Facts About Residential Solar Power Systems

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Information regarding the installation and efficiency of photovoltaic (PV) solar panels.

Residential solar panel installations have been around since the 1970's but in recent years these installations have become increasingly prevalent due to utility, state, and federal government rebate programs. Residential solar installations doubled from 2007 to 2008 and continue to grow.

Photovoltaic (PV) devices generate electricity directly from sunlight via an electronic process that occurs naturally in certain types of material, called semiconductors. Electrons in these materials are freed by solar energy and can be induced to travel through an electrical circuit, powering electrical devices or sending electricity to the grid.

Typical residential PV systems commonly have a peak power production of between 1,200 and 5,000 watts, AC - requiring from between 150 to over 1,000 square feet of installed area depending on the efficiency of the PV technology used. Most often, PV panels are installed on roofs, but they can also be installed as free-standing units, on a pole on the ground, or even on complex tracking structures that change with the sun's angle during the day.


PV Systems

There are two types of solar power systems you can install in your home, grid tie and off grid. Grid tie solar systems tie into your local utility company and offset your usage from your local power provider. Grid tie is the most popular type of solar power system for homeowners in the United States. Off grid solar power systems are typically used in homes located remotely or for cottages and cabins which are far from the utility grid. With an off grid solar power system you're completely independent from the power grid, so you need to ensure the system can produce enough energy to run your residence.

Solar Power System Components

Solar panels are the most critical part of a home's solar power system, but they will not be useful unless they are part of a system; what good is an engine if there are no wheels, transmission, or steering systems in a car? The panels capture the sun's light and convert that light into DC (Direct Current) electricity. Homes run on AC (Alternating Current) electricity so a solar inverter is just as important as the panels. Inverters receive the DC energy from the panels and convert the power to AC, which can be used in your home. After the energy is converted to AC, the power flows into your home through your current electrical box.

If you were to install an off grid solar power system you'd need two other critical components. Solar batteries to store power for later use and a solar charge controller, ensuring your batteries aren't being over loaded or under loaded by the power flowing out of the solar panels and inverter.

Types of PV Panels

Mono crystalline solar cells are cut from a chunk of silicon that has been grown from a single crystal. These cells are used in high quality solar panels and are very efficient in converting the sunlight into electricity. Mono crystalline solar panels produce the same power as a polycrystalline solar panel but the cells of a mono crystalline solar panel have a uniform color and appearance.

A polycrystalline solar cell is cut from multifaceted silicon crystal. Polycrystalline solar panels produce the same power as a mono-crystalline solar panel; however the cells of a polycrystalline solar panel tend to have a unique appearance, similar to that of shattered glass.

Thin Film amorphous panels are about half the efficiency of monocrystalline panels, but production costs are cheaper. Thin film requires more space to install the same size system as crystalline panels would. These types of PV cells are typically incorporated on otrher surfaces, such as roof tiles or shingles.

Ratings and Information

Minimum Warranted Power

This is a manufacturer rating. Essentially, the manufacturers will guarantee that their panel’s actual power rating will not be less than a specified amount coming out of the box. This may have several names including “peak tolerance rating, minimum warranted power”, and “negative tolerance rating.” With either rating, it means that a 200-Watt solar panel, with a negative tolerance rating of 10% (or a minimum power rating of 90%) is only warranted for 180 Watts out of the box.

Remember to look for the highest minimum warranted power rating and/or the lowest negative tolerance rating.

STC vs. PTC Rating

California and many other states require solar panel manufacturers to submit operating specifications for their products. These products are then further tested by an independent laboratory. Many states use an approved testing standards such as PVUSA. The results of PVUSA tests are dubbed a panel’s PTC rating (PVUSA Test Conditions).

PVUSA: Photovoltaics for Utility Systems Applications. Application and Validation of a New PV Performance Characterization Method - http://photovoltaics.sandia.gov/docs/PDF/IEEE5.PDF


Therefore every panel will have both an STC rating (standard test conditions) and PTC rating. The PTC rating involves more extreme, real world test conditions, so you’ll find it to be lower than the STC rating and more indicative of how a solar panel will hold up. So the higher PTC rating, the better.

Also, it is the PTC rating that the state uses to calculate system rebates (i.e., expected performance). So while the STC rating for a panel may be 175 Watts, if the PTC rating is only 160 Watts, then that will be the benchmark for your cash rebates — an important tip when figuring expenses and payback period for your solar system.

Efficiency Ratings

This is the rating we’re all familiar with and the one that gets so much attention these days as solar panel developers and researchers struggle create the high efficiency solar panel. Efficiency rating is vital because the higher the efficiency, the more power you’ll get per square inch of panel surface. In other words, the higher the rating the less area you’ll have to use to achieve the same amount of power production. Be sure to ask your dealer for a factory printed spec sheet that lists solar module efficiency (not simply solar cell efficiency) and the other ratings listed above and below.

UL Listing

Underwriters Laboratories (UL) (www.ul.com ) is a product rating company that has been testing products for safety for over a century. In terms of solar panels they test for mounting method, weather resistance, performance, and other safety considerations. In 2008, UL opened a massive PV testing site in Silicon Valley. Always look for products that pass UL’s testing as indicated by being “UL listed.”

Also verify that systems are UL listed. Individual components may be UL listed, but the system may not meet standards if improper system designs are used or components not rated for the power requirements or environmental conditions.


System prices are drop each quarter and according to the Solar Energy Industries Association, www.seia.org , average national installed costs were $3.85/W for the 1st Quarter of 2011.

Highly Rated Solar Panel Manufacturers

New manufacturers are coming into the market every year. Well established companies also invest in PV systems, buy smaller companies, or develop their own systems and processes. Further research is important when selecting a PV system for your home and always look for warranties of at least 25 years. Many companies also guarantee a minimum power produced by the PV system for a certain number of years. The percentage depends on the technology and the rated peak power of the PV system and is generally about 80% of the rated power.

Some of the top selling systems are:

Sunpower Corporation


BP Solar


Evergreen Solar



First Solar

Suntech Power, Inc.



Roberta Baxter
Posted on Aug 9, 2011
Roberta Baxter
Posted on Jul 12, 2011