Solar panels often produce less electricity than their rated capacity due to real-world conditions differing from the lab's standard test settings. During peak hours, a 10 kW solar system typically generates only 8 to9kW, which surprises many buyers.
The panel's power capacity, known as Pmax, is determined through a flash test conducted in a controlled environment with specific conditions. However, factors like changing light intensity, panel temperature, and air mass affect real-world performance.
Heat reduces panel efficiency, with performance decreasing by 0.3-0.4% per degree above 25°C. NOCT (Nominal Operating Cell Temperature) represents the panel's efficiency under real-world conditions. For example, a 415W panel may produce around 311W under NOCT conditions.
How does temperature affect solar panels and what are Pmax and NOCT?
Many people find it surprising that when they purchase and install solar panels, the panels generally a bit less electricity than their rated capacity. This disparity can be attributed to multiple reasons, but chief among them are the real-world conditions that are often different from the controlled environment under which the panels are tested. Normally, during the peak hours of the day, a 10 kW solar system will generally only produce a maximum power output of 8 to 9 kW. This raises the question, how does temperature affect solar panels and is the heat influencing their output?
Not just a mathematical calculation
Solar energy, while environmentally friendly, is still subject to the whims of Mother Nature. This means that a range of factors can affect its performance. They include cloud cover, air pollution, and atmospheric conditions. All these elements can play a crucial role in determining the amount of sunlight that actually reaches a solar panel. Furthermore, the angle at which sunlight hits the panel, known as the “angle of incidence,” also affects energy generation. A panel that’s directly facing the sun will naturally produce more energy than one at a sideway angle.
What is Pmax?
Solar panels receive their capacity badge or rating, denoted as “Pmax” or “Pmpp,” with many current panels having a capacity of approximately 430 to 450W. This rating is obtained through measurements conducted under Standard Test Conditions (STCs) in a laboratory setting.
Establishing these conditions aims to provide a benchmark for comparing the performance and capacity of solar panels across the industry. That way we can compare “apples with apples”. These Standard Test Conditions consist of 1000W/m2 of light, panel temperature of 25⁰C, and Air Mass AM 1.5.
The panel rating, meaning the label that says 430W or 450W in our example, results from a “flash test” conducted to determine the panel’s power capacity under these conditions.
The test determines the panel’s power output and applies it to the panel’s rating. It’s important to note that experts optimize these laboratory conditions to achieve the best performance from the solar panels, but replicating such conditions in real-world settings is rare.
The intensity of light, the angle of the sun, the temperature of the panel, and the mass of the air are constantly changing throughout the day, so the power produced by the panel will rarely reach that of the Standard Test Conditions.
One reason is that the light level achieved in most of the Australian continent during the day does not reach 1000W per m2, but even on a sunny day in the middle of the day if one reaches 800 to 900W per square meter one would do well. So there is straight away 10% of the potential output lost.
Solar energy adopters must have realistic expectations of the energy generation they can achieve. Given the variables at play, it’s more pragmatic to consider the average sunlight hours and energy generation rather than the peak potential or consult an output calculator as found on this website in the calculator and tools section.
How does temperature affect the solar panel output?
In particular, the temperature of the solar panel and cells will be higher than that of the ambient STC air temperature. Since the sun shining on the panel creates heat, heat reduces the panels’ performance. So on a 25⁰ C day, the panel will be at a higher temperature.
This is usually around 20⁰C to 25⁰C higher than the ambient air temperature. Solar panel’s performance will decrease by between 0.3-0.4% per degree above 25⁰C that the panel gets to. So if a panel is at 20⁰C to 25⁰C above ambient air temperature, the panel is around 10% down on the performance rating at STCs.
The role of temperature in solar panel efficiency
Heat is one of the critical factors that impact solar panel efficiency. While solar panels are designed to absorb sunlight and convert it into electricity, they aren’t as effective at dissipating the heat that comes with it.
Prolonged exposure to very high temperatures can also affect the structural integrity of the solar panel and can lead to faster degradation over time. It’s worth noting that not all solar panels are affected by heat in the same way; this is where the quality of the solar panel, the materials used, and its design come into play. Panels made of monocrystalline silicon, for instance, tend to be more efficient and heat-tolerant than their polycrystalline counterparts.
What is NOCT?
NOCT is an abbreviation for Nominal Operating Cell Temperature, the photovoltaic (PV) cell temperature in a solar panel under standard test settings. The NOCT rating is significant since it determines the panel’s efficiency at a specific temperature, usually about 20°C.
The calculation of NOCT (Nominal Operating Cell Temperature) commonly expresses the result in degrees Celsius (°C). It considers various variables such as ambient temperature, solar irradiation, and wind speed to determine the operating temperature of solar cells under nominal conditions. In simple terms, it shows the panel’s performance in real-world conditions.
They measure this at 800W/m2 of light, 20⁰C ambient temperature, module operating temperature at approximately 43-45⁰C, and Air Mass AM 1.5. Under these conditions, a panel rated at 415Wp at Standard Test Conditions will have an expected power output of around 311W. This will vary a little for different brands of panels.
This means a solar system with a 16 x 415W panel would be rated at 6640W (6.64kW peak). However, the power you will generally see out of the system during peak conditions is closer to 16 x 311W = 4976W.
Incorporating the NOCT into the solar system design
When planning a solar installation, particularly in regions where temperature fluctuations are significant, the NOCT becomes an invaluable tool. Understanding the NOCT allows for better system design, ensuring that the panels operate at or near their optimum efficiency for most of the year. Furthermore, by knowing the NOCT, homeowners and businesses can have a clearer picture of the energy generation they can expect, thus making financial projections more accurate.
Oversizing a system
For the reasons outlined above, it is perfectly okay in Australia to oversize a solar system. Oversizing in this circumstance means putting a higher capacity of panels onto an inverter, than what the inverter capacity is. For example, a 10kW inverter is allowed to be oversized by 1/3 as per the Clean Energy Council design rules, meaning we could put up to 13,33kW of panel capacity onto that inverter and the system would still work very well.
Strategies behind oversizing solar systems
Oversizing a solar system may seem counterintuitive, but it has distinct advantages. The primary benefit of oversizing is to counter the losses associated with real-world conditions – as discussed, solar panels seldom operate at their peak efficiency in the field.
By installing more panels than the inverter’s rated capacity, system owners can ensure a steady supply of electricity even on less-than-ideal days. This strategy also provides a buffer for potential future degradation of the panels, ensuring the system remains productive for many years. Lastly, oversizing is an economical choice; with solar panel prices continually decreasing, and the Australian solar rebate increasing the more panel one adds, increasing the PV system capacity is more affordable than ever and can lead to faster returns on investment.
So is temperature affecting my solar panel?
In conclusion, Pmax and NOCT are two critical parameters that influence the performance of solar panels, particularly in high-temperature circumstances. High temperatures can affect a solar panel’s efficiency and in certain extreme heat situations, the lifespan of solar panels.
Still, manufacturers can mitigate these effects by using temperature-sensitive materials, incorporating cooling systems, and designing panels to operate more efficiently at high temperatures.
Furthermore, the ongoing evolution of solar technology introduces new innovations and materials that constantly advance solar efficiency and enhance its resilience against external factors. To guarantee the best performance in all conditions, it is critical to consider both the Pmax and NOCT ratings when selecting solar panels for a specific application.