How Do Solar Panels Work?

Solar panels work through the photovoltaic effect of silicon semiconductors: when sunlight (photons) hits the cells, it excites electron flow, generating direct current (DC).

Currently, the conversion efficiency of mainstream monocrystalline silicon solar panels is about 18%-22%, and their service life generally exceeds 25 years.

The generated DC power is converted into alternating current (AC) by an inverter, providing stable, low-carbon electricity for household appliances.

The Photovoltaic Effect

Photons Knocking

The atomic structure inside monocrystalline silicon wafers is determined by a bandgap width of 1.12 eV (electron volts); only photons with energy exceeding this value can excite electron-hole pairs.

When solar radiation with a wavelength distribution between 380 nm and 1100 nm reaches the surface of the silicon wafer, more than 10^17 photons penetrate the 160 μm thick silicon substrate every second.

Photons with energy greater than 1.12 eV are completely absorbed by silicon atoms, causing electrons in the valence band to transition to the conduction band, forming freely moving charge carriers.

If the photon wavelength is shorter than 400 nm, its high energy (about 3.1 eV) will dissipate as heat after exciting the electrons, leading to a rise in module temperature.

Under standard air mass (AM1.5) lighting conditions, about 45% of the 1,000 W of radiant energy received per square meter cannot be utilized because the energy is lower than the bandgap threshold.

Internal Electric Field

By doping 9N-grade purity silicon wafers with boron atoms at a concentration of 10^16/cm³, a P-type base layer is created, where the hole concentration is much higher than that of free electrons.

Within a depth of 500 nm on the surface of the silicon wafer, phosphorus atoms are doped at a concentration of 10^19/cm³ using a high-temperature diffusion process to form an N-type thin layer.

In the depletion region with a width of 0.5 μm to 1.0 μm at the interface of these two regions with opposite polarities, carrier diffusion occurs, establishing a self-built electric field of about 0.7 V.

This electric field has strong directionality and can push newly generated electrons to the N region and holes to the P region at a drift speed of thousands of kilometers per second.

The potential difference in this space charge region is the physical source of the photovoltaic effect, determining that the output voltage of a single cell in an open-circuit state remains stable between 0.6 V and 0.7 V.

Charge Transport

Excited electrons must complete directional movement within a minority carrier lifetime of 200 μs (microseconds) before leaving the P-N junction.

The electron mobility inside monocrystalline silicon is usually 1450 cm²/Vs, while the hole mobility is about 450 cm²/Vs. This difference in mobility determines the asymmetric design of the front and back electrodes of the cell.

To ensure that electrons are not recombined while moving a physical distance of 150 μm, the oxygen and carbon content inside the silicon wafer must be controlled below 5 × 10^17/cm³.

If metal impurities exist inside the silicon wafer, the carrier diffusion length will plummet from 300 μm to 50 μm, resulting in a more than 30% drop in photocurrent collection efficiency.

In the highly doped N-type emitter, surface recombination rates can be reduced to below 10 cm/s through 10 nm thick tunneling oxide layer passivation technology, thereby increasing the probability of current extraction.

Loss Breakdown

Current single-junction crystalline silicon cells are limited by the Shockley-Queisser physical limit, with the theoretical maximum efficiency locked at around 33.7%.

In actual production, since front and back metal grid lines block about 3% to 5% of the light-receiving area, the short-circuit current (Isc) generated per square centimeter is usually maintained between 40 mA and 43 mA.

The series resistance (Rs) of the cell must be controlled below 0.5 Ω·cm²; otherwise, in high-current operation, internal resistance losses will cause the fill factor (FF) to drop from the theoretical 85% to below 78%.

Furthermore, for every 1 degree Celsius rise in ambient temperature, the lattice vibration of silicon atoms increases, narrowing the bandgap and increasing the internal carrier recombination probability, causing the output voltage to decay continuously at a slope of -0.3%/℃.

Collection Process

After passing through the P-N junction and reaching the surface, electrons need to be gathered from fine grid lines with a width of only 30 μm onto main grid lines with a width of about 1 mm.

Current mainstream 16BB (Multi-Busbar) technology reduces power loss caused by ohmic contact by more than 0.5 W by shortening the lateral transmission path of electrons in silver paste circuits.

The contact resistivity between the silver paste busbar and the silicon wafer surface needs to be maintained at the 10^-3 Ω·cm² level to ensure that current with 550 W of power can flow out smoothly.

When hundreds of millions of electrons flow to the back aluminum field electrode through an external load circuit, the entire physical loop is closed, completing the final transformation from photon kinetic energy to electrical energy.

Efficiency Upper Limit

In practical applications of the photovoltaic effect, TOPCon technology utilizes a 1.5 nm thick silicon dioxide tunneling layer to significantly increase the open-circuit voltage (Voc) to over 730 mV.

HJT heterojunction technology achieves ultimate surface passivation by depositing a 20 nm thick intrinsic amorphous silicon thin film on both sides of monocrystalline silicon, making conversion efficiency easily cross the 25% mark.

For future tandem technology, by covering 1.12 eV silicon wafers with perovskite materials with a bandgap of 1.7 eV, short-wave and long-wave spectra can be absorbed separately, pushing the theoretical power generation efficiency of single modules toward the physical limit of over 40%.

Under the current 182 mm or 210 mm large-size silicon wafer standards, every successful capture of a high-energy photon represents an effective output of about 1.5 × 10^-19 Joules of electrical energy.

Beyond the Panel: The Solar Ecosystem

Voltage Transformation and Current Conversion

As the central brain of the photovoltaic system, the primary responsibility of the inverter is to convert the fluctuating DC power generated by the modules into 50 Hz or 60 Hz sine wave AC power.

Currently, the weighted conversion efficiency of string inverters on the market generally falls in the range of 97.5% to 98.8%, with internal integrated MPPT (Maximum Power Point Tracking) channels usually numbering from 2 to 10, each with a tracking accuracy as high as 99.9%.

Under rated power operation, the Total Harmonic Distortion (THD) of high-quality inverters must be controlled within 3% to prevent harmonic interference with precision inductive loads such as refrigerators and air conditioners.

The operating frequency of the IGBT power tubes inside the inverter is set between 16 kHz and 20 kHz, achieving power conversion through high-frequency pulse width modulation technology.

Regarding the installation cost per kilowatt, the inverter accounts for about 8% to 12% of the total system investment, with a design life typically between 120 and 180 months.

In actual operation, the startup voltage of the inverter is usually set between 120 V and 180 V. When the morning light intensity reaches 100 W/㎡, the system automatically wakes up and enters power generation mode.

Storage Modules

To solve the volatility and intermittency of photovoltaic power generation, lithium iron phosphate (LiFePO4) batteries form the physical core of modern energy storage systems.

The gravimetric energy density of this type of cell is between 140 Wh/kg and 170 Wh/kg, with a nominal voltage for a single cell of 3.2 V.

A typical 10 kWh home energy storage solution consists of dozens of cells connected in series and parallel, with a Depth of Discharge (DoD) that can be stably maintained above 90%.

This means that for a 10 kWh cell pack, the actual recyclable electricity exceeds 9 kWh.

At a charge-discharge rate of 0.5C (meaning fully charged or discharged in 2 hours), the cycle life of the cell is usually between 6000 and 8000 times.

Through real-time monitoring by the BMS (Cell Management System), the voltage difference between individual cells is strictly locked within 0.02 V. Once a cell temperature is detected to exceed 60 degrees Celsius, the system triggers thermal runaway protection logic within 10 milliseconds.

The introduction of an energy storage system can increase the household's photovoltaic self-consumption rate from an average of 30% to over 75%, significantly shortening the overall investment recovery period.

Bracket Fixation

The racks supporting the panels are usually made of AL6005-T5 high-strength aluminum alloy or Q235B hot-dip galvanized steel. The thickness of the galvanized layer on the surface must reach 65 μm to 85 μm to ensure 30 years of corrosion resistance in salt spray environments.

When calculating bracket strength, designers must refer to local 50-year maximum wind pressure (usually 0.5 kN/㎡) and snow pressure (about 0.4 kN/㎡) parameters.

The tensile strength of the mid-clamp and end-clamp connectors between the module and the bracket must exceed 200 Newtons per square millimeter.

On concrete flat roofs, the weight of each bracket base ballast is usually between 40 kg and 60 kg to resist instantaneous typhoons with speeds of 180 km/h.

The installation tilt angle of the bracket directly determines the total annual radiation received. In regions at 30 degrees north latitude, fixed brackets are usually set at 25 to 30 degrees, which can yield about 15% more power generation than horizontal installation.

The 304 stainless steel bolts used when installing brackets must be pre-tightened to 16 Nm to 20 Nm with a torque wrench to prevent thermal expansion and contraction from causing micro-cracks in the module frame under long-term wind loads.

Slender Cables

Current transmission throughout the power station relies on specialized DC photovoltaic cables, with the most common specifications being 4 mm² and 6 mm².

This type of cable uses a cross-linked polyethylene (XLPE) double-layer insulation structure, possessing strong UV and aging resistance, with a design service life of up to 300 months.

In standard designs, the voltage drop loss on the DC side should be controlled within 1%, meaning that the wiring distance from the farthest module to the inverter is generally not recommended to exceed 35 meters.

The rated operating current of the cable needs a redundancy of 1.5 times. For example, for a system with a rated current of 13 A, the selected cable's current-carrying capacity should be above 20 A.

On the AC output side, to prevent induced lightning strikes from damaging electronic modules, a surge protective device (SPD) with a nominal discharge current of no less than 20 kA (8/20 μs) must be configured.

All wiring terminals must be waterproof-sealed through MC4 standard connectors, with a protection level reaching IP68, ensuring that contact resistance remains below 0.5 mΩ even in heavy rain environments.

Smart Manager

Daily operation and maintenance of photovoltaic power stations rely on data collectors and cloud monitoring platforms, with sampling frequencies usually set to once every 5 minutes or 1 minute.

The system uploads the voltage, current, inverter temperature, and ambient irradiance of each module string to the server via RS485, Wi-Fi, or 4G signals.

Using big data comparison algorithms, the system can automatically identify abnormal nodes with failure rates exceeding 5%.

For example, when the current of a certain string is 15% lower than the average, the background will automatically push an SMS alert, indicating potential risks such as leaf shading or module micro-cracks.

Professional EMS (Energy Management Systems) can even combine local time-of-use electricity price data to prioritize using cell power during peak price periods and charging during off-peak price periods.

This smart scheduling can reduce the electricity costs of industrial and commercial users by about 20% to 30%, shortening projects that originally took 7 years to break even to around 5.5 years.

The monitoring system is extremely sensitive to drops in power generation caused by dust accumulation. When real-time power deviates by more than 10% from the weather station's predicted value, the system automatically generates a cleaning work order suggestion.

Grid-tied Protection

The electricity generated by the power station finally needs to be connected to the external grid through a bidirectional smart meter. The measurement accuracy of this meter usually reaches 0.5S class, capable of separately counting self-consumption and surplus electricity sent to the grid.

Inside the distribution box, an anti-islanding protector with an automatic reclosing function must be installed.

Once the public grid loses power due to a fault, this protector must rapidly cut off the output of the photovoltaic system within 100 milliseconds to prevent electricity from being fed back to the grid and threatening the lives of maintenance personnel.

The air switch on the AC side should be selected for a rated value based on 1.25 to 1.5 times the maximum current.

For systems above 5 kW, the grounding resistance must be strictly controlled below 4 Ω to ensure that leakage current can be quickly discharged into the ground, reducing human contact voltage to a safe range within 36 V.

Efficiency & Environment

Power Generation Capability

The efficiency labeling of photovoltaic modules is usually based on Standard Test Conditions (STC), which are a light intensity of 1,000 W/㎡, a cell temperature of 25 ℃, and an AM1.5 spectral distribution.

The efficiency of mass-produced N-type TOPCon modules is stable between 22.5% and 23.2%, meaning each square meter of panel can output about 225 W to 232 W of power at peak state.

However, in actual outdoor operation, modules are often in the Nominal Operating Cell Temperature (NOCT) environment, where light intensity drops to 800 W/㎡, ambient temperature is 20 ℃, and the module's own temperature naturally climbs to about 45 ℃.

Under NOCT conditions, the actual conversion efficiency of the same module will drop below 20%, and the output power will generally decrease by 20% to 25% compared to the rated value.

Only the part of the solar spectrum whose energy matches the bandgap of monocrystalline silicon (1.12 eV) can be effectively converted.

Infrared rays with wavelengths greater than 1100 nm account for about 20% of solar radiant energy, but because the energy is too low, they will directly penetrate the silicon wafer or be absorbed by the backsheet, eventually converting into heat.

Although ultraviolet rays shorter than 400 nm have extremely high energy, after exciting electrons, the excess energy is released in the form of hot phonons within 10^-12 seconds, causing the cell surface temperature to accumulate rapidly.

This physical limitation determines that the theoretical limit of single-layer silicon cells is locked at around 33%, while current engineering optimization has approached the mass-production dividend period of 25%.

Heat-sensitive Nature

As a semiconductor material, monocrystalline silicon has significant temperature sensitivity characteristics. The ratio at which power decreases with rising temperature is known as the temperature coefficient.

The temperature coefficient of mainstream P-type modules is usually -0.37%/℃, while advanced N-type modules can optimize this coefficient to -0.29%/℃ to -0.31%/℃ due to their higher open-circuit voltage.

This means that when the ambient temperature on a roof reaches 35 ℃ in summer, the actual operating temperature on the module surface often soars to 65 ℃.

Compared to the standard test environment of 25 ℃, this 40 ℃ temperature difference will cause the module output power to shrink directly by more than 12%.

The carrier recombination rate inside high-efficiency cell cells increases exponentially as temperature rises, causing the open-circuit voltage (Voc) to drop significantly, usually by about 2 mV for every 1 ℃ increase.

To mitigate this thermal loss, installers must leave a ventilation gap of at least 10 cm to 15 cm between the module backsheet and the roof surface, using air convection to carry heat away.

Measured data show that systems with good ventilation conditions can output 5% to 8% more annual power than systems installed directly against the roof.

In extreme heat, if the module temperature exceeds 85 ℃, the EVA adhesive film encapsulated inside may undergo physical degradation, leading to a permanent drop in light transmittance of more than 0.5%.

Shading Damage

If 3% of the area on a 550 W module is blocked by fallen leaves, bird droppings, or utility pole shadows, the resistance in that area will instantly soar from a few milliohms to hundreds of ohms.

The originally generated current will be blocked when passing through the shaded area and converted into heat, forming a hot spot phenomenon with temperatures exceeding 150 ℃.

To protect the module from burning out, the three bypass diodes in the junction box will be forced to conduct, skipping the damaged cell string. However, this causes the output power of that module to drop to zero or decrease by one-third.

Suspended particulate matter and dust accumulation in the air are another invisible source of loss. In arid regions with little rain, an accumulation of 5 grams of dust per square meter on the panel surface leads to a roughly 10% drop in light transmittance.

Dust not only blocks light but also changes the refractive index of the glass surface, causing more oblique light rays to undergo total internal reflection.

According to 36 months of monitoring data, power stations that undergo professional cleaning every quarter have a Return on Investment (ROI) 15% higher than those without maintenance.

Near industrial areas, acidic dust deposition may also chemically erode the surface of the 3.2 mm thick tempered glass, causing the anti-reflective (AR) coating to physically peel off within 120 months, resulting in a permanent 3% power generation loss.

Aging Speed

The design service life of photovoltaic modules is usually set at 25 to 30 years, but their power output is not constant.

Within the first 24 hours of operation, silicon wafers undergo Light-Induced Degradation (LID) because boron and oxygen atoms inside the silicon combine under light to form recombination centers, causing an initial power drop of about 1% to 2%.

During subsequent long-term operation, influenced by UV radiation, thermal expansion and contraction caused by day-night temperature differences, and moisture penetration, modules undergo a linear degradation of about 0.4% to 0.55% per year.

To ensure reliability over 300 months, the module backsheet must have a water vapor transmission rate of less than 1 g/m²·day.

Once the seal fails, external moisture reacts electrochemically with the silver paste grid lines on the cell's surface, producing acetic acid and corroding the metal circuits, leading to an increase in series resistance (Rs).

Actual tests found that when the series resistance increases from 0.5 Ω to 1.5 Ω, the fill factor (FF) of the module drops from 82% to 65%, meaning that even if sunlight is abundant, electrical energy cannot flow out efficiently.

Current double-glass modules have pushed the annual degradation rate down to below 0.4% by using 2.0 mm + 2.0 mm tempered glass encapsulation.