☀️ Solar PV · 7 min read

PV String Sizing — Panels in Series Within MPPT Voltage Limits

String sizing is a temperature problem. The panel datasheet values are at 25 °C; your roof is not, and the difference decides how many modules you can wire in series.

The two limits

Maximum panels per string — limited by string V_oc at the coldest cell temperature staying below the inverter’s max DC input voltage.
Minimum panels per string — limited by string V_mp at the hottest cell temperature staying above the MPPT minimum voltage.

Temperature correction

V_oc(T) = V_oc,STC × [1 + β_Voc × (T_min − 25)]
V_mp(T) = V_mp,STC × [1 + β_Vmp × (T_max − 25)]

β is the temperature coefficient (negative, ~−0.27%/°C for V_oc on modern modules). Use record-low ambient for T_min and cell temperature up to ~70 °C for T_max in the tropics.

Worked example

Panel: V_oc 41.5 V, V_mp 34.8 V, β_Voc −0.27%/°C. Inverter: max DC 600 V, MPPT 90–550 V. Site: T_min 12 °C, cell T_max 68 °C.

V_oc(12°C) = 41.5 × [1 + (−0.0027)(12 − 25)] = 43.0 V
Max panels = 600 / 43.0 = 13.9 → 13 panels

V_mp(68°C) ≈ 34.8 × [1 + (−0.0037)(68 − 25)] = 29.3 V
Min panels = 90 / 29.3 = 3.1 → 4 panels

Any string of 4–13 panels works; choose the length that fits the roof and balances the MPPT inputs.

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