Designing a Photovoltaic System: Basic Principles and Methods

A photovoltaic system consists of a set of photovoltaic modules (panels) and an electrical component (cables) and an electronic component (inverter). The modules harness the incident solar energy to produce electricity through the photovoltaic effect.

The use of this technology arises from the need to combine:

• compatibility with architectural needs and environmental protection;
• no noise pollution;
• a reduction in fossil fuel consumption;
• electricity production without emissions of pollutants.

The aim of the photovoltaic system is to achieve a significant energy saving for the served structure. If you make calculation errors, your project will fail and will not provide any of the mentioned benefits. To avoid designing a low-yield photovoltaic system, I suggest you try a photovoltaic software for free that can help you understand how each type of system works.

In this article, we outline the technical characteristics of a photovoltaic system and touch upon the design criteria.

What is photovoltaic system design and why is it fundamental?

The design of a photovoltaic system is not simply arranging modules on a roof but is a multidisciplinary engineering process that aims to convert solar irradiation into usable electricity, maximizing the return on investment (ROI). Designing means predicting the behavior of a dynamic system in a variable environmental context, balancing physical, regulatory, and economic variables.

The value of accurate design

In the current context, where architectural integration and energy efficiency are pillars of the Green Deal, approximate design is a luxury that no technician can afford. Here’s why this process is crucial:

• optimization of the Performance Ratio (PR) – only a thorough study of shading analysis and mismatching can reduce energy losses, ensuring that the system produces as estimated in the business plan;
• safety and regulatory compliance – from the structural integrity of support structures to protection against surges (SPD), each component must meet rigorous technical standards (such as CEI standards in Italy) to ensure the longevity of the system;
• integration into the BIM flow – today, photovoltaic design must interact with the building’s information model. Considering the system as a “smart object” allows predicting installation interferences and facilitating future maintenance (Asset Management).

Why “do it yourself” is a risk

Relying on empirical calculations or unverified software exposes the designer to sizing errors that can compromise the profitability of the project. A correct calculation methodology is the only guarantee to provide the end customer with a reliable data on the payback period of the investment.

To transform this complexity into a smooth and error-free workflow, it’s essential to equip yourself with a photovoltaic software that allows managing every phase, from surveying to economic simulation, with extreme precision.

How to approach photovoltaic system design for the first time

The secret to a successful debut in the sector lies in methodological rigor. You never start with the choice of panels, but with the understanding of the context and needs.

Energy need analysis

The first step is not technical but analytical. You need to examine the consumption profile of the user (residential or industrial) through electricity bills or, even better, through hourly withdrawal data. The goal is to size the system to maximize self-consumption since the energy produced and consumed instantly ensures the greatest economic savings.

Technical survey and environmental constraints

Before proposing a solution, the “container” must be analyzed. The critical elements to evaluate are:

• orientation and inclination (Tilt and Azimuth) – a south-facing exposure with optimal inclination is ideal, but modern design allows for excellent results even with east-west orientations;
• shadow analysis – distant (mountains, buildings) or nearby (chimneys, antennas, trees) obstacles can cause drastic voltage drops;
• roof conditions – checking the static integrity and suitability of materials (e.g., presence of asbestos or need for structural reinforcements).

Choosing technology and components

Only at this point does one move to system configuration. The designer must choose between monocrystalline modules (more efficient) or alternative technologies, and match the most suitable inverter (string inverter or with optimizers). At this stage, the electrical compatibility between strings and inverters is crucial to avoid faults or inefficiencies.

Integration between project and regulatory reality

A common mistake among novices is ignoring the bureaucratic process. Every project must interface with the grid operators’ portals and comply with landscape or municipal regulations. Designing with appropriate photovoltaic software allows for easier generation of technical documentation and single-line diagrams, drastically reducing the risk of application rejections.

Key factors to consider in the design of photovoltaic systems

Modern design requires a holistic approach. Every technical decision has a direct impact on specific productivity (kWh/kWp) and the lifespan of components. Below are some aspects you need to consider to proceed with a more informed design.

Installation site

The energy sizing of the photovoltaic system, connected to the distributor’s network, as well as financial availability, is carried out considering the following aspects:

• availability of spaces on which to install the photovoltaic system
• availability of solar resources
• morphological and environmental factors (shading and albedo)

Identification of spaces for installing the photovoltaic system

The first operation is to identify the spaces where to place the photovoltaic modules. Generally, in the case of residential installations, panels are sought to be placed on the slopes of the roof. If possible, an integrated installation in the roof is preferred. Therefore, it is necessary to identify the slope facing SOUTH.

The optimal exposure coincides with full SOUTH; therefore, maximum productivity is achieved at azimuth equal to 0°.

Additionally, it is necessary to identify the optimal inclination of the panels with respect to the horizontal plane; this value is defined as tilt.

Generally, maximum productivity values occur for tilt = latitude -10°.

Solar irradiation and local climate data

Not all “SOUTHs” are alike. Design must be based on certified climatic databases (such as PVGIS or Meteonorm). It is essential to consider:

• direct and diffuse components – in areas with high cloud cover, diffuse radiation plays a key role.
• albedo – the reflection of the ground or surrounding surfaces (especially in the case of white roofs or snow) can increase yield, particularly if bifacial modules are used.

Azimuth

Tilt

Availability of the solar source

The availability of the solar source for the installation site must be verified using data related to monthly average daily values of solar irradiation on the horizontal plane.

Average monthly photovoltaic energy production

Morphological and Environmental Factors (Shading and Albedo)

The effects of shading caused by volumes on the horizon, due to natural elements (reliefs, trees) or artificial ones (buildings), determine the reduction of solar gains and the payback time of the investment.

It is also necessary to take into account the additional radiation due to the reflectance of surfaces (the ability to reflect part of the incident light on a given surface or material) of the area in which the system is installed. Therefore, the average monthly albedo values are defined.

An inadequate assessment of these elements could compromise the efficiency of the entire system. To achieve the desired results and correctly size a photovoltaic system, I recommend using the photovoltaic software capable of assisting you throughout the design phase.

Operating Temperatures and Ventilation

A common mistake is to underestimate the effect of heat. Photovoltaic modules lose efficiency as temperature increases (temperature coefficient P_max).

• Floating vs. Integrated: an integrated rooftop system heats up more than a raised one.
• Spacing: providing an adequate air gap between the modules and the mounting surface is vital to ensure passive cooling and protect the inverter’s electronics.

System Losses

In addition to the efficiency of the modules, it is necessary to account for inevitable losses:

• voltage drop in DC/AC cables: it should preferably be kept below 1% to prevent energy dissipation as heat;
• mismatching: minimal constructive differences between the modules that penalize overall performance;
• soiling: accumulation of dust, droppings, or pollution, varying based on the area (agricultural, industrial, or coastal).

Wind Actions and Snow Loads

From a structural point of view, a system acts as a “sail.” The design must include calculations for wind forces and snow overload, correctly defining the number and type of anchors to prevent damage to the building envelope or, worse, the detachment of the panels.

Classification of Photovoltaic Systems

Below are some basic concepts for the design of photovoltaic systems.

• photovoltaic modules: devices for converting solar energy into electrical energy
• charge controller: device for the protection of batteries (regulates their charging and discharging)
• batteries: devices for storing energy from the photovoltaic system in order to supply it to users when the modules cannot produce it due to lack of solar irradiation
• inverter: machine for converting direct current from the modules and/or batteries into conventional alternating current at 220  Note: If the devices to be powered are of direct current type, this component can be omitted
• electrical panel: for energy distribution. In case of high consumption or in the absence of power from the photovoltaic modules, the current is drawn from the public grid. Otherwise, the excess photovoltaic energy is fed back into the grid. Moreover, it measures the amount of energy provided by the photovoltaic system to the grid.
• photovoltaic cell: the single photovoltaic cell, approximately 10 x 10 cm in size, constitutes the elementary device at the base of every photovoltaic system.

We can have different types of photovoltaic systems: grid-connected systems (grid connected) and off-grid systems (stand alone).

Connected systems

Isolated stand-alone systems

Photovoltaic Plant Project: Calculation Procedures

The design principle normally used for a photovoltaic system is to maximize the annual solar radiation capture available.

In most cases, the photovoltaic generator must be optimally exposed to sunlight, prioritizing a South orientation and avoiding shading phenomena. Depending on any architectural constraints of the structure hosting the generator itself, however, different orientations are adopted, and shading phenomena are permitted as long as they are adequately evaluated.

Energy losses due to such phenomena impact the cost of the kWh produced and the payback time of the investment.

From an architectural insertion perspective, in the case of applications on sloped roofs, the choice of orientation and inclination should be made taking into account that it is generally advisable to keep the plane of the modules parallel or even coplanar to that of the roof itself. This is to avoid altering the shape of the building and not to increase the wind action on the modules themselves. In this case, it is useful to promote air circulation between the back of the modules and the surface of the building, in order to limit temperature losses.

Criteria for Estimating Produced Energy

The generated energy, therefore, depends on the following factors:

• installation site (latitude, available solar radiation, temperature, reflectance of the surface in front of the modules)
• module exposure: angle of inclination (Tilt) and angle of orientation (Azimuth)
• any shading or defacement of the photovoltaic generator
• module characteristics: nominal power, temperature coefficient, losses due to decoupling or mismatch
• characteristics of the BOS (Balance Of System)

The value of the BOS can be estimated directly or as a complement to the total loss unit, calculated using the following formula:

Total losses [%] = [1 – (1 – a – b) · (1 – c – d) · (1 – e) · (1 – f)] + g

for the following values:

1. losses due to reflection
2. losses due to shading
3. losses due to mismatching
4. losses due to temperature effect
5. losses in DC circuits
6. losses in inverters
7. losses in AC circuits

Electrical Verification Criteria

At minimum operating temperature values of the modules (-10 °C) and maximum operating values (70 °C), the following quantities are verified:

• MPPT voltages (Maximum Power Point Tracker), the MPPT values represent the minimum and maximum values of the voltage window useful for searching for the operating point at maximum power:
  • voltage at maximum power point, Vm, at 70 °C greater than or equal to the minimum MPPT voltage (V_{mppt min})
  • voltage at maximum power point, Vm, at -10 °C less than or equal to the maximum MPPT voltage (V_{mppt max})
• maximum voltage: open-circuit voltage, Voc, at -10 °C less than or equal to the maximum input voltage of the inverter
• maximum module voltage: open-circuit voltage, Voc, at -10 °C less than or equal to the maximum system voltage of the module
• maximum current: maximum generated short-circuit current, Isc, less than or equal to the maximum input current of the inverter
• dimensioning: the percentage ratio between the nominal power of the inverter and the power of the photovoltaic generator connected to it (between 70% and 120%)

Photovoltaic Plant Project: Sizing the System

To correctly size a system, it is necessary to define the general data:

• client data (name and address)
• latitude (N)
• longitude (E)
• altitude (m)
• annual solar irradiation on the horizontal plane (MJ/m²)
• shading coefficient

and the technical data:

• total module area (m²)
• total number of modules
• total number of inverters
• total annual energy (kWh)
• total power (kW)
• phase power (L1-L2-L3) (kW)
• energy per kW (kWh/kW)
• storage system
• useful storage capacity (kW)
• BOS (%)

External Storage System

The external storage system used allows for storing the excess energy produced by the system for reuse during periods when the system does not produce energy.

It is necessary to define the type of battery (code, brand, model, and characteristics) and the configuration of the storage system.

How Solarius PV Simplifies Photovoltaic Plant Design

Below we present the advantages of designing with the help of a photovoltaic software:

• design of the surfaces available for the photovoltaic system with specific graphic objects, which include all necessary data for sizing the photovoltaic system, or import them directly from DXF/DWG files
• placing the photovoltaic panels to be arranged on the roof or on the ground, selecting them from those suggested automatically by the software to achieve the best result
• simulation of the photovoltaic shading produced by elements on the horizon placed at a considerable distance from the photovoltaic system with a simple photographic survey of the location, the photovoltaic shading from nearby obstacles, and that caused by parallel strings of photovoltaic modules
• configuration of the photovoltaic system with low voltage single-phase inverters (BT), three-phase low voltage (BT) and medium voltage (MT), multi MPPT inverters, inverters with integrated or independent storage (external photovoltaic batteries). Photovoltaic batteries and inverters will be selected and suggested automatically by the software to achieve the best result
• calculation of the photovoltaic system yield (total annual output with hourly production detail) and evaluates the profitability and payback period of the photovoltaic system
• calculation results, in numerical and graphical form, always updated with every modification of project data. Always supported by operational diagnostics with alerts that can quickly highlight any anomalies, so that everything is processed correctly.
• possibility of photo-insertion allows to visualize the designed system even with photorealistic representations to show the client the visual impact of the designed system

For the proper drafting of a photovoltaic system project, it is therefore necessary to use a photovoltaic software that assists you in all phases of design.

FAQ about the Photovoltaic System Project

What is photovoltaic system design and why is it essential?

Photovoltaic system design is a multidisciplinary engineering process aimed at transforming solar irradiation into usable electrical energy, maximizing the return on investment (ROI). It is essential because it allows forecasting the behavior of a dynamic system in a variable environmental context, balancing physical, regulatory, and economic variables. Accurate design ensures the optimization of the Performance Ratio (PR), safety, regulatory compliance, and integration into the BIM flow.

How to approach photovoltaic system design for the first time?

For a correct approach, it is necessary to follow a methodological rigor that starts from understanding the context and needs. Key steps include:

1. Analysis of the user’s energy needs to maximize self-consumption;
2. A technical survey to assess environmental constraints such as orientation (Azimuth), inclination (Tilt), shadow analysis, and roof conditions;
3. Choice of technology and components (modules and inverters);
4. Integration between project and regulatory reality for managing the bureaucratic process.

What are the key factors to consider in the design?

Key factors include identifying suitable spaces, the availability of the solar source based on certified climatic data (direct, diffuse, and albedo components), and morphological factors. It is essential to evaluate operating temperatures and ventilation (to avoid losses in module efficiency), system losses (voltage drop, mismatching, dirt), and mechanical actions such as wind and snow loads.

What are the electrical verification criteria for a photovoltaic system?

Electrical checks must be performed considering extreme temperatures (-10 °C and 70 °C). MPPT voltages are checked (the voltage at the maximum power point must fall within the window of the inverter), the open-circuit maximum voltage (Voc) should not exceed the maximum input voltage of the inverter or module, and the maximum short-circuit current (Isc) is verified. Additionally, the correct sizing of the ratio between the power of the inverter and the power of the generator is verified (usually between 70% and 120%).

How does Solarius-PV simplify the design of photovoltaic systems?

Solarius-PV, defined as the ‘photovoltaic software’, simplifies the work by allowing the design of surfaces with specific graphic objects or DXF/DWG import, optimal automatic placement of the panels, and advanced simulation of shading through photographic surveys. The software suggests the best configuration for inverters and storage systems, calculates hourly yield and economic profitability, provides real-time operational diagnostics, and allows photorealistic photo-insertions for the client.