Photovoltaic Power

How the system works

The photovoltaic effect describes the ability of photons of light (predominantly from the sun) to excite electrons into a higher state of energy, allowing them to act as charge carriers for an electric current.

Most photovoltaic systems use flat cells of silicon that have been “doped” with an additional electron on one side and one less electron on the other. Photons strike and excite the extra electron, causing it to “jump” to conductors embedded in the cell, where it completes whatever circuit has been wired to the cell. Multiple cells are linked together as modules (panels), which are protected by a glass covering and typically installed in an aluminum frame. Multiple panels can be wired together to provide a desired output.

PV modules produce direct current (DC). In most systems, an inverter is employed to change DC to alternating current (AC), the type of current used for grid power and household appliances.

System output

Modules from different manufacturers will have varying output depending on the size, number and efficiency of the cells. Modules are rated for their output in watts in peak sun conditions. Typical output for current photovoltaic technology is in the range of 1000kWh per year per 1kW peak module sizing, or an average continuous output of 114W.

Current PV technology converts between 13–18 percent of the available solar energy to electric energy.

Environmental impacts: low

PV emits no carbon or other pollutants to produce power. The US Department of Energy and the National Renewable Energy Laboratory both estimate that a PV module generates as much energy as the input required to manufacture the module in about two years. Given life expectancy that exceeds thirty years, the overall energy output far exceeds the embodied energy.

As PV only produces power in the presence of direct sunlight, no power is produced at night and output is reduced in cloudy weather and at around dusk and dawn. Environmental impacts can arise from the need to either store the PV power or use other, less clean forms of power when PV is not producing.

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Material costs: Moderate to high

Labor input: moderate to high

Racks and mounts are prefabricated for straightforward installation. Labor requirements will be based on the placement of the mounts. Roof mounts may require additional bracing of the roof framing, and care must be taken while making multiple penetrations through the roof to avoid creating leaks. Ground mounts can require substantial anchors.

Skill level required for the homeowner

Installation — Moderate to Difficult. An understanding of basic wiring and PV configuration is required. Roof mounting may require special equipment.

Use — Easy.

Maintenance — Easy. Panels should be kept clean for maximum output.

Sourcing/availability: Easy

PV has become widely available in the past decade, with sufficient suppliers and installers to ensure competitive quotes.

Code compliance

All North American electrical codes now address PV installations. Building codes may offer accepted solutions for roof mounting systems, or may require engineered designs.

Durability: very high

PV manufacturers offer a standard 25-year output warranty. Inverters are typically warrantied for five to ten years.

Future development

A huge amount of research and development work is being applied to improving the efficiency of current PV technologies and experimenting with new approaches. Hardly a week goes by when the news media doesn’t trumpet a laboratory “breakthrough” in solar technology. We will likely see improvements and perhaps even major advances in this field in the upcoming decades.

In the meantime, existing PV technology has experienced a cost revolution, with increased demand and production lowering prices dramatically in the past five years, and little sign of imminent decline. Per watt PV prices are approaching parity with other, dirtier forms of power.

The number of utilities and governments offering incentives to homeowners and businesses for PV installations are making the technology familiar and common in many regions.

Resilience

PV panels cannot be manufactured in a low-energy scenario, but can be operated for decades with no further input.