How Solar Panels Work — Photovoltaic Energy Explained

Blog › Engineering · 10 min read · Published 2026-03-05

Understand the science behind solar panels, how they convert sunlight to electricity, and how to calculate solar energy output for your home.

The Science of Photovoltaics

Solar panels convert sunlight directly into electricity through the photovoltaic effect, first discovered by French physicist Edmond Becquerel in 1839. When photons (light particles) from the sun strike a solar cell made of semiconductor material (typically silicon), they knock electrons from their atoms. These free electrons flow through an external circuit, creating an electric current. This is the fundamental mechanism that powers the entire solar energy industry.

A typical solar panel contains 60–72 individual solar cells, each producing approximately 0.5 volts. Connected in series, these cells produce the panel's rated voltage (typically 30–40 volts DC). An inverter then converts this DC power to AC power for household use or grid export.

How a Solar Cell Works (Step by Step)

1. Silicon Semiconductor Structure

Solar cells are made from two layers of silicon: the N-type (top layer, doped with phosphorus to have extra electrons) and P-type (bottom layer, doped with boron to have "holes" — missing electrons). Where these layers meet is the P-N junction, which creates an electric field.

2. Photon Absorption

When sunlight hits the cell, photons with sufficient energy (at least 1.1 eV for silicon) are absorbed, knocking electrons loose. Photons below this threshold pass through without generating electricity, which is one reason solar cells can't convert all sunlight to power.

3. Electron Flow

The electric field at the P-N junction pushes freed electrons toward the N-type layer and holes toward the P-type layer. This separation of charges creates a voltage difference. When connected to an external circuit, electrons flow from N-type through the circuit to P-type, creating usable electric current.

4. Power Collection

Metal contacts on the cell's surface collect the current. The grid-like lines you see on a solar cell are these conductors. The current flows to a junction box, then to wiring, and ultimately to an inverter.

Types of Solar Panels

Monocrystalline (Mono-Si): Made from a single silicon crystal. Highest efficiency (20–24%), most expensive, black appearance. Best for limited roof space.
Polycrystalline (Poly-Si): Made from melted silicon fragments. Lower efficiency (15–20%), lower cost, blue appearance. Good balance of cost and performance.
Thin-Film (CdTe, CIGS, a-Si): Semiconductor material deposited in thin layers on glass or flexible substrate. Lower efficiency (10–13%) but cheapest per watt. Used in large commercial installations and building-integrated applications.
Bifacial: Generate power from both sides by capturing reflected light on the rear surface. Up to 30% more energy than single-sided panels in optimal conditions.

Calculating Solar Panel Output

The energy a solar panel produces depends on several factors:

Daily Energy (kWh) = Panel Wattage × Peak Sun Hours × System Efficiency

Panel Wattage: Rated power under Standard Test Conditions (STC: 1,000 W/m² irradiance, 25°C cell temperature). Typical residential panels are 350–450 watts.
Peak Sun Hours: Hours of equivalent full sunlight per day. Varies by location: Arizona gets ~6.5 hours, New York ~4 hours, London ~2.5 hours, Sydney ~5 hours.
System Efficiency: Accounts for losses from inverter conversion (~3–5%), wiring (~2%), shading, temperature, soiling, and degradation. Typically 75–85% overall.

Example: A 400W panel in Phoenix, AZ (6.5 peak sun hours) with 80% system efficiency:

Daily output = 0.400 kW × 6.5 hours × 0.80 = 2.08 kWh/day

Annual output = 2.08 × 365 = 759 kWh/year per panel

How Many Panels Do You Need?

Number of Panels = Annual Energy Need / Annual Output per Panel

Average US household uses ~10,500 kWh/year. Using our Phoenix example:

10,500 / 759 = ~14 panels (about 5.6 kW system)

In cloudier climates like Seattle (~4 peak sun hours, same 80% efficiency): each 400W panel produces ~467 kWh/year, requiring ~22–23 panels (~9 kW system) for the same household.

Factors That Reduce Output

Temperature: Counterintuitively, solar panels lose efficiency in extreme heat. For every 1°C above 25°C, silicon cells lose about 0.3–0.5% efficiency. A panel at 65°C (common on hot rooftops) can lose 12–20% of rated power.
Shading: Even partial shading on one cell can dramatically reduce the entire panel's output because cells are connected in series. Modern panels use bypass diodes to mitigate this, but shading should be minimized.
Orientation and tilt: Optimal tilt equals your latitude (e.g., 34° in LA, 51° in London). South-facing (northern hemisphere) or north-facing (southern hemisphere) maximizes annual production.
Age degradation: Panels degrade approximately 0.5–0.8% per year. After 25 years, expect 85–90% of original output. Most manufacturers guarantee 80% output at 25 years.
Dust and soiling: Dirty panels can lose 5–25% efficiency depending on environment. Regular rain usually provides adequate cleaning in most climates.

The Economics of Solar

Average residential solar cost in 2026: $2.50–3.50 per watt installed (before incentives). A 7 kW system costs $17,500–24,500 before the federal Investment Tax Credit (30% through 2032), bringing net cost to $12,250–17,150. At average US electricity rates ($0.16/kWh) and production of ~10,000 kWh/year, annual savings are ~$1,600, yielding a payback period of 8–11 years — with panels lasting 25–30+ years.

Net metering policies (where your utility credits you for excess power sent to the grid) significantly affect economics. States with strong net metering (California, Massachusetts, New Jersey) offer faster payback than states with less favorable policies.

Key Takeaways

Solar panels are a proven, increasingly affordable technology for generating clean electricity. Understanding how they work — from the photovoltaic effect to system sizing calculations — empowers you to evaluate proposals from installers, estimate your potential savings, and make informed decisions about going solar. Use a solar panel calculator to model your specific situation based on location, roof space, energy consumption, and local electricity rates.