PV String Voltage Calculator (Max & Min Modules per String)
Enter the module datasheet values, the site's record low temperature and the inverter limits. The tool corrects Voc for cold and Vmp for heat and tells you the longest and shortest string the inverter will accept — then you know whether every DC component must be 1000V or 1500V class.
Module, Site and Inverter
Datasheet value, e.g. −0.28
Use the Pmax coefficient if Vmp is not listed
Ambient max + 25–30 °C for rooftop
Worked example (default inputs)
A 49.5 V Voc / 41.5 V Vmp module with −0.28 %/°C Voc coefficient at a site with −10 °C record low: Voc per module rises to 54.4 V, so a 1000 V inverter takes at most 18 modules per string (978 V). At 70 °C cell temperature Vmp falls to 35.2 V, so a 200 V MPPT minimum needs at least 6 modules. Press Calculate String Voltage to run your own numbers.
Voc cold / module
54.4 V
Vmp hot / module
35.2 V
Max modules (1000 V)
18
Min modules (200 V MPPT)
6
How the string voltage is calculated
Cold limit (maximum modules): Voc_cold = Voc × (1 + |β_Voc| × (25 − T_min)). The string Voc at the record low must stay below the inverter maximum DC input voltage and the DC component class (600 / 1000 / 1500 V).
Hot limit (minimum modules): Vmp_hot = Vmp × (1 − |β_Pmax| × (T_cell − 25)). The string Vmp at the hottest cell temperature must stay above the MPPT minimum, or the inverter stops tracking on summer afternoons.
Rooftop cell temperature is typically 25–30 °C above ambient; ground-mount arrays with free airflow run cooler. IEC 60364-7-712 and NEC 690.7 use the same correction; NEC refers to the ASHRAE extreme annual minimum design temperature.
Buying for a whole system?
Standard solar kits, matched and priced
Modules, inverter, battery, cables and protection in one kit for 230 V, 120/240 V and 400 V grids. Choose a Premium Deye build or a lower-cost Value build. FOB, CFR or CIF.
Frequently Asked Questions
How do I calculate the maximum number of solar panels in a string?
Correct the module Voc to the record low temperature at the site: Voc_cold = Voc × (1 + |β| × (25 − T_min)), where β is the Voc temperature coefficient in %/°C. Divide the inverter maximum DC input voltage (or the 1000V / 1500V system class) by Voc_cold and round down. Example: Voc 49.5 V, β −0.28 %/°C, −10 °C → 54.4 V per module → 18 modules on a 1000 V inverter (978 V).
How do I calculate the minimum number of panels per string?
Correct Vmp to the hottest cell temperature: Vmp_hot = Vmp × (1 − |β_Pmax| × (T_cell − 25)). Divide the inverter MPPT minimum (start) voltage by Vmp_hot and round up. Example: Vmp 41.5 V, −0.34 %/°C, 70 °C → 35.2 V per module → at least 6 modules for a 200 V MPPT minimum.
Which temperature should I use for the cold Voc check?
Use the record low ambient temperature at the site, not the average winter temperature. NEC 690.7 refers to the ASHRAE extreme annual minimum; IEC 60364-7-712 asks for the lowest expected temperature. If the coefficient is unknown, IEC allows Voc × 1.2 as a conservative factor.
Does the string voltage change which DC BOS I need?
Yes. A string that approaches 1000 V needs 1000 V-rated cable, fuses, isolators, SPD and combiner; above 1000 V you move to the 1500 V class for every DC component. Fix the string length first, then size the DC protection and cable.