Starfire Energy - Solar Electric Basics

Solar Electric Systems

Solar Electric Basics

Solar electric systems use two basic pieces of equipment: photovoltaic (PV) modules and an inverter. Photovoltaic modules capture photons and convert them to direct current or DC electricity. The inverter then changes this DC electricity into alternating current or AC electricity which is what you use in your home. For optimal performance, solar panels need to be mounted in a location which receives full sun from 9 am to 3 p.m. each day. There are two main types of Solar Electric Systems: grid-tied and off-grid systems.

Grid-Tied Basics

Most home and business are supplied with electricity by their local utility through an electricity grid. A grid-tied solar electricity system works in concert with the grid to supply the home or business with as much electricity as it requires at any point in time. During the day, the photovoltaic modules will produce electricity and supply it directly to the home or business. Whatever is not consumed is fed into the grid, turning your utility meter backwards to credit you for your power production.

At night, on cloudy days, or during high-consumption times, the grid supplies whatever additional electricity is required by the home or business. During these times, the utility meter runs forward in the normal way.
Batteries are not required for a grid-tied solar electricity system. The utility's power lines, already hooked to your home or business, provide any additional power required during the day and at night.

Since grid-tied systems do not use batteries, their maintenance is very low and they should last between 40 and 50 years when installed properly. Normally, the only thing a home or business owner needs to do to keep a system operating properly is trim trees and bushes over time so they don't shade the photovoltaic modules.

Off-Grid Systems

You may be interested in an off grid, or grid tied PV system with a battery back up if you:

Want to live completely off the electricity grid.
Have certain types of appliances which require an uninterruptible power supply.
Want to make sure you have power even when the electricity goes out.

These types of systems use all of the same types of components as a grid
tied PV system but are more expensive because of the addition of a few other items:

Large deep-cycle batteries to run all of the electrical appliances in your home.
A charge controller to manage the use and recharge of the batteries to ensure their long life.
A special type of inverter that does not need to be connected to an electricity grid.

What you should consider if you are interested in an off grid or grid tied with battery back up system:

What electrical appliances must I have power to at all times?
Where can I reduce my demand for electrical usage?
How many days of autonomy (days without charging the batteries) do I need?

By contacting Starfire Energy we can help you answer these and many other questions.

System Production

The amount of electricity that a system will generate depends on a variety of factors. The specifics include such considerations as the direction it faces, its tilt angle, any shading it may experience, its temperature coefficient of performance (how much power the modules lose when they get hot), resistive losses in the wiring, how often they are covered with snow in the winter, and the efficiency of the inverter. The effects of direction, tilt, and shading must be quantified for each specific installation. If an array in the Denver metro area facing south tilted at 40 degrees has no shading, it will received an average of 5.5 hours of full intensity sunlight each day. That is the average over the course of a year; in the summer it will receive more and in the winter it will receive less. Over the years, solar electric system installers have developed derating factors to compensate for considerations such as the temperature coefficient of performance, inverter efficiency, and other system inefficiencies. When these derating factors are taken together, the actual power output of an array is approximately 67%-70% of the nameplate value calculated from the number of modules in the array. For example, an array with 5 200 W modules would have a nameplate power of 5 x 200 W = 1000 W, but its actual derated output would be 670 - 700 W. This comes about because the power of an individual module is measured by itself at 25 C, rather than in an array of modules at a higher temperature that is more representative of actual field conditions. Using the average 5.5 hours of sunlight and a derated power of 700 W for a nominal 1 kW solar electric system, such a system would be expected to produce an average of 3.85 kWh of electricity each day (more in the summer, less in the winter), or about 1,405 kWh each year. Larger systems would produce proportionally more electricity.

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