PV: Photo-Voltaic means electricity from light.
Basically, there are three major types of solar cells:
The Chart Explained:
Life Span: History is our best indicator of lifespan.The first monocrystalline cells made by Bell Laboratories in the 50s still work! Vice versa, the first amphorous cells made in the 70s deteriorated rapidly in sunlight, and gave solar a bad name. All modules deteriorate slightly over the years, and the new amorphous panels carry 20 year warranties, so you’ll worry more about whether the sun will shine. Warranty accounts for normal degradation. Notice Amorphous have fewer years and allow double the degradation.
Efficiency: is how much of the sun’s energy that is converted to electricity. Our rating shows how many square feet of space 1000 watts of solar modules take. Notice the mono-crystal panels take less than half the space of an amorphorous array. That means smalller mounting racks, thus saving money.
Early Morning, etc Performance: this indicates how much power the module makes when the sun isn’t hitting the module straight on or it is cloudy. While all modules should make their rated power under perfect conditions, single crystal modules put out more power under more conditions to be worth their extra cost.
Partially Shaded Performance: Normally, a solar module should be located so that it is never shaded from 9 AM till 3 PM year round. Just the shadow from a TV antenae or tree branch can keep an entire monocrytalline module from working. Amorphous have an edge here, and are a good choice for an RV that may park in shade. However, its usually better to relocate the module, or remove the obstruction instead.
Hot Weather Performance: Solar modules lose voltage as their temperature goes up. A module can be close to 100 deg F , when it is only 65 deg F outside . They can get as hot as 160 deg F in the summer in the desert on a roof. However, modules are rated at 77 deg F (25 C), so modules are going to perform their best in colder weather. Luckily most systems have a surplus of power in the summer, so the power loss then is tolerable. In extremely cold weather with bright sun and snow reflection, modules can actually exceed their rated output!
All modules are rated at Standard Test Conditions (STC). These are alomst perfect conditions in which full sun is shinning straight on the module, & the module temperature is 77 deg F. These conditions actually occur in the winter for a few hours on clear days, but are not a complete picture of the module’s performance year round in a real life situation. Another standard is being used (PTC) which measures air temperature (usually a module is much hotter than the surrounding air). Modules are usually derated from 10% to 25% under this rating system.
The following table will explain the different module ratings listed on the back of each module:
Its easiest to compare modules on a dollars per watt basis.(Divide the module’s price by its power rating) Just because one module is cheaper than another means nothing without comparing the price per watt. Also monocrytalline is worth 10% more per watt, because it will make more power throughout the day and throughout the year than the same wattage module in polycrystalline.
Solar modules are wired in different combinations to get the voltage & current needed. They are wired in series to increase voltage; up to 30 modules can be strung together for voltages as high as 600 v (the highest voltage most modules are rated for). The voltages of each module are added together.
Modules are also paralleled to increase current. The voltage stays the same, but their currents are added. Some 12 volt systems can have dozens of modules in parallel for currents close to 100 amps. To avoid too much current in a single module that can overheat and melt connectors, the modules are wired into subarrays. Each subarray carries as much current as the modules are designed for including a safety factor. This factor includes both a 25% safety factor and a 25% factor in case the modules produce more than their rated power in special sunlight conditions (see edge of cloud effect in glossary).
Example: If a module’s series fuse rating is 20 amps, then you divide that by the combined safety factor of 1.56. so you could safely wire 12.8 amps of modules together in parallel. If each module’s short circuit current is 4.2 amps, then you could parallel 3 modules. Regardless, we don’t recommend ever wiring more than 4 modules in parallel in one subarray, and some inspectors don’t allow any parallel wiring within a subarray.