How many panels per string — frost, MPPT and input current
Why a string that works in summer can damage the inverter in January. Four checks to pass for every MPPT input.
A string is sized on a warm day from a datasheet, and it is tested by frost. A panel's open-circuit voltage rises as it gets colder. A string that sits within limits in July can exceed the inverter's maximum input voltage on a hard frost. That is no longer lost yield — it is a risk to the equipment. So the check has to be made for the coldest morning, not for the datasheet.
What you need before you calculate
- Panel: Voc and Isc at standard test conditions (STC), the operating voltage Vmp and the voltage temperature coefficient α (from the datasheet, %/°C).
- Inverter: the maximum input voltage, the MPPT range and the maximum input current — for each MPPT separately.
- Site: the absolute minimum air temperature. The absolute minimum, not the winter average.
Check 1. Voltage in frost
Open-circuit voltage in frost is computed per HD 60364-7-712 (Annex B):
U_oc max = U_oc · (1 + α · (T_min − 25) / 100)
T_min is the site's absolute minimum temperature and α the panel's coefficient. α is negative and T_min is below 25 °C, so the product is positive and the voltage rises.
If T_min or α is unknown, the standard gives a simpler rule: 1.2 · U_oc. It is conservative. In a project with a location, Solispec takes T_min from the site's climatic region (ДСТУ-Н Б В.1.1-27:2010, Table 1); the 1.2 · U_oc rule applies when there is no site temperature (for example, in the calculator without one) or the panel datasheet gives no α.
The number of panels in the string times U_oc max must stay below the inverter's maximum input voltage. That is a hard limit.
Check 2. The lower MPPT limit in heat
In summer the cells heat up and the operating voltage drops. If the string is too short, its voltage in the heat falls below the lower MPPT limit. The inverter then cannot hold the string at its maximum power point, and yield drops in exactly the sunniest hours. This check bounds the panel count from below.
Check 3. The upper MPPT limit in winter
The operating voltage Vmp rises in frost too. If it goes above the upper MPPT limit, the inverter is not damaged, but it loses some energy. It is a warning, not a ban — still better known in advance.
Check 4. Input current
Parallel strings on one input add their currents. The design maximum current is taken with a margin: I_SC MAX = 1.25 · I_SC. If it is above the input maximum, check how the inverter maker rates that value in the datasheet. Some makers give the operating current and the short-circuit current separately.
If the operating current exceeds the input limit, the inverter will clip the power.
Then — protection
The string length decides more than inverter compatibility. U_oc max drives the choice of the DC equipment:
- the DC surge protective device — its voltage U_cpv must be at least U_oc max;
- string fuses — needed when three or more strings run in parallel on one input, on both poles;
- the DC switch-disconnector — rated for the same voltage.
The installation requirements are in ПУЕ:2026 and ДБН В.2.5-23:2025; what is specific to PV installations is in HD 60364-7-712.
When this calculation does not apply
With optimisers in the system, size the string in the manufacturer's configurator. Optimisers change the string voltage, and the simple formula no longer holds.
How to do it quickly
Choose the panel, the inverter and the site's minimum temperature in the free string calculator. It shows the allowed range of panels per string and checks your choice against all four points. In a Solispec project, the same rules drive the automatic string layout.
The calculation is preliminary. A qualified engineer must check it before installation.