1. Inverter Types
The inverter is the "brain" of a solar PV system. It is not simply a converter that transforms the DC current from the panels into AC current, but rather the central unit that determines the stability, efficiency, and protection of the system's operation.
String inverter – the classic setup
The string inverter is the most common solution for residential systems.
Its operating logic is simple: several panels are connected in series, forming a string, and the inverter handles the entire DC side of this string. The inverter searches for a common maximum power point (MPPT) for the entire string. This means that the panels work together as one large unit. If all panels have the same orientation, the same tilt angle, and are free from shade, this solution is efficient and reliable.
The advantages of a string inverter are its simplicity and high efficiency. Little active electronics are placed on the roof, which results in a stable system in the long run. However, its limitation is that it cannot handle variations within a string. If one panel produces less power, it affects the performance of the entire string.
Multi-MPPT inverter – for multiple roof surfaces
A multi-MPPT inverter is essentially an upgraded version of the string inverter. It contains multiple independent MPPT inputs within one unit, allowing several strings to be controlled separately. This is particularly useful, for example, when there are both east and west-facing roof surfaces. In such cases, the two sides can be connected to separate inputs, and both operate at their own optimal operating point.
It is important to understand, however, that a multi-MPPT inverter handles differences between strings, not within strings. A group of panels connected to a given MPPT still move together. This solution is therefore more flexible but does not replace panel-level control.
Hybrid inverter
A hybrid inverter not only converts but also controls. In addition to solar panels, it also manages a battery and can decide whether the energy goes to consumers, into the battery, or to the grid.
Such an inverter performs simultaneously:
- MPPT control for solar panels,
- battery charge management,
- grid-synchronized AC operation,
- and in some cases, off-grid supply.
A hybrid inverter is an ideal choice if the goal is to increase self-consumption, store energy, or provide backup power during outages. However, it requires a more complex system logic. Sizing errors appear more quickly here than in a simple string system.
Microinverter
The microinverter follows a completely different philosophy. Here, each panel has its own inverter, which converts DC energy to AC on the roof. The panels are thus completely independent of each other. Shade or different orientations only affect the performance of that specific panel. This provides great flexibility, especially for complex or shaded roofs.
The cost, however, is due to greater complexity. Many active electronic components are placed on the roof, which are exposed to heat and environmental influences. In addition, the system cost is typically higher.
2. MPPT Operation
MPPT (Maximum Power Point Tracking) is one of the most important control functions of an inverter. Its task is to ensure that the solar array delivers the maximum available power at any given moment, given the current irradiation and cell temperature.
A solar panel is not an ideal voltage source. Its I-V characteristic is non-linear and constantly changes due to environmental conditions. For this reason, a fixed operating point does not exist. The position of the maximum power point (MPP) shifts with the time of day, temperature changes, and even cloud cover.
MPPT tracks this continuously changing operating point.
How does it work in practice?
The power of a string is the product of voltage and current. The MPP is the point where this product is maximized. The inverter does not "see" the entire curve, but works based on measurement feedback. The inverter slightly modifies the input voltage, then observes how the power changes. If it increases, it continues the intervention in that direction. If it decreases, it reverses direction. This is a continuous, dynamic search process.
Modern inverters use more advanced algorithms that react faster to changes in irradiation and can also handle characteristics resulting from partial shading. In such cases, multiple local maxima may appear on the P-V curve, and the MPPT must find the global maximum.
If an inverter has multiple MPPT inputs, it means that several independent control loops are operating within it. Each MPPT separately measures and controls its associated string. This is not about increasing power, but minimizing losses. In the case of strings with different orientations or tilt angles, the electrical characteristics of the individual arrays will be different. If these are connected to a single MPPT, the inverter will choose a compromise operating point. Neither array will operate at its own optimum.
However, when connected to separate MPPTs, both strings can operate at their own MPP. The difference can mean a measurable production surplus on an annual basis. It is important to note, however, that multiple MPPTs handle differences between strings. Within a string connected to a given MPPT, a common operating point still applies.
Orientation and Control
Orientation directly affects the I-V characteristic of the string. The peak performance of a south-facing array occurs at a different time of day than that of an east or west-facing array. The MPPT does not "know" the geometry of the roof; it only sees the input electrical parameters. If strings with different characteristics are connected to one MPPT, the inverter cannot optimize them separately. Therefore, separating arrays with different orientations is not a convenience feature, but a design principle.
Shade and MPPT limitations
In case of partial shading, the string characteristic is distorted. Due to the operation of bypass diodes, multiple power peaks may occur. In such cases, the MPPT must decide which point yields the greatest actual power. A simpler algorithm may stabilize at a local maximum, causing a reduction in production. More advanced control can scan the curve and find the global maximum, but it cannot override physical limitations.
If the string characteristic is significantly distorted, the MPPT's scope of action also narrows. In such a case, a separate MPPT, optimizer, or conscious compromise is necessary.
3. Voltage and Current Limits
The DC side voltage and current limits of the inverter are not recommendations, but absolute technical thresholds. Exceeding them can lead not to a decrease in efficiency, but to shutdown, error codes, or permanent damage. The essence of correct design is that the solar array remains within these limits under all expected environmental conditions (cold, hot, low light, strong irradiation), while providing sufficient room for the MPPT to operate.
Maximum DC voltage
The maximum DC voltage of the inverter (Max. DC Voltage) is the highest permissible input voltage that the inverter's electronics can withstand without damage. This value must never be exceeded, not even momentarily. The critical state here is not the operating Vmpp, but the string's open-circuit voltage (Voc). In cold weather, the Voc value of solar panels increases because the semiconductor material produces a higher voltage as the temperature decreases. Therefore, when designing, calculations must always be based on the Voc corrected for the coldest expected temperature.
Electrically, this can be interpreted as follows: in the morning, at sunrise, the panels are cold, there is no load yet (the inverter has not started), the string is "idle," so the maximum voltage appears at the inverter's input. If this exceeds the inverter's limit, the inverter will shut down or be damaged. Therefore, when designing long strings, the max. DC voltage is the most stringent limit, often determining the maximum number of panels per string.
MPPT operating range
The MPPT operating range (MPPT voltage range) is the voltage range within which the inverter can actively regulate and find the maximum power point. This is not the same as the maximum DC voltage, nor is it the startup voltage.
If the string's Vmpp value is within this range, the inverter can operate efficiently. If it is below or above, the inverter:
- either does not start,
- or starts but does not operate at an optimal operating point,
- or encounters an error.
The goal of the design is that the string's operating Vmpp falls close to the middle of the MPPT range throughout the year. This leaves room for temperature and irradiation changes, and the MPPT algorithm operates stably.
Important difference:
- the max. DC voltage is a safety limit,
- the MPPT range is an operational efficiency limit
Maximum string current
The DC input of the inverter is limited not only by voltage but also by current. The maximum string current refers to the current that the inverter can continuously withstand on a given MPPT input.
This becomes particularly critical when:
- high-current panels are used,
- parallel strings are connected to one MPPT,
- or an optimizer system is used, where current conditions may differ from classical string logic.
It is important to understand that the inverter does not "regulate down" excessively high current without penalty. If the input current exceeds the permissible value, the inverter either clips the power or encounters an error. In case of continuous overload, the heating of the electronics accelerates, which reduces its lifespan. Therefore, when paralleling, it is always necessary to check that the Imp/Isc values of the individual strings, and their sum, do not exceed the current limit of the MPPT input.
Cold start voltage
The cold start (start-up) voltage is the minimum DC voltage required for the inverter to even turn on and begin MPPT search. This value is typically higher than the inverter's minimum operating voltage.
This is important because in low irradiation conditions (morning, cloudy weather, winter), the panel voltage can drop. If the string is too short, or the panels are hot and the light is weak, the voltage will not reach the startup threshold, so the inverter will not produce, even if there is some available energy.
This phenomenon typically occurs with short strings and causes noticeable production losses, especially during winter. Therefore, the correct string length is critical not only for the upper (overvoltage) limit but also for the lower (startup) limit.
4. Panel-Inverter Matching
Panel-inverter matching is one of the most crucial design points in a solar PV system. This is where the theoretical performance of the solar panels meets the real, physical limitations of the inverter.
A poorly matched system is often not obviously faulty. It works, but not optimally. It may limit too often, start too late, or generate unjustified losses under certain circumstances. This is an area where "rules of thumb" alone are no longer sufficient. Here, calculations are necessary.
DC/AC Ratio
The DC/AC ratio is the quotient of the nominal DC power (kWp) of the solar panels and the nominal AC power (kW AC) of the inverter. This ratio indicates the potential the solar array provides relative to the inverter.
In reality, panels almost never deliver their nominal STC power. High cell temperature, non-ideal irradiation, soiling, and aging all reduce actual power. The inverter, however, can stably deliver its nominal AC power.
Therefore, the DC/AC ratio is rarely 1:1 in practice. In most well-designed systems, the DC side is larger than the inverter's AC side power. Electrically, this means we utilize the inverter more effectively: it operates closer to its nominal range for a larger part of the day, not just during short, ideal peak times.
Oversizing
DC side oversizing is not a mistake, but a strategy. In the morning, afternoon, and cloudy weather, panel performance is lower. If the array is slightly larger than the inverter, the inverter reaches the startup voltage and the MPPT range earlier, thus spending more time in efficient operation.
In ideal summer conditions, however, it may happen that the panels would deliver more power than the inverter can pass through. This is when clipping occurs – the inverter simply cuts off the excess. This is not a dangerous phenomenon, but normal operation. The question is always how often and with what amount of energy loss it occurs.
A moderate DC/AC ratio, for example around 1.1–1.2, is generally an optimal compromise both economically and technically.
Example
Suppose in a system:
- the panel power is 400 Wp,
- the inverter's nominal power is 5 kW,
- a total of 14 panels are installed.
The PV array power is thus 14 × 400 W = 5600 Wp.
The DC/AC ratio is 5600 / 5000 = 1.12.
This means 12% oversizing, which is a completely acceptable value.
But we cannot stop there. We also need to look at the electrical matching. For example, if the operating voltage (Vmpp) of the panels is around 40 V, then 7 panels connected in series give an operating voltage of approximately 280 V. Two such strings connected to separate MPPTs remain stably within the operating range.
Thus, matching is not just about power, but also about voltage and current control.
Clipping
Clipping typically occurs for short periods during strong summer sunlight. Annually, the loss is usually a few percent, while the extra energy gained during periods of lower irradiation often compensates for this. Problems begin if the DC/AC ratio is too high, and the inverter clips for a significant portion of the day. In such cases, we are no longer talking about optimization, but an undersized inverter.
Common design errors
One of the most common mistakes is to only look at the kWp-kW ratio, ignoring voltage and current limits. A system that looks good on paper can still be electrically flawed if the string voltage falls outside the MPPT range or approaches the maximum DC limit.
Another typical mistake is a string that is too short. In such cases, the inverter does not start in low light, so the advantage of oversizing is lost.
Finally, the local climate is often overlooked. In colder regions, cold-weather Voc is critical, while in warm, flat-roof environments, the lower MPPT limit can become a bottleneck.
