Once the panel layout is done, the most critical electrical step in the design begins: deciding which modules are wired in series and which inverter input each group goes to. This is called stringing. Poor stringing can push an inverter past its voltage limit on a cold winter morning, drop a string below the MPPT window on a hot summer afternoon, or let modules facing different directions drag each other's output down. This article covers the basic concepts, the effect of temperature on voltage and a step-by-step example calculation.
Series and parallel
When you wire modules in series, their voltages add up and the current stays at the current of one module. A group of modules in series is called a string. When you wire strings in parallel, the voltage stays at the voltage of one string and the currents add up.
- A 20-module string has roughly 20 times the voltage of a single module.
- Two identical strings in parallel on the same MPPT input carry twice the current of one string.
Strings connected in parallel need similar voltages. That is why strings sharing an MPPT should have the same module count, module model, orientation and tilt.
Voc, Vmp and temperature
A module datasheet gives two key voltages at standard test conditions (STC, 25 °C cell temperature):
- Voc (open-circuit voltage): the voltage across the terminals when no current is drawn. It is the highest voltage the module will present.
- Vmp (maximum power point voltage): the voltage at the operating point where the module delivers maximum power.
In silicon modules, voltage rises as temperature falls and drops as temperature rises. The datasheet gives this as the Voc temperature coefficient (%/°C), which is negative. In practice this has two consequences:
- On the coldest morning of the year, cells are still at ambient temperature when the sun comes up, so string Voc peaks. This value must not exceed the inverter's maximum DC input voltage.
- On a hot summer afternoon, cell temperature rises well above ambient and Vmp drops. This value must not fall below the lower limit of the MPPT voltage window.
Inverter limits to check
- Maximum DC input voltage: the absolute limit that must never be exceeded. Cold-weather string Voc is compared against it.
- MPPT voltage range: the window in which the inverter can track the maximum power point. String Vmp should stay in this range all year.
- Maximum input current per MPPT: the combined current of parallel strings must not exceed it; some inverters also specify a maximum short-circuit current per MPPT.
- Number of MPPTs and string inputs per MPPT: determines how many orientations or string groups can be tracked independently.
- Start-up voltage: the lowest voltage at which the inverter starts operating.
Worked example: modules per string
The values below are example values chosen to illustrate the method; for a real project, use the datasheets of the actual module and inverter and local temperature data.
| Parameter (example) | Value |
|---|---|
| Module Voc (STC) | 49.5 V |
| Module Vmp (STC) | 41.5 V |
| Module Imp / Isc | 13.2 A / 13.9 A |
| Voc temperature coefficient | −0.27 %/°C |
| Lowest ambient temperature | −10 °C |
| Highest summer cell temperature (assumed) | 70 °C |
| Inverter max. DC input voltage | 1,000 V |
| MPPT voltage range | 200–850 V |
| Max. input current / short-circuit current per MPPT | 32 A / 40 A |
1. Module Voc in the cold
Temperature difference: −10 − 25 = −35 °C. Correction factor: 1 + (−0.0027 × −35) = 1 + 0.0945 = 1.0945. Cold module Voc: 49.5 × 1.0945 ≈ 54.18 V.
2. Maximum modules in series
1,000 / 54.18 ≈ 18.46. Round down: at most 18 modules per string. Check: 18 × 54.18 ≈ 975.2 V, below the 1,000 V limit. With 19 modules, 19 × 54.18 ≈ 1,029.4 V would exceed it.
3. Hot Vmp and minimum modules in series
Where the datasheet gives no separate Vmp coefficient, this example uses the Voc coefficient as an approximation. Temperature difference: 70 − 25 = 45 °C. Factor: 1 − 0.0027 × 45 = 0.8785. Hot Vmp: 41.5 × 0.8785 ≈ 36.46 V. 200 / 36.46 ≈ 5.49; round up: at least 6 modules per string (6 × 36.46 ≈ 218.7 V).
4. Upper MPPT limit and current
Vmp also rises in the cold: 41.5 × 1.0945 ≈ 45.42 V. For an 18-module string, 18 × 45.42 ≈ 817.6 V, inside the 850 V upper limit. With two strings in parallel on one MPPT, operating current is 2 × 13.2 = 26.4 A (below the 32 A limit) and short-circuit current is 2 × 13.9 = 27.8 A (below the 40 A limit).
In this example, anywhere from 6 to 18 modules per string is electrically acceptable. In practice you pick a number that keeps the voltage in the middle-to-upper part of the MPPT window, reduces cable losses and fits the roof layout.
Why do different orientations go to different MPPTs?
On a roof with east and west faces, the east modules receive more irradiance in the morning and the west modules in the afternoon. When irradiance differs, the two groups have different currents and different maximum power points. If you put both groups in parallel on one MPPT, the inverter has to pick a single operating point and neither group runs at its own optimum. That is why faces with different orientations or tilts are connected to separate MPPT inputs wherever possible. Mixing orientations within one string is even worse, because in a series string the least-irradiated module limits the current.
Shading and string layout
In a series string, a partially shaded module pulls down the current of the whole string until its bypass diodes kick in. When you know where the shade comes from, you can arrange strings around it:
- Group modules shaded by a chimney or parapet into the same string where possible, so the shade affects only one string.
- Connect the shaded string to its own MPPT so it does not disturb the operating point of unshaded strings.
- Check how the shade moves by month and hour; morning and afternoon shade fall on different modules.
- With heavy or scattered shading, consider module-level power electronics (optimisers, microinverters) as a separate option.
We cover how shading analysis affects design in our shading analysis article.
Common mistakes
- Checking Voc at its STC value: a calculation based on 25 °C Voc can exceed the limit on a winter morning.
- Looking only at the upper limit: short strings can drop below the lower MPPT limit in summer, so the inverter starts late and shuts down early.
- Paralleling strings of different lengths on one MPPT: strings with different voltages reduce each other's output.
- Mixing orientations in the same string or MPPT.
- Forgetting the MPPT current limit: with high-current modules, fewer strings fit on one MPPT.
- Ignoring the cable route from the survey: the distance from strings to the inverter drives cable length and losses; see our site survey checklist.
- Ignoring leftover modules: if the total module count does not divide evenly into the string length, plan what to do with the remaining modules.
Stringing with PVAGE
In PVAGE's roof design screen, once the layout is done you can choose an inverter from the catalogue. The catalogue lists On-Grid, Off-Grid and hybrid inverters with brand, phase, power, MPPT count and Voc.
Automatic stringing shows the number of modules in series, the number of strings, used and leftover modules and the allocation per inverter and per MPPT. When you want a different arrangement, manual stringing lets you assign modules to strings yourself. Per-module shading loss and a sun simulation with month and hour selection help you see shaded modules while you build the string layout. On the financial analysis side, module-inverter matching and an MPPT limit check are also carried out. See the 3D roof modelling feature page for details.