1. Design Principles
The goal of design is not to "install X kW" on the roof, but rather to ensure that the system operates safely, predictably, and stably over the long term at the given location, with the given consumption habits, and within the chosen system type (grid-tied, hybrid, or off-grid).
A good design is both an energy and an electrical design. If energy demand is treated superficially, or electrical limitations are ignored, the system will either underperform or be technically unstable. The three pillars of design are accurate interpretation of energy demand, correct determination of system size, and conscious management of expandability.
Energy Demand Analysis
Energy demand analysis is not about a single number. The annual kWh value is only the starting point; the key is when and at what power consumption occurs.
Suppose a family home has an annual electricity consumption of 4800 kWh. This gives an order of magnitude, but in itself does not tell us what kind of system should be built. The same annual consumption may require a completely different system depending on whether the load occurs mainly during the day or in the evening, whether there is significant winter heating demand, or whether high-power equipment, such as an electric car charger, is in operation.
Solar production occurs during the day, so the proportion of self-consumption is a key issue. The greater the daytime consumption, the more efficiently the generated energy is utilized locally. In hybrid or off-grid systems, however, not only energy (kWh) but also instantaneous power demand (kW) is crucial. An induction hob, a heat pump, or an EV charger can demand very high power for a short period, which the system must be able to serve.
Seasonal behavior is also important. In Hungary, PV production is high in summer and low in winter, while electricity heating demand typically increases in winter. This is not a disqualifying factor, but a design reality. In off-grid systems, examining the minimum winter production is particularly important, as this determines the required solar panel and battery size.
System Size Determination
System size determination starts with energy demand but does not end there. First, an energy approximation is made, and then this must be aligned with physical and electrical constraints.
The basic formula is as follows:
P_PV ≈ E_ev / H
where:
- P_PV = the required installed power (kWp)
- E_ev = the annual energy demand (kWh/year)
- H = the specific yield (kWh/kWp/year)
In Hungary, the specific yield typically ranges between 1100–1250 kWh/kWp/year, depending on orientation and shading.
Let's take a concrete example. If the annual consumption is 5000 kWh, and the specific yield of the site is 1200 kWh/kWp/year, then:
P_PV = 5000 / 1200 ≈ 4.17 kWp
This means that approximately 4.2 kWp of installed power is needed to cover the annual energy. If a panel has a power of 420 Wp, then roughly 10 panels are needed.
However, this is only the first approximation. The next step is a physical check: can this many panels fit with proper orientation, without shading? The segmentation of the roof, chimneys, maintenance corridors, and mounting options all influence the actual installable size.
Then comes the electrical matching. Series connection of panels increases voltage, parallel connection increases current. However, the inverter is not "infinitely flexible": it has a maximum DC voltage, a defined MPPT operating range, and input current limits. Thus, the system size is considered appropriate only as long as it is in line with these limits.
Planning for Expandability
Consumption rarely remains constant for 20–25 years. Electric cars, heat pumps, new living areas can all increase energy demand. Therefore, it is worth thinking ahead during the design phase.
At the physical level, it must be examined whether there will be a suitable roof area later, with appropriate orientation and without shading, to which a new panel array can be added. On the electrical side, important questions are whether there is a free MPPT input, whether the string configuration can be expanded, and whether the AC side connection can handle the additional load. In hybrid and off-grid systems, the expandability of energy storage requires special attention. For batteries, compatibility, voltage level, and support for the control electronics (BMS) determine whether future expansion will be simple or problematic.
Therefore, design not only responds to the current state but also anticipates future needs. A well-sized system not only works well today but can also adapt to changing circumstances later.
2. Design Software
The purpose of solar design software is not to "tell you what a good system is," but to model the physical and electrical reality. The program calculates, but the decision always remains with the designer. If the input data is inaccurate, such as shading or consumption profile, even the best software will produce misleading results.
In Hungary, most contractors do not use a single universal program but choose a tool for the specific task. In practice, three main approaches have become widespread: manufacturer-specific inverter designers, complex energy simulation software, and visual layout programs.
Manufacturer Design Programs
For the vast majority of residential systems, the inverter manufacturers' own design programs are the primary tool. Examples include SMA Sunny Design, Huawei FusionSolar Design, Fronius Solar.creator, or SolarEdge Designer.
These programs are primarily used for electrical compatibility checks. They do not provide a detailed annual energy model, but rather examine whether the selected panels and inverters are electrically compatible.
In practice, this is where it is decided, for example, whether:
- the maximum DC voltage is not exceeded in cold weather,
- the string's operating voltage falls within the MPPT range in warm weather,
- the input current limits are not exceeded.
This is the most common design method in the Hungarian market because it is fast, manufacturer-specific, and safe from a warranty perspective. Most residential systems are electrically checked exclusively with such software.
PV*Sol – when an energy model is needed
PV*Sol is a well-known and used program in Hungary, but it appears more in complex projects. These can include tender-based systems, hybrid solutions, or higher-power public installations.
The program performs an annual simulation, in hourly or even finer time steps. It takes into account geographical location, radiation data, temperature losses, shading, and – for hybrid systems – battery operation.
Here, it's not just about how much annual production there is, but also when production and consumption occur. Overtproduction, evening energy deficits, and the proportion of self-consumption become visible.
However, it is important to emphasize: detailed simulation is only valuable if the consumption profile is based on real data. Otherwise, the model is mathematically accurate but energetically misleading.
Geometric and Visual Design
For segmented roofs, industrial halls, or complex shading conditions, 3D modeling becomes important. In such cases, 3D-based software, such as Helioscope, is used.
Their strength is not electrical calculation, but the visualization of panel array placement. They clearly show:
- how the panels fit on the actual roof geometry,
- the effect of a chimney or structure,
- how the shadow moves throughout the year.
This is particularly useful for larger projects or customer communication, but it does not replace electrical compatibility calculations.
Free or Paid?
The real difference is not in price, but in the depth of modeling. Simpler programs are suitable for quick configuration and basic checks. More complex systems offer detailed loss models and documented energy calculations.
In Hungarian practice, the safest approach is for the designer to combine several tools: checking electrical compatibility with manufacturer programs and using a more detailed model for energy issues.
3. String Design
String design is the electrical backbone of a solar power system. It determines how the panels work together, at what voltage and current the system operates, and whether the inverter can truly extract maximum power from the array.
A poorly designed string will perform weakly even if all components are premium quality.
A string is simply an electrical unit of series-connected solar panels, managed by the inverter at a specific input, i.e., an MPPT. Design always starts from the electrical behavior of the panels, not the inverter.
Series and Parallel Connection
Solar panels are essentially current source devices, so how they are connected matters.
In series connection:
- voltages add up,
- current remains the same.
This is the basis of string design. The goal is for the system to reach a voltage range where the inverter operates efficiently and line losses remain low.
In parallel connection:
- currents add up,
- voltage remains unchanged.
This is necessary when multiple strings of the same voltage need to be connected to one input, or when power needs to be increased on the current side.
In practice, however, we primarily think in terms of series connection. Too much current:
- requires thicker cabling,
- causes greater losses,
- and puts more strain on the inverter input.
Requirement for Identical Panels
One of the most important and often violated rules of string design is that the panels within a string must be electrically identical. In series connection, the current is the same for every panel. This means that the string's performance is always determined by the weakest panel.
If a panel is shaded, of a different type, has a different nominal current, or is aging differently, then the entire string will operate at that lower current. Therefore, the basic principle is that within a string, panels of the same type, with the same nominal power, the same cell layout, the same orientation, and the same tilt angle should be used.
Bypass diodes reduce the effect of shading, but do not eliminate it. They cannot be considered as a basis for design.
Orientation and MPPT
One of the key issues in string design is the proper utilization of MPPTs. An MPPT (Maximum Power Point Tracker) is the part of the inverter that finds and continuously tracks the maximum power point. An MPPT can handle only one common operating point at a given moment.
This means that strings connected to one MPPT must have similar electrical behavior.
If to one MPPT:
- an east and west-facing array are connected,
- panels with different tilt angles are connected,
- or arrays with different shading patterns are connected,
then their maximum power points will not be the same. In such cases, the inverter chooses a compromise, and neither array operates at its optimal point.
Therefore, the basic principle is:
One MPPT = array with identical orientation and identical irradiation.
Modern inverters contain multiple MPPTs precisely for this reason. Not so that "more panels can fit," but so that arrays with different behaviors can be optimized separately.
4. Power and Voltage Calculations
The electrical design of a solar system is not done by intuition.
The connection between panels and the inverter is based on specific calculations (especially regarding voltages).
Most serious design errors do not stem from panel quality, but from someone failing to check:
- how much the string voltage increases in cold weather,
- how much the operating voltage decreases in warm weather,
- and whether all of this fits within the inverter's limits.
In this chapter, we will cover the three key issues:
cold Voc, hot Vmpp, and inverter DC limits.
Voc Cold Correction – why is winter more critical?
The Voc (open-circuit voltage) value given in the datasheet is measured at 25 °C cell temperature. In reality, however, panels can be much colder in winter – and in such cases, the voltage increases.
This is not a fault, but a physical law:
as temperature decreases, voltage increases.
However, the inverter's DC input has a maximum voltage limit. Exceeding this is prohibited, even for a short time. Therefore, the maximum string voltage in cold conditions must be calculated.
The basic calculation formula is:
Voc_cold = Voc_STC × [1 + (|αVoc| × (25 − T_min))]
Where:
- Voc_STC is the value from the datasheet,
- αVoc is the temperature coefficient (in decimal form),
- T_min is the expected lowest cell temperature.
Important: we do not calculate with air temperature, but with cell temperature.
Example
Data:
- Voc = 49.5 V
- αVoc = −0.29 %/°C → 0.0029
- T_min = −10 °C
- Temperature difference: 25 − (−10) = 35 °C
Calculation:
Voc_cold = 49.5 × [1 + (0.0029 × 35)]
Voc_cold ≈ 54.5 V
If 16 panels are connected in series: Voc_string_cold = 16 × 54.5 ≈ 872 V
This value must not exceed the inverter's maximum DC voltage.
This determines the upper limit of the string length.
Vmpp Operating Range – where the system actually works
While Voc is more of a safety check, Vmpp (maximum power point voltage) is the voltage at which the system actually operates.
In summer, cell temperature can reach 60–70 °C. In warm weather, Vmpp decreases, and if it becomes too low, the inverter:
- starts late in the morning,
- or falls out of the MPPT range,
- and does not produce at the maximum point.
The calculation of Vmpp in warm conditions:
Vmpp_hot = Vmpp_STC × [1 − (|αVmpp| × (T_cell − 25))]
Where:
- Vmpp_STC is the value from the datasheet,
- αVmpp is the temperature coefficient,
- T_cell is the operating cell temperature.
Example
Data:
- Vmpp = 41.5 V
- αVmpp = −0.35 %/°C → 0.0035
- T_cell = 65 °C
- Temperature difference: 65 − 25 = 40 °C
Calculation:
Vmpp_hot = 41.5 × [1 − (0.0035 × 40)]
Vmpp_hot ≈ 35.7 V
For 16 panels: Vmpp_string = 16 × 35.7 ≈ 571 V
This must be within the inverter's MPPT operating range, otherwise the system cannot operate optimally.
Inverter DC Limits – these are not recommendations
The inverter's datasheet specifies three important DC limits:
- Maximum DC voltage: this must be checked based on the cold Voc
- MPPT voltage range: the hot Vmpp value must fall within this
- Maximum input current per MPPT: mainly important for parallel strings
The essence of good design, simply summarized, is that the maximum DC voltage is not exceeded in cold weather, the system does not fall out of the MPPT range in warm weather, and the inverter is not overloaded on the current side.
If all three conditions are met, the string is electrically correct.
Power – what does nominal kW mean?
If 16 pieces of 450 W panels are connected in series:
P_string = 16 × 450
P_string = 7200 W
This is the nominal peak power.
This does not mean that the system always outputs 7.2 kW.
It means that under ideal conditions, this is the maximum potential.
Actual power depends on:
- irradiation,
- temperature,
- and inverter control.
The purpose of the calculation is not to "get a nice number," but to ensure that the system operates safely and stably under all circumstances.
