String Sizer
Safe min/max modules per series string
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Inverter & Module Data
Voc/Vmp are prefilled for a typical 545 Wp Mono PERC module — edit them from your module datasheet for exact results.
Assumes a 1100 V max-DC string inverter with a 200–1000 V MPPT window, site temperature range 0–75 °C (cell), voltage coefficient −0.27%/°C, and a DC/AC oversizing ratio of 1.35. Always confirm against the specific inverter datasheet before ordering.
Minimum modules / string
6
Keeps hot-weather string Vmp ≥ 200 V (217 V at 6 modules)
Maximum modules / string
20
Keeps cold-morning string Voc ≤ 1100 V (1,059 V at 20 modules)
Corrected voltages
- Voc at 0 °C
- 52.9 V
- Vmp at 75 °C cell
- 36.2 V
Array-level guide
- Max total modules (DC/AC 1.35)
- 123
- ≈ Strings at max length
- 6
String Voltage vs Cell Temperature
At 20 modules/string — the shaded band is the inverter's safe operating window.
About This Calculator
String sizing is where paper designs fail in the field. Too many modules in series and the cold-morning open-circuit voltage (Voc) exceeds the inverter’s maximum DC input — a safety violation that can destroy the inverter. Too few, and the hot-afternoon operating voltage (Vmp) sags below the MPPT window, collapsing generation exactly when the sun is strongest.
This calculator applies the standard temperature-correction method: Voc is scaled up for the coldest expected morning and checked against the inverter’s DC limit; Vmp is scaled down for peak cell temperature and checked against the MPPT minimum. Module Voc/Vmp fields come prefilled with typical 545 Wp Mono PERC values — replace them with your module’s datasheet numbers for exact results.
How the Calculation Works
A string must stay inside the inverter’s voltage limits in the worst weather. The maximum modules per string is set by the coldest morning: voltage rises as temperature falls, so cold-condition Voc × modules must stay below the inverter’s maximum DC voltage. The minimum is set by the hottest afternoon: voltage drops as modules heat up, so hot-condition Vmp × modules must stay above the bottom of the MPPT window.
The calculator applies the module’s voltage temperature coefficient to both limits and also checks the total module count against the inverter’s capacity at a typical DC/AC ratio. Always confirm against the exact datasheets — our inverter reviews show real MPPT windows and limits, such as the Growatt MIN 5000TL-X.
Worked example (the calculator’s defaults)
- Module: 545 Wp, Voc 49.6 V, Vmp 41.8 V, voltage coefficient −0.27%/°C. Inverter: 1,100 V maximum DC, 200 V lower MPPT limit.
- Cold (0 °C) Voc: 49.6 × (1 + 0.0027 × 25) = 52.95 V, so the maximum is 1,100 ÷ 52.95 = 20 modules per string.
- Hot (75 °C cell) Vmp: 41.8 × (1 − 0.0027 × 50) = 36.16 V, so the minimum is 200 ÷ 36.16 = 6 modules per string.
- Strings of 6 to 20 modules are therefore valid for this pairing.
Go Deeper
Frequently Asked Questions
- Why Does Temperature Matter in String Sizing?
- Silicon voltage rises when cold and falls when hot (≈ −0.27%/°C). A string that measures 950 V at 25 °C can exceed 1,020 V on a cold winter morning — which is why calculations must use temperature-corrected voltages, not STC datasheet values.
- Can Strings of Different Lengths Connect to One Inverter?
- Only on separate MPPT inputs. Strings paralleled on the same MPPT must be identical in module count, model, and orientation, or mismatch losses and reverse-current risks follow.
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