Photovoltaic cells, which are comprised of a semi-conductor material (typically crystalline silica), convert sunlight (photons) into electricity (voltage). A photovoltaic module (panel) is comprised of a series of cells. Direct current (DC) power generated by the photovoltaic modules is either supplied directly to batteries or converted into alternating current (AC) at an inverter for supply to electrical loads.

The amount of electricity a photovoltaic system will generate at a specific location is largely dependent on the insolation value or annual average number of sun hours per day for that location. The insolation value can be directly measured with field equipment or approximated based on a number of reference databases that track this information.

A common misconception is that photovoltaics are only viable in the desert or southern locales where cloudless skies are the norm. However, photovoltaic modules actually operate more efficiently in colder climates. Excessive heat degrades system performance, so while a photovoltaic module in southern California may see more sunshine on an annual basis, that same module will produce electricity more efficiently in a cooler region such as the northeast US.

The size and corresponding amount of power that may be produced by a photovoltaic system is primarily contingent on the amount of available south (or southeast/southwest)-facing roof space or ground mounting space for photovoltaic modules. The average household in Massachusetts consumes approximately 8,000 kWh/year. A 2 kW PV system will provide approximately 30% of the annual power needs the household, while a 5 kW system may produce up to 75% of electrical power needs.

While beauty will always be in the eye of the beholder, today’s photovoltaic systems are designed with specific attention to enhancing, or at least blending in with a building’s appearance. Racking systems for photovoltaic modules are designed for low or no visibility and minimal relief from the roof surface. Module frames are available in different finishes to complement most any roof. If photovoltaic panels are not preferred, other options may include thin-film photovoltaic modules which adhere directly to specific roof-types and integral photovoltaic roofing materials such as ‘solar shingles’.

Many photovoltaic systems will continue to produce nominal power under cloudy or low-light conditions. If the photovoltaic system is a battery-based system, the diminished power production from the photovoltaic modules during cloudy periods may require electrical loads to dip into battery reserves. However, a majority of systems are grid-interactive (grid-tied) and not battery-based. During prolonged periods of cloudiness, electrical loads will rely more on power supplied by the grid and less on the photovoltaic modules. When the sun returns, power needs during the day time will once again be supplied by the photovoltaic system with little or no reliance on the grid.

Photovoltaic systems can be as effective or more effective in the winter months as in summer. While there are fewer sunny days in the winter and days are shorter, any reduction in power generation as a result of less sunshine may be offset by more efficient power production in colder temperatures. Voltage output of photovoltaic modules increases notably as temperatures decrease. Furthermore, sunlight reflection off snow cover can enhance system performance.

Photovoltaic modules will often be mounted at an angle above horizontal. For most residential systems, the photovoltaic array will be mounted at the same angle as the roof pitch. The dark anti-reflective coating and smooth surface of the modules typically prevent any snow that has accumulated on the modules from remaining for any extended period of time. It would rarely be necessary to manually clean snow off the array.

Excess power generated by the photovoltaic array may be stored in battery banks for use in an emergency or during periods when the photovoltaic system is not producing power. The battery banks, typically consisting of multiple lead-acid, AGM or gel batteries, are sized in accordance with designed power output of the photovoltaic array. Excess power generated by a grid-interactive system is fed onto the grid by a process known as ‘net-metering’. During periods when the photovoltaic system is producing more power than is required, the electrical meter will actually ‘spin’ backward as power is fed onto the grid. While currently in New England, a residential photovoltaic system owner cannot sell excess power back to the electric company, they can ‘zero-out’ their electric bill from the power company with a productive photovoltaic system.