|Get the most Power out of your Solar Electric Panels: Electric Resistance, Cell Temperature, Shading Effects and Panel Orientation
Voltage Drops in Solar Panel Systems
The current (amps) and power output (watts) of photovoltaic solar panels are approximately proportional to the sun’s intensity. At a given intensity, a solar panel's output current and operating voltage are determined by the characteristics of the load. Many materials naturally provide an electric resistance to the flow of electrical current. If that load is a battery, the battery's internal resistance limits the module’s operating voltage. This is also known as voltage drop.
A solar panel rated at 17 volts will yield between 12 and 15 volts when used in a battery backup system. Because wattage (or power) is the product of volts multiplied by the amps, the module output is reduced. For example, a 50-watt solar panel working at 13.0 volts will products 39.0 watts (13.0 volts x 3.0 amps = 39.0 watts).
To reduce these voltage drops in off grid systems we utilize MPPT charge controllers. All charge controllers will regulate the amount of voltage to the battery bank within acceptable limits. This keeps the battery from being undercharged or overcharged and prolongs battery life. Additionally, an MPPT charge controller optimizes the voltage of the system array to maximize power output. This can result in a boost of system performance by as much as 30%. Larger solar modules can be used with this MPPT technology without the associated battery voltage drop. This is ultimately more economical on a per watt basis.
An I-V curve shows all of a solar panel’s possible operating points (current/voltage combinations) at a given cell temperature and light intensity (irradiance). “I” is the amperage/current. Increases in cell temperature will slightly increase a solar panel’s current while significantly decreasing voltage
Maximum power in watts is shown at the knee of the curve. Most I-V curve charts use optimal operating cell temperature of 25° C (77° F). To calculate the true power generated from a battery/array system without a charge controller, check the amperage produced during operating battery voltage (12, 24 or 48 volts) and actual cell temperatures.
PV solar panels are very sensitive to shading.
Many brands of PV solar panels do not tolerate shading, even from the branch of a leafless tree. Shading obstructions may be from “soft” or “hard” sources.
If a tree, roof vent, chimney or any other item is blocking the sun’s rays from a distance, the shadow is diffuse or dispersed. A bit of dust directly on the panel surface will also cause soft shading. These soft sources reduce the amount of light reaching the solar panel’s cells and can significantly reduce power output.
Shading obstructions are considered “hard” when objects are in direct contact with the glass, completely blocking the sun’s rays. Bird droppings, broken tree branches and wayward frisbees are examples of hard shading that can affect a panel’s performance. Partial hard shading of one cell in a photovoltaic panel can create a power drop as much as 50%. Because all the cells in a panel are connected in a series string, all the cells will be affected, ultimately reducing the panel output by about the same percentage.
Examples of partial cell shading that reduce solar panel power by half.
When a single cell is fully shaded by hard sources it can pull power from the remaining cells throughout the series string, again likely resulting in a 50% power loss. If the bottom row of cells is fully hard shaded, as can happen with accumulated snow on ground-mounted panels, this can result in a complete power loss. If enough cells are hard shaded the panel can become a drain on an entire system, or on one string of panels within the system.
To reduce this shading effect, roof-mounted panels wired in the same string of a system should be installed on roof sections that face the same directions and set at the same tilt angle. Most manufacturers use bypass diodes that allow current to flow in only one direction to reduce the effects of shading, but they often cannot prevent the effects completely. When shading cannot be avoided (or you do not have enough space to position all solar panels in the same direction), you'll want to consider using:
- The SolarEdge System – SolarEdge PowerBox optimizers allow solar panels to operate independently by maximizing energy throughput of each and every module individually. Because SolarEdge Power Box optimizers automatically maintain a fixed string voltage, the SolarEdge inverter can work at optimal efficiency.
- Micro inverters – In a system using micro-inverters, each panel has its own inverter, eliminating system power drains. Each panel can still be individually affected by shading. See Enphase inverters and AUO AC solar panels.
To compare the energy output of your array to its optimum value, you will need to know the actual tilt angle (which may be the slope of your roof if your array is flush-mounted) and the directional angle of your array. If your solar array tilt is within 15% of the latitude angle, you can expect a reduction of 5% or less in your system's annual energy production. If your solar array orientation is greater than 15 degrees off either angle – the latitude angle and/or the east/west angle, the reduction in your system's annual energy production may fall by as much as 15% from its peak available value. During the winter months at higher latitude, the reduction will be greater, as much as 25% or more.
Optimal Tilt Angle
At any given instant, an array will generate maximum output when pointed directly at the sun. The optimal tilt angle is perpendicular, or 90 degrees, to the sun’s rays at true solar noon. True solar noon is when the sun is at its highest during its daily east-west path across the sky. This is also known as 0° Azimuth.
To capture the maximum amount of solar radiation over the course of a year, the tilt angle should be adjusted seasonally. During spring and fall the angle should be approximately equal to a site's latitude at solar noon. To optimize winter performance, the solar array can be tilted 15 degrees more than the latitude angle, and to optimize summer performance, 15 degrees less than the latitude angle.
Optimal Direction Angle
True solar south is the optimal direction for maximizing the power output of your PV solar panels. Using a compass or a magnetic declination chart does not give you true solar south. See tech tip (top right of page) to find true solar south. The closer you get to the north and south poles the more the earth’s magnetic field skews your compass readings. Other geological magnetic forces can affect the compass needle as well. Remember playing with magnets and a compass in science class?
A magnetic declination map will not give you "true solar south". See tech tip to the right.
If a south-facing roof is unavailable, an east or west-facing surface is the next best option. SolarEdge and Microinverter Solar Power Gridtied Systems allow for your solar panels to be facing more than one direction, while centralized gridtied inverter systems allow for only one orientation.
Local Climate Effects
Typical weather patterns may change your optimal panel orientation. For instance, those on the West Coast with early morning fog may want to adjust their array towards a southwest angle to take advantage of the clear skies later in the day.
The type of solar panel mounting that can offer the greatest flexibility is top of pole mounting. This type of mounting allows you to take advantage of the optimal panel angles for your climate, latitude and site conditions.