Solar Electric Systems

Solar electric systems generate electricity from sunlight. This is different from solar thermal systems (also known as solar hot water systems or solar heating systems), which absorb heat from sunlight. This page will briefly explain how solar electric systems work, how the solar modules can be mounted, and how much electricity you can expect a system to make. It will also give a "ballpark" estimate of how much a residential solar electric system can cost, although it must be emphasized that the actual cost will depend on the specifics of each system.

How Grid-Tied Solar Electric Systems Work

Grid-tied solar electricity systems use two basic pieces of equipment: photovoltaic (PV) modules and an inverter. Photovoltaic modules turn sunlight into direct current electricity. For optimal performance, they need full sun from 9am to 3pm each day. An inverter takes the direct current electricity produced by the photovoltaic modules and converts it to alternating current electricity so it can be used by normal appliances.

Most home and business owners already have electricity supplied by "the grid", which is run by a utility company. 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 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 forwards in the normal way.

No batteries are required for a grid-tied solar elecricity system. The utility's power lines, already hooked to your home or business, act as a back-up for the system. Since the grid-tied system does not use batteries, its maintenance is very low. Normally, the only thing a home or business owner needs to do is trim trees and bushes so they don't shade the photovoltaic modules.

Mounting Solar Electric Modules

There are a variety of options available for mounting solar electric modules. The main consideration is that they need to be unshaded from about 9am to 3pm each day because that is when they generate most of their electricity. The most common place to mount solar electric modules in a residential or commercial installation is on the roof of the building, such as in the photo to the right. However, arrays can also be mounted in other ways. Ground mounted systems utilize a frame anchored into the ground in multiple locations. Pole mounted systems are attached to a single large pole that is anchored in the ground. Awning-like systems attach to the side of a building. Ideally, they can be mounted above the windows and thus shade the windows in the summer while also producing electricity.

The optimal direction for a solar electric array in the Denver metro area (or anywhere in the northern hemisphere) to face is usually south. Generally, the electricity production is not reduced very much if the array faces within 15 degrees east or west of south. There are some situations in which it is helpful to turn the arrays farther to the southeast or southwest (for example, to compensate for morning or afternoon shading that cannot be removed), but these are the exception rather than the rule.

The optimal tilt angle for grid-tied solar electric systems in the Denver metro area is 40 degrees. However, the tilt angle of the module can vary from 25 degrees to 55 degrees without changing the annual electricity production very much. Arrays with steeper angles will produce a larger portion of their electricity in the winter, while those with shallower angles will produce a larger portion of their electricity in the summer.

How Much Electricity a System Will Generate

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.

An Example Residential Solar Electric System

The Beach's home is a 1000 sq. ft. home in the Denver metro area. It has an asphalt shingle hip roof, the front side of which faces south with a slope of about 22 degrees. Starfire Energy designed and installed a 1.6 kW solar electricity array for its south facing roof. The Beach's use an average of about 6.5 kWh of electricity each day. Accounting for the mild shading from the nearby trees, this installation is projected to produce about 80% of the household's annual electricity consumption each year. The Beach's plan to add insulation and upgrade some electrical appliances to reduce their electricity usage. Once those improvements are complete, their photovoltaic installation should produce as much electricity as they use each year. The photo (click on it to see a larger image) shows the photovoltaic installation just after it was installed in October 2007.

Starfire Energy monitor's this array's performance via a web connection. Web monitoring of solar electric system production isn't necessary and most people do not have it installed on their systems, but we find it useful to demonstrate system performance to people who are interested in solar electric systems. It also helps to satisfy our "inner geek". We can install this type of monitoring on new or existing solar electric systems. The plot to the right (click on it for a larger image) shows the system's daily energy production for the past week. On a sunny day in December, the array can produce 3.5 - 4.0 kWh of electricity. In March, that increases to 8.5-9.0 kWh. In the winter, snow covering the array can reduce its production to nearly zero, but it melts off quickly when the sun starts shining again. A snowfall of up to 2" will usually melt off after one day of sunshine.

Starfire Energy can also install equipment to monitor a how much electricity a home draws from the grid and how much electricity it sends to the grid. The plot to the right (click it to see a larger image) shows the following data for the Beach's home today:

• DC solar power produced by the solar array (red line)
• AC solar power produced by the inverter (blue line)
• Solar power sent to the grid (green line)
• Utility power consumed from the grid (orange line)

The difference between the DC solar power and the AC solar power is due to power losses in the inverter. The difference between the AC solar power and the solar power sent to the grid is due to loads in the home consuming solar electricity. The regular up-and-down cycles in the power from the grid at night are from their refrigerator's compressor cycling on and off. In the winter months, an increase is seen at about 4:30am when the programmable thermostat turns on their furnace to warm the house before they wake up. Large spikes in consumption from the grid (power consumption in excess of 2 kW) are generally from running an electric clothes drier. In the summer, the Beach's use a solar clothes drier (one of the most common forms of solar power, also known as a clothesline) to largely eliminate that electricity usage.

Since the Beach's home is grid-tied and net-metered, their solar electric system runs the meter backwards when it produces more electricity than they use that day. The plot to the right (click on it for a larger image) shows the amounts of utility electricity taken from the grid (red bar) and solar electricity sent to the grid (blue bar) each day for the past week. On days when the blue bar is larger than the red bar, their system produced more electricity than they used that day. When that happens, their electric meter reading is smaller at the end of the day than it was at the beginning of the day. If their electric meter reading at the end of the year is smaller than it was at the beginning of the year, Xcel will pay them for the net production at the wholesale electricity price (about half of the retail electricity price).

Getting Started on Your Solar Electric System

1. First, call us at 303-363-7848. We will pre-qualify you over the telephone.
2. After pre-qualification, we will set up a time to come on out for a FREE site evaluation. Evening (before sunset) and weekend appointments are available.
3. You will receive a system quote within a few days. Questions anytime during the process are always welcome.
4. Once you accept the quote and sign our contract we will start the interconnection agreement process with your utility.
5. With the interconnection agreement in place, we'll work with your local building department to get the necessary permits.
6. Once we're all set, installation usually takes only a few days (weather permitting).

Why wait? Let's get started on your system today!

Contact us at 303-363-7848, we're happy to help you start using clean electricity!