One of the most important considerations when you are planning your installation is locating your home solar kit to maximize solar exposure. You should start your planing by positioning your solar panels so they are not shaded from 9 a.m. to 3 p.m. Many installers and homeowners alike fail to consider the impact of even negligible shade caused by overhangs, second stories, trees, exhaust vents, power lines and chimneys.
Each solar panel consists of dozens of cells that, when even partially shaded, will result in decreased performance, which is like throwing KWH and money down the drain. Lower performance means more electricity purchased from the utility, and less financial return on your solar investment. For those grid-tied home solar systems that qualify for performance-based incentives, even more revenue is lost with poor performance. Because of the resistance caused by an inactive portion of a series circuit, the impact of shading across a series of cells can be severe.
Having said that, there are a couple of things you can do if you have shading issues. The first thing going for you is most solar panels today incorporate bypass diodes that can route power around a shaded portion of the module, thus minimizing power losses from localized shading. The second thing is consider using micro-inverters instead of a regular central string inverter. Micro-inverters isolate power down to a single or pair of panels (depending on the inverter selected) which creates a more forgiving system. You do loose some efficiency in the system, but it is not significant and even that small loss is becoming less of a factor with more efficient micro-inverters.
Shading is the number one solar system performance problem and should be avoided. Our solar installers take great care so that when the solar panels are installed in rows and tilted up from the roof plane there is not inter-row shading. Calculating the distance needed between the rows can be complex, but at least for flat roofs, there is a simple design rule the space between a row of modules should be at least three times the height of the row in front of it.
For example, if your home solar south-facing array is mounted on a flat roof and stands 2 feet tall, each row would start 6 feet behind the row in front of it. This will provide a clear solar window from at least 9 a.m to 3 p.m., even as far north as 45 degrees latitude. In the southern half of the U.S., closer spacing may be possible, but minimum spacing should not be less than two times the height of the adjacent row. Those are minimums wider spacing may be used to squeeze out a bit more energy production in the early morning and late afternoon. In snowy regions, drifts or accumulated snow can further complicate row spacing and array placement. Be sure to provide ample clearance under and around the array to help keep it clear.
Do-it-yourself solar installers should take care when planning an solar array’s orientation and tilt because it can make a difference in an panels energy production. For best year-round performance in most locations, fixed arrays should be oriented to true south as opposed to magnetic south which means taking into consideration the site’s magnetic declination.
Solar panel array tilt also plays an important role in energy production. For optimal production, arrays generally should be tilted at an angle equal to your latitude. However, most solar panels are mounted parallel to the roof plane, and have the same tilt as the roof, which is typically pitched at an angle less than the latitude. An array mounted parallel to the roof surface at a tilt less than latitude will produce more energy in summer, when some utilities have higher per-KWH rates.
If your home solar installation site does not allow for true south orientation or tilt equal to latitude, you can simply factor the production losses into your system design and compensate by using a slightly larger array. To determine the KWH impact of various tilts and other factors for any solar PV system at any site, use the National Renewable Energy Laboratory’s PVWatts online calculator.
Solar panel power ratings (nameplate ratings) are determined at “standard test conditions” (STC) 1,000 watts per square meter of solar irradiance at a PV cell temperature of 25°C (77°F). A system’s size is nominally stated by multiplying the STC rating by the number of modules but you shouldn’t count on this being an accurate reflection of the system’s actual output.
STC testing is performed in a laboratory setting where solar modules are flashed with a light source and power output is measured. This measurement doesn’t account for temperature or wind variations, which can drastically affect performance. Like any material exposed to sunlight, PV modules heat up as they absorb solar infrared radiation, becoming less efficient at converting light to electrical energy. For solar cell temperature to be 77°F, the same as STC, the ambient air temperature has to be much lower (about 23°F to 32°F) unusually low temperatures in most circumstances.
Another standard, PTC (PVUSA test conditions) was developed to better simulate real-world installations. PTC is conducted at the same irradiance, but at a somewhat more realistic ambient temperature of 68°F (with cell temperature about 113°F), and at a wind speed of 1 meter per second(2.24 mph). Because temperature-related power loss averages -1/2% per °C rise for crystalline solar panels, their PTC ratings typically range from 85% to 90% of the STC rating.
The underlying lesson is to provide for sufficient airflow around mounted PV modules to minimize production losses due to heat. In general, allow a 3- to 5-inch unrestricted air gap between the roof and flat-mounted modules. Modules tilted up from the roof plane fare even better but to some, tilting can be aesthetically undesirable.
To most accurately project long-term performance, also consider the module manufacturer’s warranted minimum solar power rating. After modules are placed into service, their power output will decrease over time. Besides initial photon degradation due to the physical process that generally occurs within the first few hours of a PV module’s operation, the longterm effects of weather and photon degradation influence module performance over its lifetime. One report estimates that initial degradation will be 0% to 3.9% of a crystalline PV module’s performance, while continuous degradation can reduce performance from 0.1% to 1.0% per annum. Reported degradation values will vary.
Most solar modules are warranted for minimum peak power output within two different time frames 90% of minimum peak power for 10 years, and 80% for 20 to 25 years. While you can’t do much to prevent module degradation, you can select wisely. Before you buy, compare the rated power tolerance for various solar panels. Most solar panels have a tolerance of ±5% (or better) of STC-rated power. For example, if a 100 W module has a specified power tolerance of ±5%, then the minimum peak power value for this module will be 95 W, and the module warranty will be based on this value rather than the STC rated power of 100 W. The tighter the tolerance, the more you can be assured that you’re getting the wattage that you paid for.
Lastly, because module degradation can result in lower voltage output, be careful when matching a solar panel to a particular inverter’s input voltage range. Say a grid-tied inverter input window will accept eight to twelve modules in series. After many years in the field, the voltage of the solar array could degrade to the point that on a hot, sunny day, eight modules in series no longer stay within the inverter’s input voltage window. To avoid this problem, aim for the higher end of an inverter’s input voltage window when you’re determining the number of modules in series strings.
Solar panel manufacturing tolerances mean that modules of the same make and model will have slightly different current-voltage characteristics resulting in a decreased efficiency when the modules are connected together you can figure in a loss of up to 2% because of mismatch. “Module mismatch,” as discussed here, is not referring to modules of differing make or models being wired together. This is a separate issue and if mixing modules is done incorrectly it can result in much more significant power loss. If modules are wired in series, then all within the series string should be of the same model and with the same tilt and orientation.
The next consideration in planning your do-it-yourself residential solar system loss is inverters, which convert the solar array’s DC into AC for household use. Unlike modules, inverters should be installed out of direct sun. Too much heat is a deadly enemy of all electronics, and inverters are no exception. Installing in a high temperature environment can cause a unit to operate less efficiently and may lead to premature component failure. Even inverters that have weather protection and are rated to be installed outdoors must be kept shaded, even if it means installing an awning over them. Likewise, be sure that inverters installed in closets or small rooms have sufficient air circulation to help remove heat buildup.
Most modern inverters are rated at efficiencies of 90% or greater, but actual operating efficiency can vary. One factor that can affect power production is an inverter’s maximum power point tracking (MPPT) performance. Solar module voltages fluctuate as light and temperature conditions change, and the inverter must be able to work efficiently within a range of voltages. If an inverter’s effective MPPT voltage range is too narrow, then production can drop accordingly.
Always consider the efficiency of inverters as well as solar chargers before buying. A 1% improvement in efficiency can mean thousands of KWH gained over the lifetime of your system, and more money in your pocket. Each inverter lists maximum efficiency in its specifications, but a more realistic value is the “weighted efficiency” a useful comparison tool for designers and consumers. A weighted efficiency is estimated by assigning a percentage of time the inverter resides in a particular range of operation to approximate its efficiency over the full day.
Because available sunlight and array operating conditions are constantly fluctuating, actual array power will vary throughout the day, so weighted efficiency can be a better predictor of system performance. Line Losses. The amount of energy lost in conductors and electrical connections is known as line loss. The wasted energy from resistive losses voltage drop in the electrical circuit from source to load should be designed to be less than 5%. Since voltage drop is additive for each individual wire run within a circuit, keeping the overall voltage drop from source to load under 5% means voltage drops of the individual wire runs will have to be much lower (2% or less). Maximize performance by evaluating and sizing each wiring run individually. Of particular importance is the output circuit from a grid-tied inverter to the main service panel. This wire run usually needs to have a 1% or lower voltage drop to ensure that the inverter has enough excess voltage to be able to push its energy onto the utility grid and to make sure the voltage from the grid stays within the inverter’s AC operating window. Using higher voltages and larger conductors means less resistance losses.
Additionally, reliable, low-resistance connections between custom conductors and equipment will help minimize losses. Soiling. Dirt, dust, bird excrement, and snow can filter out some sunlight from the solar cells. According to the National Renewable Energy Laboratory, modules in areas that experience high levels of particulate pollution and infrequent rain can experience soiling losses of up to 25%, especially on flat-mounted arrays. Isolated soiling that remains for an extended time can cause “hot spots” that prematurely degrade or damage PV cells. It’s nothing a squeegee, some water, and a little elbow grease can’t conquer keep your modules clean. In many areas, a periodic rainfall can do the job. But if it’s been a long time since the last rain and you notice a fine layer of dirt or dust building up, you will boost your system’s energy production with a little cleaning.
Performance Check System owners should periodically inspect their solar system and check performance to make sure all is functioning correctly. Physically inspect for broken modules and potential shading issues (for example, a growing tree). For checking performance, most grid-connected inverters have a meter that displays the amount of power (W) being produced and how much energy (KWH) has been produced that day and over the lifetime of the inverter.
Paying attention from when you first plan your solar kit purchase through the installation details monitoring your home system’s output over its lifetime will give you the most value from your investment. Source used with permission; Home Power by Jeremy Taylor.