy days—but to operate at rated capacity, they must be placed in direct sunlight, allowing photons to pass directly into the cells. Sunlight is “direct” when it is within a 15-degree angle (in any direction) of being perpendicular to the solar panel. This translates to about a 30-degree angle “window” where the panel will operate at maximum output.

Sunlight hitting a solar PV panel directly is absorbed most efficiently. If sunlight hits panel at an angle, the photons will be reflected more and absorbed less. Only absorbed photons can be converted into electricity. The amount of photons passing through a solar panel corresponds directly to its output current, and how this happens is explained by the elemental substances contained in a solar PV cell.

Solar cells consist of four main substances: silicon, phosphorus, boron, and copper. Each substance serves a purpose in the operation of converting sunlight to electrical energy. Silicon is essential to the function of solar cells because it is a semiconductor. The octet rule states that for an atom to become stable it must react or combine with other atoms to achieve a stable outer shell configuration of eight electrons. Because silicon has four valence electrons, it can share its electrons with other silicon atoms to form a solid crystalline structure. However, this characteristic of silicon means that no electrons are free to move, rendering silicon a poor conductor. This is why phosphorus is added to a piece of the silicon in a solar cell in a process known as doping. Because phosphorus has five valence electrons, four of those electrons are shared with four silicon electrons, but there is still one phosphorus electron free to move around so electrons can flow and electricity conducts better. The resulting silicon from this addition of phosphorus is known as N-type (negative type) silicon due to the prevalence of free electrons.

The other piece of the silicon is doped with boron because it has only three valence electrons (one less than silicon). This means that when a silicon atom and a boron atom share electrons, they will each have seven valence electrons, creating a gap for an electron to fill. The resulting silicon from this addition of boron is known as P-type (positive type) silicon due to the space available for electrons to fill.

When the N-type silicon exposed to a source of light absorbs light in the form of photons, the silicon atom absorbs the quantum of energy and releases an electron. Because the electric field causing the electrons to want to travel from the N-type silicon to the P-type silicon is blocked by the electron barrier, copper wires are run through the solar cell so the electric field is channeled throughout them, causing loose electrons to be drawn through the wire. The flow of electrons produces electricity, and the copper wires become the negative and positive terminals of the solar cell.

When the electrons are released by light energy, they flow through the wire and to the P-type silicon, doing work on the way. Eventually enough electrons accumulate in the P-type silicon to disrupt the equilibrium. This causes some of the electrons at the PN-junction to flow through to the N-type silicon and some of the free electrons in the P-type silicon to re-form the barrier, so the equilibrium at the barrier is constantly maintained as the electrons flow. The current output of the solar cell depends on the flow of electrons, and the voltage is determined by the intensity of the electric field.

Temperature plays a key role in a solar PV panel’s ability to produce power. Remember this rule when using solar PV panels: Cooler panels generate more electrical power than hot panels. As a panel becomes hotter, its efficiency is degraded. When solar PV panels are exposed to the sun’s rays, they will always experience a rise in temperature to a point known as the panel’s “operating temperature”. Panels are rated for a specific amount of power generation in relation to this temperature.

Two factors that affect heat degradation are solar panel (PV module) construction and mounting of the solar PV panel.

A charge controller should always be used between a solar PV panel—or array—and a battery to ensure that a safe charging voltage is applied to the battery. Maximum Power Point Tracking (MPPT) charge controllers serve both the solar PV panel and the battery by algorithmically maximizing the output of the panel and acting as a buffer between panel and the battery.

The MPPT charge controller is “smart” and recognizes the solar PV panel and the battery as individual circuits with completely different characteristics. When MPPT controllers are used in a solar PV system, they allow panels to operate at their rated voltage instead of the battery’s voltage, resulting in about a 25% increase in power to the battery bank when compared to using less efficient charge controllers such as series or shunt-type charge controllers.

Connecting Additional Panels

Use only the same PV panel types when building an array—especially if using MPPT charge controllers. Different PV panel technologies often operate at different voltages, so using mismatched panels cancels out the benefit of an MPPT charge controller and makes maximum power production unachievable.

For instance, a flexible amorphous silicon panel should not be connected to a circuit that uses rigid monocrystalline panels. While this connection will not result in damage, it will significantly decrease the total power output of the panel array.

How MPPT and other solar charge controllers work is explained in detail, with examples, in the Power Management module.