The Photovoltaic Effect

The photovoltaic (PV) effect is the science behind every solar panel. Discovered in 1839 by Edmond Becquerel, it describes how certain materials generate voltage when exposed to light. In modern solar cells, silicon is the key material.

Silicon atoms are arranged in a crystalline structure and treated with impurities through a process called doping, creating two layers: a negatively charged n-type layer and a positively charged p-type layer. The boundary between them is the p-n junction. When photons from sunlight strike the cell, they knock electrons loose. At the p-n junction, these electrons are swept in one direction, creating a flow of charge — a direct current (DC). Connect the cell to a circuit and you have electricity.

A single silicon solar cell produces about 0.5 volts. Solar panels contain 60–72 cells wired in series to produce 30–40 volts at useful wattages (typically 300–450W per panel in 2026).

Types of Solar Panels

Three main panel technologies are used in residential and commercial installations:

Monocrystalline silicon panels are cut from a single continuous silicon crystal ingot. The uniform crystalline structure gives them the highest efficiency: 20–23% for standard panels, up to 25%+ for premium models. They have a uniform black appearance and 25–30+ year lifespans. Most residential and commercial installations use monocrystalline panels.

Polycrystalline silicon panels are made from multiple silicon fragments melted together. The grain boundaries between crystals slightly reduce efficiency to 15–18%. They cost less than monocrystalline and have a characteristic speckled blue appearance. Less common in new installations as monocrystalline costs have fallen.

Thin-film panels deposit photovoltaic material (typically cadmium telluride or CIGS) in thin layers onto glass, plastic, or metal. Efficiencies range from 10–13%, but they are lightweight and flexible — useful for large commercial roofs, building-integrated PV (BIPV), and utility-scale installations where cost per watt matters more than efficiency per square foot.

Key Components of a Solar Energy System

A complete solar energy system includes more than just panels:

  • Solar panels (modules): Convert sunlight to DC electricity. Wired in series strings to match inverter input voltage requirements.
  • Inverter: Converts DC to grid-compatible AC electricity. String inverters handle multiple panels; microinverters attach to individual panels for better performance under partial shading.
  • Mounting hardware: Roof mounts, ground-mount racking, or ballasted systems for flat commercial roofs.
  • DC and AC wiring: Connects panels to inverter and inverter to the electrical panel. Includes safety disconnects and overcurrent protection.
  • Production meter: Monitors system output, often integrated with inverter monitoring software.
  • Battery storage (optional): Lithium-ion battery systems (Tesla Powerwall, Enphase IQ Battery, LG RESU) store excess solar energy for use at night or during grid outages.
  • Net meter: Utility-installed meter that measures both energy drawn from and exported to the grid for net metering billing.

How Solar Panel Efficiency Works

Efficiency is the percentage of sunlight energy converted to electricity. A 420W panel rated at 21.5% efficiency produces 420 watts under Standard Test Conditions (STC): 1,000 W/m² irradiance and 25°C cell temperature. Real-world output is typically 80–85% of STC due to heat, wiring resistance, inverter losses, and soiling — a factor called the performance ratio (PR).

Higher efficiency panels produce more electricity per square foot, which matters when roof space is limited. For large ground-mount commercial projects where land area is available, lower-cost panels with lower efficiency can achieve better economics per dollar invested. The IEA projects average solar panel efficiency to reach 25%+ for commercially available monocrystalline panels by 2030.

Factors That Affect Solar Panel Output

Several variables determine how much electricity your solar panels produce in practice:

  • Solar irradiance (GHI): The total solar energy reaching a surface, in kWh/m²/day. Phoenix, AZ averages 5.7–6.0; Seattle, WA averages 3.0–3.5; Miami, FL averages 5.0–5.3. See our solar data by city for location-specific values.
  • Panel tilt and orientation: South-facing panels at an angle equal to your latitude produce the most energy annually in the northern hemisphere. Deviations reduce output by 1–5% per 10°.
  • Shading: Even partial shade on one cell of a string-wired panel can reduce the entire string's output. Microinverters and DC power optimizers significantly mitigate shading losses.
  • Temperature coefficient: Solar cell efficiency decreases as temperature rises. Premium panels have a lower temperature coefficient (e.g., −0.26%/°C vs −0.40%/°C), reducing hot-weather losses.
  • Soiling: Dust, pollen, and bird droppings reduce output by 1–5% on average; more in arid, dusty environments. Annual cleaning or self-cleaning glass coatings help maintain output.
  • System age and degradation: Panels degrade approximately 0.5–0.7% per year. A panel rated 400W today produces roughly 350W after 25 years.

How Solar Data Helps Predict Production

Accurate production estimates require location-specific irradiance data that accounts for local cloud patterns, atmospheric conditions, and seasonal variation. NREL's National Solar Radiation Database (NSRDB) provides satellite-derived irradiance data at 4km resolution for the full US. NASA POWER provides complementary global data.

The standard production formula is:

Annual kWh = System Size (kW) × Annual PSH (peak sun hours) × Performance Ratio (0.80)

For a 10 kW system in Phoenix with 5.8 PSH: 10 × 5.8 × 365 × 0.80 = 16,936 kWh/year

SolarScope's Site Studio automates these calculations — enter a location, specify system size, and receive NREL-backed production estimates and financial analysis instantly. Explore solar GHI data for 200+ US cities to compare locations before your site visit.