The current and power output 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. If that load is a battery, the battery’s internal resistance will dictate the module’s operating voltage.
A solar panel, which is rated at 17 volts will put out less than its rated power when used in a battery system. That’s because the working voltage will be between 12 and 15 volts. Because wattage (or power) is the product of volts multiplied by the amps, the module output will be 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). This is important to remember when sizing a PV system.
An I-V curve (see image on right) is simply all of a solar panel’s possible operating points (voltage/current combinations) at a given cell temperature and light intensity. Increases in cell temperature increase a solar panel’s current slightly, but significantly decrease voltage output.
PV solar panels are very sensitive to shading. Unlike solar thermal panels used in hot water heating that can tolerate some shading, many brands of PV solar panels cannot even tolerate shading from the branch of a leafless tree.
Shading obstructions can be from “soft” or “hard” sources. If a tree branch, roof vent, chimney or other item is shading from a distance, the shadow is diffuse or dispersed. These soft sources significantly reduce the amount of light reaching a solar panel’s cells. Hard sources are defined as those that stop light from reaching solar cells, such as a blanket, tree branch, bird dropping sitting directly on top of the glass. If even one full cell is hard shaded, the voltage of that module will drop to half of its un-shaded value in order to protect itself. If enough cells are hard shaded, the module will not convert any energy and will, in fact, become a tiny drain of energy on the entire system.
Partial shading of even one cell on a 36-cell solar panel, will reduce its power output. Because all cells are connected in a series string, the weakest cell will bring the others down to its reduced power level. Therefore, whether 1/2 of one cell is shaded, or 1/2 a row of cells is shaded, (as shown above), the power decrease will be the same and proportional to the percentage of area shaded, in this case 50%.
When a full cell is shaded, it can consume energy produced by the remainder of the cells, and trigger the solar panel to protect itself. The solar panel will route the power around that series string. If even one full cell in a series string is shaded, as seen on the right, it will likely cause the module to reduce its power level to 1/2 of its full available value. If a row of cells at the bottom of a solar panel is fully shaded, as seen in Figure 7, the power output may drop to zero. The best way to avoid a drop in output power is to avoid shading whenever possible.
To capture the maximum amount of solar radiation over the course of a year, a solar array should be tilted at an angle approximately equal to a site’s latitude, and facing 15 degrees of due south. 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. At any given instant, an array will output maximum available power when pointed directly at the sun.
To compare the energy output of your array to its optimum value, you will need to know the site’s latitude, and actual tilt angle of your array–which may be the slope of your roof if your array is flush-mounted. 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 tilt is greater than 15 degrees off the latitude 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.
Maximum power is derived at the knee of the curve. Check the amperage generated by the solar array at your battery’s present operating voltage to better calculate the actual power developed at your voltages and temperatures.
Azimuth Angle and Magnetic Declination
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 grid-tied inverter systems allow for only one orientation.) Be aware that solar power output decreases proportionally with a horizontal angle or “azimuth,” greater than 15 degrees from due south. The decrease in annual power output from a latitude-tilted east or west-facing array may be as much as 15% or more in the lower latitudes or as much as 25% or more in the higher latitudes of the United States. Avoid directing your tilted solar panels northwest, north or northeast, as you’ll get little power output.
Magnetic declination, the angle difference between magnetic south and true solar south, must also be taken into account when determining proper solar array orientation. If a magnetic compass alone is used to determine where to point the array, you may not capture the maximum declination field lines in North America, see the map on the right.