Solar Panels for Homes — How Sunlight is Turned Into Electricity
- Solar Panels: Solar modules convert sunlight to direct current (DC) electricity.
- Inverter: An inverter converts the solar electricity into alternating current (AC) power used your home.
- Utility Meter: The utility meter spins backwards when you generate more electricity than you use. The excess energy is sent back to the utility, accumulating credits that will offset your next bill. This is called ‘net metering.’
- Utility Grid: Your home remains connected to the utility grid to supply you with electricity when you need more power than your solar power system has produced, such as at night.
A photovoltaic (PV) system consists of one or more photovoltaic (PV) modules. One PV-module consists of about 36-72 photovoltaic solar cells. The cells convert the light into electricity. The PV modules are connected in a series called an array. Because PV Arrays are built with individual, linked PV modules, photovoltaic systems are exceptionally modular, which provides for easy transportation and rapid installation, and enables easy expansion if power requirements increase.
PV systems that are grid-connected or “grid-tied” applications need an inverter or power conditioner to convert the direct current (DC), generated by the PV-modules, into alternating current (AC) for use in your home. Excess power generated and not used immediately is “sold” back to the utility for a credit to be used when sunlight is not available. This is called Net-Metering.
How PV (Photovoltaic) Cells Work
The photovoltaic cell is the component responsible for converting light to electricity. When sunlight strikes a photovoltaic cell, part of the light particles (photons), which contain energy, is absorbed by the cell. By the absorption of a photon a (negative) electron is knocked loose from a silicon atom, and a positive “hole” remains. The freed electron and the positive hole together are neutral. Therefore, in order to be able to generate electricity, the electron and the hole need to be separated from each other. A photovoltaic cell has an artificial junction layer, also called the p/n-layer. Now, the freed electronics cannot return to the positive charged holes. When the electric contacts on the front and rear are being connected through an external circuit, the freed electrons can only return to the positively charged holes by flowing through this external circuit, thus generating current. The electrical power that can be extracted from a photovoltaic cell is proportional to its area and to the intensity of the sunlight that hits the area, and is measured in watts (W).
The PV cells currently on the market convert an average of 12% to 15% of the sunlight that strikes them into electricity.
Due south (180-200 Degrees) is the optimal orientation, but facing the array to the East or West of due south is acceptable. Considerations on orientation include, site conditions, site shading issues, aesthetics, panel tilt and electrical rate schedules. If you plan to be on a Time-Of-Use (TOU) rate schedule, then a westerly facing system will produce the most energy within the “Peak” time period, where you accrue energy credits at the higher Peak rates.
These modifiers are used for the central California area and may be different for other areas of the country or different climatic circumstances.
The number one rule is to never design or install a solar electric system that faces in any portion of the 180 degree arc of the compass that faces north. When you need it the most it won’t work. Needless to say, installing solar panels in the shade of a tree or building will also not be functional. However, a newer technology using solar ‘microinverters’, allow solar panels to operate independently of each other. So if one module ends up in the shade, the others just keep on buzzing, resulting in much higher average system availability. That’s especially great news for residential solar arrays with inconsistent shading profiles. Chimneys, trees, and dormers are among the biggest culprits causing inconsistent roof shade.
Inverters are the “brains” of the solar electric system. The inverter converts DC electricity to the AC electricity that your home uses. The inverter monitors the utility grid and controls your solar energy systems production, as well as shutting your system down during a utility black-out.
Most modern Utility-Tied (also grid-tied) inverters do not utilize batteries, and are much more efficient than the older battery type systems that were design for off-grid homes.
All modern grid-tied inverters are UL Listed for grid connection (UL1741). Without the listing individual inverters are not able to be legally connected to your house’s electrical system, and do not qualify for any rebates.
An alternative to large, central inverters are Enphase microinverters, as mentioned above. Each small micro-inverter is attached to each individual solar panel. These microinverters allow each panel to operate independently, leading to significant improvements in energy production, and flexibility of array design, especially when shading is an issue.
How Net Metering Works
California’s net metering law allows residential users to get PG&E credit for extra electricity produced by their solar panels at peak retail prices during the day and draw from the grid during the night, at lower off-peak rates . During the day, the electric meter spins backwards as it feeds excess electricity back into the utility grid. At night, or during off-peak producing times, the electric meter spins forward as it returns electricity from the grid into the customer’s home.
PG&E’s standard net energy metering program offers customers the opportunity to get credit for the electricity they have produced in excess of the amount that they have used within a given month. The credit appears on the customer’s monthly utility statement, and is applied to electricity-related charges within each 12-month reconciliation period.