Solar Panels

Today, solar panels come in many varieties to meet the different cost, performance, size, efficiency, and aesthetics needs of customers.

Solar Panels utilize the following major technologies:

  • Poly-Crystalline Silicon (cheaper, moderate efficiency, usually blue colored)
  • Mono-Crystalline Silicon (considered "premium," high efficiency, requires less roof space, usually black colored)
  • Thin-Film (cheaper, low-moderate efficiency, could be flexible, has lower [better] temperature degradation)

Solar cells made from these technologies are connected together, usually in sets of 60, 72, or 96 cells to form a solar panel, or module.  The panel, or modules, in turn, are connected together to form arrays (e.g. groupings on different roofs), which are then collectively referred to as the Solar System.

Example: Panasonic N330 (VBHN330SA16) panel is composed of 96 N-Type, Mono-crystalline cells.

Power Rating (watts)

Solar Panels are rated by their DC power output, in Watts (W), under standard test conditions (STC) and under real world test conditions (PTC). The closer the PTC value is to the STC, the better since  the "real world" performance would be closer to the "marketed" performance.  When looked at as a ratio (PTC/STC), the ideal would be 100%.

Example: Panasonic N330 (VBHN330SA16) panel's STC rating is 330W and the PTC rating is 311W, resulting in a PTC/STC ratio of: 94% (which is very good for this technology).

Efficiency (%)

A solar cell's efficiency measures its ability to convert light into electricity.  The higher the efficiency the better (assuming the costs are about the same), with residential efficiencies in the low 20% range considered to be excellent at present.  Higher efficiency cells mean that fewer are needed to attain the same panel wattage - hence, requiring less roof space.  Alternatively, a panel utilizing high efficiency cells would produce higher wattage in the same space.  The comparison metric is (PTC Watt rating/sf).   STC watts could be used as well but PTC is the more realistic and conservative value.  Either way, when comparing, the value used needs to be consistent.

Example: Panasonic N330 (VBHN330SA16) panel' s efficiency is rated at 19.7% and its power is rated at 306 PTC Watts and takes up (41.5" x 62.6" = 18sf) resulting in 306W/18sf = 17W/sf.

This is why higher wattage panels do not automatically mean higher efficiencies (and vice versa) since it depends on the cell efficiency and the number of cells used. Some high wattage panels just use more lower efficiency cells, at a cost of larger panel size.

Higher efficiency cells are more expensive; so, if you have enough roof space available, it may be better to use more and/or larger panels using lower efficiency cells.  This will lower your net cost, resulting in a shorter payback period and higher ROI.

Temporal Degradation

Most solar panels’ power also degrades with higher temperature – hotter the panel, less it produces. This percentage decrease per each degree of increase over 25 deg C is referred to as the temperature coefficient. Lower the coefficient, the better. Additionally, to keep the panels cool, it is recommended to install them as high as possible above the roof surface (to provide better air circulation), usually at least 6”.

Example: Panasonic N330 (VBHN330SA16) panel's temperature coefficient is -0.258%/deg C.  On a 95 deg F day, it is about 120 deg F on the roof. 120 deg F is about 50 deg C. This is 25 deg C above the 25 deg C baseline, so the % degradation is -0.258 * 25 = 6.45% leading to a 21W drop n the output of the 330W rated panel.  The good news is that for every deg C that temperature drops below 25 deg C, the output increases.

Temperature Degradation

Most solar panels’ power also degrades with higher temperature – hotter the panel, less it produces. This percentage decrease per each degree of increase over 25 deg C is referred to as the temperature coefficient. Lower the coefficient, the better. Additionally, to keep the panels cool, it is recommended to install them as high as possible above the roof surface (to provide better air circulation), usually at least 6”.

Example: Panasonic N330 (VBHN330SA16) panel's temperature coefficient is -0.258%/deg C.  On a 95 deg F day, it is about 120 deg F on the roof. 120 deg F is about 50 deg C. This is 25 deg C above the 25 deg C baseline, so the % degradation is -0.258 * 25 = 6.45% leading to a 21W drop n the output of the 330W rated panel.  The good news is that for every deg C that temperature drops below 25 deg C, the output increases.

Warranted Tolerance

Not all panels are of the same exact efficiency and output due to inconsistencies in the material and manufacturing process.  So, a particular 330W panel may more or less than 330W.  Most manufacturers provide a Warranted Tolerance % that limits this variance.  Most premium category panels have only a positive tolerance, meaning that, on the average, the actual output will be between 0% and the warranted tolerance value.

Example: Panasonic N330 (VBHN330SA16) panel's Warranted Tolerance is: +10%/-0%.  This means that a randomly chosen panel's output will be anywhere between 330W and 360W, with an average output of 345W.

Bypass Diodes

When one or more solar cells in a panel do not produce electricity due either failure or shading, the solar panel, as a whole, is negative impacted.  To reduce this impact, the cells are broken up into groups (usually 20 cells/group in a 60 cell panel, or 24 cells/group in a 96 cell panel), with each group capable of being "bypassed" in case of low producing cells within that group, so that the remaining groups will still perform.

Example: Panasonic N330 (VBHN330SA16) panel consists of 96 cells and are broken up into 4 cell groups of 24 cells each, and having 4 bypass diodes.

Bi-Facial

Bi-Facial panels have a double glass construction that serve to generate solar energy from both sides of the solar panel, front and back. Being ultra-thin, the panels can also be frame-less and with a plastic back sheet.  They are aesthetically attractive options to install on residential roofs, terraces, and carports.

The technology can be maximized when installed on a highly reflective roof surface with the potential to produce 24% more energy than standard mono-facial modules. Bi-facial panels can generate up to 100% AC output vs. 80-85% AC output from conventional panels due their low thermal and light included degradation. Module efficiencies are currently up to 22%, with panel output between 310W – 370W. A 25-year power warranty is offered with 95% efficiency from years 1-5 and 83% efficiency from years 6-25 at 0.6% degradation.

Frameless

Frameless panels offer the most aesthetically attractive modules in the market. With almost invisible cell lines, their smooth black design will look great on any roof. These panels are not your usual solar panels. They are manufactured with high-efficiency thin-film technology with less degradation than mono or poly modules. The purpose of film is to generate consistent power year-round during cloudy and overcast day and when the sun is at a low angle during the winter months. Shading will have little effect on these panels providing better performance in such conditions compared to typical crystalline silicon modules. With lower temperature tolerance, thin film performs better in a variety climates from cold to hot weather. NREL (National Renewable Energy Laboratory) tested frameless solar modules have the same warranty as the mono or poly modules.