Embarking on solar energy is great, but knowing exactly how much your panels will produce is even better. This article guides you through calculating photovoltaic output, focusing on the famous L coefficient. We’ll break down everything, from the basics to the small details that make a difference to your installation.
Key takeaways on calculating the photovoltaic L coefficient
- Peak power (kWc) is the reference, but actual production depends on many factors such as sunshine, panel efficiency, and losses.
- Energy output can be calculated using the formula ‘Surface x Efficiency x Sunshine x Loss coefficient’ or by using peak power and equivalent sunshine hours.
- It is essential to identify and quantify all potential losses: inverter, cabling, temperature, shading, etc. These losses reduce the final output.
- The production coefficient (kWh/kWc) is a good indicator for estimating annual output and comparing different installations or regions.
- Geographical location, orientation, panel inclination, and shading are determining factors that directly influence the calculation of the L coefficient.
Understanding the fundamentals of photovoltaic calculation
To get started in the world of photovoltaics, you first need to grasp a few basic concepts. It’s a bit like learning the alphabet before writing a novel. Without these foundations, it’s difficult to make sense of the figures and calculations that follow.
Definition of peak power and standard conditions
Peak power, often abbreviated to kWp, is the maximum power a solar panel can produce. But be careful, this is under very specific conditions, known as Standard Test Conditions (STC). These conditions include a panel temperature of 25°C, an irradiance of 1000 W/m², and an air mass coefficient of 1.5. It’s a bit like the advertised power for a car under laboratory conditions. In practice, the actual power will often be different. The kilowatt-peak (kWp) serves as a reference for objectively comparing different panels, regardless of where they are manufactured.
The importance of sunshine and solar irradiation
Sunshine is the time during which the sun shines on your installation. Solar irradiation is the amount of solar energy that reaches a given surface, usually expressed in kWh/m²/year. The sunnier your region and the higher the irradiation, the more electricity your installation will produce. This is a determining factor for overall efficiency. For example, the south of France benefits from higher annual irradiation than the north, which directly influences the expected output.
The role of solar panel efficiency
The efficiency of a solar panel is its ability to convert received solar energy into electricity. It is expressed as a percentage. A panel with 18% efficiency will convert 18% of the solar energy it receives into electricity, while a 20% panel will perform better. This figure is also measured under STC conditions. It is important to know that efficiency can be affected by several factors, such as temperature or manufacturing quality. Choosing panels with good efficiency is important, but other aspects such as durability and cost must also be considered.
Methodology for calculating energy output
Several approaches exist for estimating the output of a photovoltaic installation. They all aim to quantify the electrical energy your panels can generate over a given period, usually a year.
Basic formula: Energy = Surface x Efficiency x Sunshine x Loss coefficient
This formula forms the basis of the calculation. It takes into account the total surface area of your panels, their intrinsic efficiency, the amount of sunlight received (sunshine), and a factor that aggregates all performance reductions (loss coefficient). It is important to note that panel efficiency can vary depending on their technology and age.
The output calculation can be presented as follows:
| Parameter | Description |
|---|---|
| Surface (S) | Total panel surface area in m² |
| Efficiency (r) | Panel conversion efficiency (e.g., 14% or 0.14) |
| Sunshine (H) | Annual solar irradiation on the inclined surface in kWh/m²/year (variable by region) |
| Loss coefficient (Cp) | Factor aggregating losses (inverter, cables, temperature, shading, etc.), often between 0.75 and 0.8 |
The formula then becomes: Energy (kWh/year) = S * r * H * Cp.
Calculation based on peak power and equivalent sunshine hours
Another common method involves using the installation’s peak power (expressed in kilowatt-peak, kWp) and a weighted average sunshine indicator, often called ‘equivalent sunshine hours’. The latter represents the number of hours during which the installation would receive an irradiance of 1000 W/m², the reference condition for peak power.
The simplified formula is: Output (kWh/year) = Peak power (kWp) x Equivalent sunshine hours (h/year).
This approach is more direct and often used for quick estimates. It allows you to understand the production potential based on your system’s nominal power. For a more precise estimate, it is advisable to consult specific irradiation data for your geographical location, such as those available via PVGIS.
Practical application with concrete examples
Let’s take an example: a 3 kWp installation with an average annual output of 1100 kWh/kWp (this ratio varies by region). The estimated annual output would be 3 kWp * 1100 kWh/kWp = 3300 kWh.
It is important to consider the losses that can affect this output:
- Losses related to the inverter and cabling (approximately 5-10%).
- Impact of temperature on panel efficiency (can reduce output by 5-15% in high heat).
- Losses due to shading, even partial (highly variable).
- Performance reduction due to low irradiance.
These elements reduce actual output compared to the theoretical value. A detailed analysis of these losses is discussed in the next section. For a personalised evaluation of your project, using a simulator like that of ADEME can be very useful.
Identification and quantification of installation losses
Even with the best panels and optimal sunshine, a photovoltaic installation does not convert 100% of solar energy into usable electricity. Several factors lead to losses, reducing actual output compared to theoretical potential. It is therefore essential to identify and quantify them to obtain an accurate estimate of production and to optimise the system.
Losses related to inverters and cabling
The inverter, a key component that converts DC current from the panels into AC current usable by our appliances, is not perfect. There is an energy loss during this conversion, generally between 8% and 15%. Similarly, the cables connecting the panels to the inverter, and then the inverter to your electrical panel, cause losses due to the Joule effect. These losses depend on the quality of the cables, their cross-section, and length, but are often estimated at around 2% for a well-designed installation.
Impact of temperature and low irradiance
The performance of solar panels decreases as their temperature increases. This is a physical phenomenon: the hotter a panel is, the less efficient it is. Temperature-related losses can vary from 5% to 12% depending on climatic conditions and the type of panel. Furthermore, in very cloudy weather or when sunlight is low, panel efficiency also decreases. These losses due to low irradiance are generally in the order of 3% to 7%.
Consideration of shading and reflectivity
Shadows cast by surrounding elements (trees, chimneys, neighbouring buildings) or even by elements of the installation structure can significantly reduce output. These losses, called shading losses, can be highly variable, ranging from 0% if the installation is perfectly clear, to over 50% in the most unfavourable cases. Losses due to reflectivity, which are generally low, around 3%, must also be considered, but they add to the overall balance. A good analysis of shading is therefore essential, and tools like Épices software can help model them.
Here is a summary table of common losses:
| Type of loss | Estimated losses |
|---|---|
| Inverter | 8% – 15% |
| Cabling | ~2% |
| Temperature | 5% – 12% |
| Low irradiance | 3% – 7% |
| Shading | 0% – 50%+ |
| Reflectivity | ~3% |
It is important to note that the calculation of the overall loss coefficient (Cp) is the product of the efficiency coefficients of each component. For example, a simplified calculation could give: Cp = 0.9 (inverter) * 0.98 (cables) * 0.95 (temperature) * 0.97 (low irradiance) * 0.97 (reflectivity) = 0.74, which represents a total loss of 26%.
The production coefficient: a performance indicator
The production coefficient, often expressed in kWh/kWp, is an essential measure for evaluating the actual efficiency of a photovoltaic installation. It allows for comparison of the electricity output obtained relative to the nominal power of the installed panels. In other words, it tells you how many kilowatt-hours (kWh) your system has produced for each kilowatt-peak (kWp) of its capacity. It’s a simple way to get an idea of your installation’s performance, regardless of its size.
Definition and calculation of the kWh/kWp ratio
The kWh/kWp ratio is obtained by dividing the total energy output in kilowatt-hours (kWh) by the installation’s peak power in kilowatt-peak (kWp). For example, if a 5 kWp installation produced 5500 kWh over a year, its production coefficient is 1100 kWh/kWp (5500 kWh / 5 kWp).
This figure is particularly useful because it helps put production into perspective. A 3 kWp installation producing 3300 kWh/year has the same production coefficient (1100 kWh/kWp) as a 9 kWp installation producing 9900 kWh/year. This helps understand if the installation is performing as expected relative to its potential, taking into account local conditions.
Using the coefficient to estimate annual output
Once you know the average production coefficient for your region, you can use it to estimate the annual output of your own installation. Simply multiply your system’s peak power by this regional coefficient. For example, if your region has an average coefficient of 1200 kWh/kWp and you have a 6 kWp installation, you can anticipate an annual output of approximately 7200 kWh (6 kWp * 1200 kWh/kWp).
It is important to note that this coefficient varies depending on geographical location, panel orientation and inclination, as well as local weather conditions. Online simulators can help you obtain more precise estimates for your specific situation, taking into account factors such as solar panel efficiency.
Comparison of regional performance
The production coefficient offers an excellent way to compare the performance of photovoltaic installations in different regions. For example, an installation in the south of France will generally have a higher production coefficient than a similar installation in the north, due to greater sunshine. Production data can be aggregated to show these regional differences.
Here is a simplified overview of possible variations:
| Region | Average production coefficient (kWh/kWc/year) |
|---|---|
| South of France | 1100 – 1300 |
| Central France | 1000 – 1200 |
| North of France | 900 – 1100 |
These figures are indicative and can be refined by more detailed simulation tools. Understanding these variations helps set realistic expectations and evaluate the profitability of a photovoltaic project.
Factors influencing the calculation of the photovoltaic L coefficient
The precise calculation of a photovoltaic installation’s output, and by extension its L coefficient, depends on a multitude of interdependent factors. Ignoring any one of them can lead to an underestimation, or even an overestimation, of the energy actually produced. It is therefore essential to consider them all carefully.
Influence of geographical location and orientation
The amount of solar energy received by an installation varies considerably from one region to another. The South of France, for example, benefits from significantly more annual sunshine than the North. Beyond latitude, panel orientation is a major lever. A south-facing orientation is generally the most efficient, but south-east or south-west orientations can also be very interesting. The important thing is to maximise direct sun exposure throughout the day and year. A precise orientation study is therefore a key step for a good estimate of solar production.
Impact of solar panel inclination
The angle at which the panels are tilted relative to the horizontal plays a significant role. The ideal inclination depends on the latitude and the desired seasonality for production. In mainland France, an inclination between 30 and 45 degrees is often recommended to optimise overall annual production. However, for specific needs, such as maximising winter production, a steeper angle may be considered. The right compromise must be found to capture the maximum solar radiation throughout the year.
Consideration of shading and the immediate environment
Shadows cast by surrounding elements (trees, neighbouring buildings, chimneys) or even by elements of the installation structure can drastically reduce output. Even partial shading on a single cell can affect the performance of the entire panel, or even the connected string of panels. It is therefore essential to carry out a meticulous analysis of potential shading throughout the day and seasons. The immediate environment, such as the presence of dust or snow, can also impact efficiency. A well-designed installation takes these elements into account to minimise losses. It is sometimes necessary to call on certified professionals for optimal installation, especially if one plans to resell all the electricity produced.
The efficiency of a photovoltaic installation is not just a matter of panel peak power, but a complex combination of environmental and technical factors. A thorough analysis of these elements makes it possible to establish a realistic L coefficient and to anticipate energy production as accurately as possible.
Tools and simulators for photovoltaic calculation
To estimate the potential output of your solar installation, several digital tools are available. These simulators, often free, make the calculation more accessible, even without extensive technical expertise. They take into account key elements such as the power of your panels, the sunshine in your region, their efficiency, as well as the orientation and inclination of your installation. Using these platforms allows you to obtain a reliable estimate of your future energy output.
Presentation of the ADEME simulator
ADEME offers a simulator designed to evaluate solar energy production. This tool takes into account determining factors such as installed power, roof orientation and inclination, as well as the available space for panels. It is particularly useful for adapting your project to the specificities of your home.
Using PVGIS for production estimation
PVGIS (Photovoltaic Geographical Information System) is a free application developed by the European Commission. It greatly simplifies the calculation of your solar output by allowing you to enter the geographical location of your installation. You can find monthly irradiation data there, useful for adjusting calculations based on inclination and orientation, even if the tool defaults to a south orientation. It is a valuable resource for obtaining precise data on sunshine, a key element in output calculation, just like the performance of semiconductors in photovoltaic cells.
AutoCalSol software for in-depth analysis
AutoCalSol, developed by INES (National Institute of Solar Energy), is free pre-sizing software. It goes beyond simple production calculation by also evaluating your self-consumption rate, based on your consumption habits. The tool provides a detailed economic and ecological analysis of your photovoltaic project, making it particularly comprehensive and easy to use. It helps to better size your installation to avoid over-investment or unused surplus production.
To optimise your forecasts, it is advisable to minimise your overall electricity consumption. Real-time monitoring tools can help you identify the most energy-intensive appliances and adopt eco-friendly habits. Remember that panel performance decreases slightly each year, but their lifespan of 25 to 30 years ensures the profitability of the investment.
Here is an overview of the factors to consider for estimating production:
- Panel power (in kWp)
- Sunshine in your region
- Panel efficiency
- Orientation and inclination
- Annual solar irradiation
The basic formula for estimating production is: Solar production = Panel power (kWp) x Sunshine duration (hours) x Panel efficiency (%).
Determining the electrical energy to be produced
Before embarking on the installation of solar panels, it is essential to clearly define your electricity needs. This will allow you to correctly size your system and avoid unnecessary extra costs or, conversely, an underestimation that would limit your production.
Analysis of daily energy needs
To begin, you need to make a precise estimate of your daily electricity consumption. This involves listing all the appliances you intend to power with your future photovoltaic installation. For each appliance, note its power (in Watts) and the number of hours of use per day. The daily consumption of each appliance is obtained by multiplying its power by its usage time. For example, a 5W radio used 5 hours a day will consume 25 Wh per day.
Calculation of electrical equipment consumption
Once you have the consumption of each appliance, you need to add them up to get your total daily consumption. It is also relevant to consider the maximum instantaneous power, i.e., the sum of the powers of all appliances that could operate simultaneously. This is particularly important for sizing the voltage converter.
Here is an example table to organise this information:
| Equipment | Power (W) | Usage duration (h/day) | Daily consumption (Wh/day) |
|---|---|---|---|
| Refrigerator | 150 | 24 | 3600 |
| LED lighting | 10 | 5 | 50 |
| Computer | 50 | 4 | 200 |
| Daily Total | 3850 |
Defining self-consumption objectives
It is also important to define your self-consumption objectives. Do you want to cover all your needs, or only a part? This decision will directly influence the size of your installation. For example, if you aim for complete autonomy, you will need to size your system to meet 100% of your consumption, taking into account losses and periods of low sunshine. The goal is to produce enough energy to cover your needs, while optimising the use of the solar energy produced, potentially by storing it for later use. A good estimate of the potential output of your solar panels is therefore necessary, taking into account factors such as panel power and the annual solar irradiation of your region, as suggested by production calculations.
It is recommended to slightly oversize your installation to anticipate potential increases in consumption or decreases in system performance over time.
Sizing the photovoltaic installation
Once you have a clear idea of the expected energy output and potential losses, the next step is to correctly size your installation. This involves choosing the right power for your panels, the necessary storage capacity for your batteries, and appropriate electronic components such as the charge controller and inverter. Precise sizing is key to an efficient and profitable installation.
Calculation of the total power of panels required
To determine the total power of solar panels to install, you must base it on the electrical energy you wish to produce each day, divided by the average daily sunshine at your site. Average sunshine (Hi) is expressed in kWh/m²/day and depends heavily on geographical location, orientation, and inclination of your panels. Tools like PVGIS can help you obtain precise data for your specific situation. For example, if your daily consumption, adjusted for losses, is 2615 Wh/day and the average daily sunshine for your site is 5.2 kWh/m²/day, the peak power required would be approximately 503 Wp. It is often wise to allow a margin, so choosing two 300 Wp panels for a total of 600 Wp would be a reasonable option.
Choosing batteries and determining their capacity
If your installation is not connected to the grid, energy storage is essential. Battery capacity is measured in Ampere-hours (Ah) and depends on your daily consumption, the desired number of days of autonomy (for periods without sun), and your system voltage (12V, 24V, or 48V). You must also consider the maximum recommended depth of discharge (DOD) for the chosen battery type (AGM, Gel, Lithium) to avoid reducing their lifespan. For example, for a daily consumption of 1700 Wh and an autonomy of 2 days, with a voltage of 24V and a DOD of 80%, you would need a capacity of approximately 200 Ah.
Selection of charge controller and inverter
The charge controller protects your batteries against overcharging and deep discharge, while optimising charging from the solar panels. There are two main types: PWM and MPPT. MPPT controllers are more efficient, especially in variable sunshine conditions or with panels of higher voltage than the batteries. Its capacity must be chosen according to the power and maximum current your panels can deliver. For a 600 Wp system at 24V, a controller capable of handling at least 25A (600W / 24V) would be necessary. The inverter converts the direct current (DC) from the batteries into alternating current (AC) usable by your household appliances. Its power must be greater than the maximum instantaneous power of all appliances you intend to use simultaneously. For example, if your peak consumption is 800W, a 1000W or higher inverter would be appropriate. It is important to choose quality equipment, such as those available from suppliers specialising in solar solutions.
It is important to list all appliances that will be powered and their daily usage time. This allows for the calculation of total daily consumption in Wh. Then, the maximum instantaneous power must be defined, i.e., the sum of the powers of all appliances that could operate simultaneously. This data is essential for choosing the battery system voltage (12V, 24V, or 48V) and the inverter power.
The correct sizing of each component, from panels to batteries, including the controller and inverter, is a step that requires rigour. Poor evaluation can lead to under-production of energy or premature wear of the equipment.
Analysis of tariff and degression coefficients
For those who sell the electricity produced by their solar panels, understanding the tariff mechanisms is essential. These coefficients directly influence the income you derive from your installation. It’s not just about producing electricity, but also about knowing how it is valued on the market.
Understanding the BN and KN coefficients
The BN coefficient, often set at 0.25%, represents an anticipated long-term cost reduction in the photovoltaic sector. It’s a kind of recognition of technological evolution and manufacturing process optimisation. The KN coefficient, on the other hand, is more dynamic. It is calculated based on the evolution of seven economic indices, such as labour costs in the mechanical and electrical industry, material prices (aluminium, copper, steel), transport costs, and interest rates. Its role is to reflect shorter-term cost variations. The objective is to maintain a stable return on investment for projects, even if the economic context changes. For example, a rise in interest rates can be offset by a modification of the KN so that the project remains profitable.
Mechanisms of degression coefficients
Tariff degression is a tool used to adjust the pace of photovoltaic sector development relative to national objectives. It mainly applies to installations over 9 kWp. The principle is simple: if the volume of new installations exceeds a certain threshold, purchase tariffs for new installations can be reduced. This prevents excessive over-profitability and helps control solar deployment according to network needs and capacities. Degression coefficients are calculated by comparing connection requests recorded against quarterly targets set in the Multi-year Energy Programme (PPE). If the volume of requests is higher than expected, the degression coefficient will be less than 1, leading to a tariff reduction.
Example of purchase tariff calculation
Let’s take an example to illustrate the calculation of a purchase tariff for an installation from 36 to 100 kWp. The tariff for a given quarter is calculated from the previous quarter’s tariff. First, the BN coefficient is applied (a 0.25% reduction). Then, the evolution of the KN coefficient, which reflects variations in economic indices, is integrated. Finally, the degression coefficient is applied. If, for example, the volume of connection requests was higher than expected, the degression coefficient could be 0.945, representing a 5.5% reduction in the tariff. All these adjustments allow for obtaining the final tariff for the current quarter. It is important to note that for installations with a power equal to or less than 9 kWp, purchase tariffs are frozen and are not affected by these quarterly evolution mechanisms. For more details on surplus sales mechanisms, you can consult information on the photovoltaic purchase tariff.
The precise calculation of these coefficients may seem complex, but it is crucial for correctly estimating the potential income from your solar installation. Authorities regularly publish index values and tariff decrees to enable this calculation. It is advisable to stay informed of regulatory developments to optimise the profitability of your project and fully understand the conditions for selling your production, particularly via the EDF OA scheme.
Optimising efficiency and monitoring production
For your photovoltaic installation to perform at its best over time, it is important to ensure it produces the expected electricity and to maintain that performance. This involves two main aspects: the quality of the equipment chosen from the outset and the implementation of regular production monitoring.
Importance of equipment quality
The choice of components is the first step to ensuring good efficiency. High-quality solar panels, an efficient inverter, and suitable cabling are essential. Superior quality equipment, even if it represents a higher initial investment, will result in better long-term energy production and greater reliability. Durability must also be considered, as panels have an estimated lifespan of between 25 and 30 years. A well-designed installation from the outset, taking into account all factors such as orientation and inclination, is the basis for optimal production. To help you in this phase, tools like this installation guide can be very useful.
Monitoring solutions for real-time tracking
Once the installation is in place, it is essential to be able to monitor its performance. Monitoring solutions allow you to visualise your panels’ electricity production in real-time. They inform you about the amount of energy produced, as well as your consumption. These systems, often accessible via a mobile application, alert you in case of an abnormal drop in production, allowing you to quickly identify a potential problem. For example, a simple cleaning of the panels can sometimes be enough to restore optimal production.
Here are some actions to monitor:
- Panel cleaning: Dust, leaves, or snow can reduce efficiency. Annual cleaning is often recommended.
- Shading monitoring: Growing trees or new constructions can create shadows on your panels. You must check that nothing obstructs exposure.
- Temperature: Panels lose efficiency when they get too hot. Good ventilation of the installation helps limit this effect.
Strategies for maximising energy efficiency
To get the most out of your installation, you must also adapt your consumption to your production. Using the appliances that consume the most electricity during daylight hours maximises self-consumption. Connected tools can help you manage this by informing you about your production and consumption in real-time. They can even suggest the best times to use certain appliances. The goal is to reduce waste and increase the share of solar energy you consume directly.
Optimising efficiency does not stop at the initial installation. Careful monitoring and regular adjustments are necessary to ensure your photovoltaic system operates at its full potential throughout its lifespan.
To improve how you use energy and track what you produce, we have simple solutions. Discover how to optimise your solar energy production and track every detail. Visit our website to learn more and start saving today!
Conclusion: Mastering the calculation of your photovoltaic output
So, we’ve covered the different methods for estimating your solar panels’ output. Whether you use basic formulas, taking into account efficiency and sunshine, or prefer to rely on tools like PVGIS or AutoCalSol, the goal remains the same: to have a clear idea of the energy your installation can generate. Remember that these calculations are estimates. Weather, panel maintenance, and even changes in your consumption will play a role. That’s why it’s always good to understand the factors that influence these figures. Ultimately, good planning and a grasp of the basics will help you make the best choices for your solar project.
Frequently Asked Questions
How do I know how much electricity my solar panels will produce?
To estimate your panels’ output, you need to look at their power (in kWp), the amount of sun in your region, and the panels’ efficiency. A simple formula is used: Output = Power x Sunshine x Efficiency. You also need to consider losses, such as those from cables or heat.
What is the ‘peak power’ of a solar panel?
Peak power (kWp) is the maximum power a panel can deliver under perfect conditions: plenty of sun, no shade, and a normal temperature (25°C). It’s like a car’s top speed; it can’t always drive that fast.
Why are sun and orientation important for production?
The more sun there is in a region, the more electricity the panels produce. The orientation (south-facing, for example) and inclination of the panels also help capture maximum sunlight throughout the year.
What is the ‘production coefficient’ and what is it used for?
The production coefficient is a figure that indicates how much electricity (in kWh) a panel can produce for each unit of its power (in kWp). It helps to easily compare the performance of different installations or regions.
What can cause my panels to produce less electricity than expected?
Several things can reduce production: heat, which makes panels less efficient, losses in cables or the inverter, and especially shadows cast by trees or buildings. Dust on the panels can also play a role.
Are there tools to help me calculate my production?
Yes, there are free websites and software like PVGIS or the ADEME simulator that help you estimate your panels’ production based on your address and roof configuration.
How do I know what size photovoltaic installation to choose?
To choose the right size, you first need to know how much electricity you consume each day. Then, you look at how much sun there is where you live and calculate what panel power you need to cover your needs, taking losses into account.
What is the ‘kWh/kWp ratio’ and how do I use it?
The kWh/kWp ratio shows how much electricity (kWh) an installation produces for each kilowatt of installed power (kWp). To estimate your annual production, you multiply your installation’s power by this ratio, which depends on your region.
