Understanding Solar Panels: Addressing Misconceptions and Optimizing System Performance
At Solar Spot we are always focused on the current and future performance of a complete system, instead of a quick sale of individual components. Therefore, whenever we receive an order for solar panels, we engage with our clients to understand why they ordered a certain brand or size of panel. In this post, we will discuss and explore all the important parameters of a solar panel, as well as address common misconceptions among end-users, resellers and installers. We will also explain the important specifications of solar panels and how they should be impacting the purchaser’s decision-making process. We will not be delving into the deepest technical details, those interested in learning even more can visit our Technical Library.
Standard Testing Conditions and the Effect of Temperature
The performance of Solar panels is tested at a cell temperature of 25 degrees Celsius and with a solar radiation of 1000W/m^2. The important consideration here for end-users, is that their solar panels will most probably not be operating exactly at these Standard Testing Condition (STC) for much of their lifetime, and therefore the actual output might differ considerably from what was expected. As the temperature of the cells increase, their output decreases. Thus the sunniest day might not yield the best output if the cells’ temperatures are very high. Conversely, at lower temperatures the cells will outperform their rated output if decent sunlight is available. This might sound fantastic, but it could catastrophically damage the solar charge controller (MPPT or PWM) if the system designer did not leave a margin of safety. Take note that we mention “cell temperature”, not “ambient temperature”. Just like the dashboard of your car after standing in sunlight, the cell temperatures can be much higher than the ambient temperature.
Monocrystalline vs Polycrystalline Solar Panels
The cells in Monocrystalline panels are cut from a single silicon crystal. Visually the cells all look homogenous and are usually quite dark in colour. The single crystal will refract and reflect a lower amount of the incoming sunlight, and therefore should operate more efficiently than polycrystalline cells.
The cells in Polycrystalline panels are cut from a prism consisting of many crystals orientated randomly. These panels usually have a purple or blue hue, and the individual crystals are visible to the naked eye. The refraction and reflection of sunlight by these cells is higher, and lead to a typically lower efficiency.
Efficiency will be discussed in more detail later, but it is worth noting here that the end-user need not be overly focused on this parameter. The darker cells will also absorb more heat, which affects the panel’s output negatively, thereby off-setting some of the efficiency advantages of the Monocrystalline cells.
Misconception: Polycrystalline Panels are inherently inferior to Monocrystalline Panels
Verdict: Not necessarily
Some manufacturers like Canadian Solar has invested heavily into perfecting the Polycrystalline cell manufacturing process and their Polycrystalline panels outperform some Monocrystalline cells from other manufacturers. The lighter colour of polycrystalline cells also absorb less heat and therefore tend to run slightly cooler, leading to a better output performance in real life scenarios. As with many other parameters, the end-user need not be overly concerned with the cell type because the Power output rating of the panel has already taken the performance of the cell type into account.
Rated Power Output, Voc, Isc, Vmpp and Impp
The Power Output of a Solar Panel is a product of the Voltage created by its cells, and the current flowing through them (and by extension through the solar cables). When a Solar Panel is exposed to sunlight but not connected to any load, it has the highest potential for creating electric current. This is the Open Circuit Voltage (Voc). When the panel is connected to a load and electric current starts flowing, the Voltage drops. The higher the flow of current, the lower the potential of the cells to drive further current flow, thus lower voltage. If the positive and negative terminals of a solar panel are connected by a cables with no resistance, the electric current flow would be at a maximum, this is called the Short Circuit Current (Isc).
The ideal situation in a solar panel, is to have a decent balance between Voltage and Current flow, so the Voltage can sustain the flow of current without dropping further. The solar charge controller, usually an MPPT is always searching for the best combination of Voltage and current, to extract the maximum possible power from the panel. The optimum Voltage is the Voltage at Maximum Power Point (Vmpp) and the optimum current is the Current at Maximum Power Point (Imp).
The rated Power Output is the most prominent parameter according to which solar panels are priced and advertised, and therefore becomes one of the main focuses of the end-user.
Misconception: A Solar Panel with a higher Power output, is inherently superior to lower wattage panels
Verdict: False
Of course the Power Output of a panel is an important consideration, but because they are most often used in a system of many panels, an over-emphasis is placed on the specification for individual panels. Usually the specifications of the MPPT (standalone or integrated into the inverter), is the main determining factor for panel size. Very often a grouping of smaller panels, especially considering series and parallel connection options, will allow a higher total Wattage of panels to be connected to the MPPT, than if a larger panel was to be used. The only considerations which really matter, are the total Wattage installed, and how much the total configuration cost in R/Watt. Individual panel power output becomes virtually irrelevant.
Efficiency of a solar panel
Efficiency of a solar panel is measured at Standard Testing Conditions. In basic terms, the efficiency is the percentage of the available sunlight which is actually converted to electricity. As mentioned previously, a solar irradiance of 1000W/m^2 is used for STC tests. If a solar panel has a physical size of 1m x 2m = 2m^2, then the available energy is 2000W. A solar panel of this size will typically have an output of 400W, meaning 400W of the available 2000W was converted to electricity. 400W / 2000W = 20% efficiency.
Misconception: The efficiency should be a major deciding factor for the end-user
Verdict: Mostly False
It is important to remember that when a Solar Panel is assigned an Output Power Rating at STC, the efficiency has already been taken into account by Physics. A 500W panel with a 20% efficiency does not have a lower output than a 500W panel with a 23% efficiency. Both are 500W panels as rated at STC. Therefore the end-user should not be overly concerned with this parameter at all. Certain panels do have increased efficiency under certain conditions, for example bifacial panels which can also convert reflected light from underneath into electricity. Technologies like these could be of use to the end-user, but only experienced designers will truly extract the potential from these technologies with specific panel placement and application.
Jolywood 460W Bi facial vs JA Solar 550W Mono-facial
From left to right – Jolywood 460W bi-facial panel’s back-side which can absorb otherwise reflected and lost light energy thereby increasing overall panel efficiency to 26%, Jolywood 460W bi-facial panel’s front (upper) side with the typical black Mono-crystalline cells visible, a JA Solar 550W Mono-crystalline Solar PV panel, the white back-sheet of the JA Solar 550W mono-facial panel. Photo Credit: Solar Spot Warehouse
Compatibility with Inverter, Batteries and Balance of System
The Voc, Isc and Power Output must be carefully matched to the Inverter (or standalone MPPT), and should also be sized according to the battery capacity. Failing to match these items correctly will seriously affect system performance, and can often lead to warranties being voided. The calculations for system design also have to take into account many other factors which cannot be thumb-sucked. At Solar Spot we often receive orders for Solar Panels, and after engaging with the client, understanding their needs and performing the necessary calculations, we end up supplying completely different panels and quantities than the client had in mind originally.
Unfortunately most end-users turn to Social Media forums for advice, and although there are many real experts on these forums, there are also many quacks. It is virtually impossible for an end-user to ascertain which advice comes from a real expert and which advice to ignore. It is much safer for an end-user to contact well-established experts with technical credentials directly, than to take advice from random comments on posts. As a rule, if an “expert” cannot back up their advice with information from product datasheets, manuals, calculations and experience which makes logical sense, it is safer to ignore the advice. In the words of Albert Einstein: if someone can’t explain it simply, they don’t understand it well enough.
Physical Size
The ability of a cell to generate electricity is directly related to its size – a larger surface exposed to sunlight, will be able to convert more sunlight into electricity. More cells assembled into a solar panel will thus also lead to a higher output. These two statements are of course only perfectly true when comparing cells which are identical in all other aspects. The key takeaway is that panels with a higher Power output, are physically larger and heavier as well. Given that most residential installers have to use basic access equipment like ladders to get equipment onto a roof, a larger panel is a lot more difficult to work with and installers will usually charge more installation labour per panel. In windy areas such as Cape Town and the West Coast, a larger panel is just a larger kite to hang onto.
Misconception: Available roof space will always be better utilised with higher powered panels
Verdict: It depends on the exact dimensions
Because a higher powered panel will also have a larger physical footprint, the total area covered by panels is the only important factor. How this coverage is achieved, is mostly unimportant. Very often a grouping of smaller panels can be fitted easier on a tricky roof, leading to better overall usage of the available space. For example, a row of large panels might be just a little too long to allow for a second row, whereas an entire additional row of panels with 20% lower output could be fitted, leading to 60% more usable space with the smaller panel.
Double row Canadian-Solar 455W poly
A perfect example where 2 rows of 455W Canadian Solar panels could be fitted, but only a single row of 550W panels would have fitted. In this case, the 455W panels led to a much better coverage of the available roof space. The typical blue hue of Polycrystalline panels is evident. Photo Credit: M. Muller
Shading Tolerance
Due to modern solar charge controllers moving mostly towards high-voltage, low-current specifications, panels are most frequently wired in series. This has many advantages, but the main drawback is that shading on one panel, will reduce the electric current flowing through the entire string of panels. This is analogous to a hosepipe which is kinked in one spot; the flow rate of water through the entire length of hosepipe is reduced. Similarly, within the panel the cells are also connected in a number of series “strings”. Panels with normal cut cells typically have 3 “strings” of cells, with 20 – 24 cells per “string”. Each string can be removed from the flow equation with the use of a bypass diode within the panel (almost like a branch connector in a hosepipe which can divert flow around the kinked pipe). Thus any partial shading on the panel, will only affect the third of the panel where the shading occurs. If 2 of the 3 strings are experiencing shading, only the remaining third of the panel will be producing electricity, but at least the entire string of panels isn’t throttled completely.
Half-cut cell technology does exactly what it says; each cell is cut in half, and thus 6 strings of cells are created instead of 3. This means that partial shading can now be managed in sixths of a panel, instead of thirds. This is very useful in areas where partial shading occurs, for example via tree leaves or other small artificial structures.
Misconception: Half-cut cells are a necessity in all Solar Panels
Verdict: It depends on the shading conditions
Shading tolerance is a parameter which can be of valid concern to the end-user, but only if shading is a reality at the installation site and only if the type of shading will allow the bypass diodes to circumvent the shade. A fully shaded panel will not benefit from half-cut cells.
Shading on solar panels
An unfortunate example of a chimney casting a shadow on 2 parallel strings of panels, negatively affecting the output of both strings even with only partial shading on the panels. Photo Credit: E. Carbutt.
Brand / Manufacturer
Brand consciousness is important, but it is very important to understand how to measure different brands against each other. Choosing a brand purely because a friend or relative purchased it, does not always constitute thorough research. The important factors to consider are:
Manufacturing Quality Control: A formal certification for quality control, such as ISO 9001 awarded by a reputable assessor such as TUV Rheinland, is the best indicator of sufficient Quality Assurance procedures throughout the manufacturing process. It also confirms compliance to other important standards such as Safety Management and ethical business practices.
Bloomberg Tier rating: This is an indication of the financial sector’s confidence in a manufacturer to supply solar panels for large, financed projects. This list is reviewed quarterly and there is no guarantee that any manufacturer will remain on this list for consecutive quarters.
Number of Patents held: This is an indication of a manufacturer’s commitment to Innovation, Research and Development.
In-house capability and process control: A Manufacturer which controls the manufacturing process from start to end, including cell-manufacturing, has an inherently higher probability of controlling the quality of the products leaving the facility.
Ranking in the Renewable Energy Sector: Renewable Energy companies are ranked in indices such as the Global Top 500 Energy Enterprises. Appearing on a list like this indicates sufficient scale and stability, which in turn indicates a high probability of being able to provide support and honour their warranties for years to come.
Industry Awards: Various publications and industry bodies award accolades to manufacturers with a high level of innovation and quality products. A manufacturer recognized by industry players and peers is almost certainly a quality manufacturer.
Misconception: Tier-1 solar panels are the best and should be the only consideration
Verdict: False
As mentioned above, a “Tier-1” panel doesn’t actually exist, only a manufacturer can be rated as Tier-1. Also, a manufacturer which is ranked Tier-1 while panels are purchased, might not appear on the Tier-1 rankings in the following quarter/s. Many excellent and well-known Solar Panel manufacturers have never appeared on the Bloomberg Tier-1 list.
Reputable, Authorised and Legitimate Import and Distribution Channels
Currently the biggest problem in the South African Solar Panel retail sector, is the circulation of counterfeit and grey-import Solar Panels.
Counterfeit panels are fake panels, much like other fake retail products. They are often so well-copied from the original, that only an expert who works with various brands of Solar Panels on a daily basis, will be able to tell the difference. The counterfeit market is a massive and lucrative one, with lots of effort and resources behind it. The only way an end-user can be certain, is by purchasing from legitimate sellers and installers with sufficient technical knowledge and established supply chains.
Grey-import panels are original, legitimately manufactured panels, but which has entered the South African market via unauthorized channels. As with many premium solar related items (inverters, batteries etc), the import channels are controlled tightly to ensure product quality and protection of the market. Currently many importers with some spare cash will import a few containers of original solar panels, but without the necessary authorisation and backing from the manufacturer. This is usually because these importers have little to no experience in the Solar industry, thus cannot offer the Technical support, but are using their purchasing power to enter the lucrative renewable energy market. The result is that the manufacturers such as JA Solar and Canadian Solar will not honour the warranties or support any grey-import panels. Clients usually only realise this when requesting support with their product serial numbers, which might be a decade later, at which time the importer and seller will be long-gone.
In a similar fashion to non-industry importers, many new resellers also have little to no technical knowledge or experience in the industry. Due to legitimate importers having stringent requirements for resellers to enter the market (such as requisite Technical knowledge), these resellers will (knowingly or unknowingly) end up purchasing from unauthorised importers. Illegitimate importing channels will often yield cheaper pricing, so if the price looks too good to be true, it probably is.
Warranties
The warranties applicable to Solar Panels are split into 2 categories:
Material Quality and Workmanship: This part of the warranty relates to the type of warranties which consumers are used to when purchasing other retail products. It reflects the confidence of the manufacturer that the materials are of good quality, and that the manufacturing and assembly process was executed to standard. Typical flaws which may occur are loose corners on the aluminium frame, delamination between glass and cells, discoloured cells, etc. However these flaws are very scarce and in our experience account for less than 0.1% of all solar panels from reputable manufacturers. As with all warranties, a number of conditions must be met, ie correct handling and installation procedures.
Linear Output Degradation: This part of the warranty predicts the expected performance of the panel over time. Solar PV cells continuously lose a portion of their output capacity, in effect they “wear out” electrically. Since the investment in a Solar PV system is significant, it is important to incorporate the expected future output into any financial calculations and considerations. Top quality panels typically degrade at 0.5% (+/- 0.1%) per year, meaning that the user can expect the panels to deliver approximately 85% of their original output after 30 years of service.
In Summary
It is clear from all the discussed topics, that it is very difficult for the end-user to gauge the suitability of a Solar PV Panel for their intended use. With so many variables at play, it is best to purchase from a legitimate, established reseller or installer with proven technical credentials, and follow their advice. The key learnings and guidelines are:
• The legitimacy of the import and distribution channel is just as important as the brand name itself. Without this, no support or warranty will be honoured by the manufacturer.
• If the price looks too good to be true, it probably is
• Polycrystalline panels are not always inferior to Monocrystalline panels
• Panel sizing and output ratings must be determined through proper design and calculation, taking into account the inverter / MPPT specifications, battery capacity etc.
• High-powered panels are not inherently better than lower powered panels.
• “Tier-1” panels do not exist, only Tier-1 manufacturers. This Tier rating is only a small part of the quality indication of a Solar Panel and therefore should not be the main purchasing consideration
• Warranties have 2 components, it’s important to know what the expected future output degradation is
