Intermediate
Solar String Sizing Formula
Calculate maximum and minimum modules per string for solar PV systems — Voc temperature correction and inverter voltage window.
Formulas
N_max = V_inv_max / Voc_cold
Maximum modules per string
N_min = V_inv_min / Vmpp_hot
Minimum modules per string
Voc_cold = Voc_STC × [1 + (Tc_Voc × (T_cold − 25))/100]
Temperature-corrected open circuit voltage
Variable Definitions
N_max=Maximum modules per string(count)
N_min=Minimum modules per string(count)
V_inv_max=Inverter maximum DC input voltage(volts (V))
Voc_cold=Module Voc at coldest expected temperature(volts (V))
Vmpp_hot=Module Vmpp at hottest expected temperature(volts (V))
Tc_Voc=Temperature coefficient of Voc(%/°C (negative value))
Worked Example
440W module (Voc=49.5V, Vmpp=41.7V, Tc_Voc=−0.27%/°C), inverter MPPT range 200-600V, min temp −10°C, max temp 45°C.
- Voc_cold = 49.5 × [1 + (−0.27 × (−10 − 25))/100] = 49.5 × 1.0945 = 54.2V
- Vmpp_hot = 41.7 × [1 + (−0.27 × (45 − 25))/100] = 41.7 × 0.9460 = 39.4V
- N_max = 600 / 54.2 = 11.07 → 11 modules max
- N_min = 200 / 39.4 = 5.08 → 6 modules min
Answer: 6 to 11 modules per string
When to Use
- •Designing solar PV string configurations
- •Ensuring string voltage stays within inverter MPPT window
- •Accounting for extreme temperature effects on module voltage
- •Complying with NEC 690 maximum system voltage limits
Standards & References
- NEC 690.7 — Maximum Voltage
- NEC 690.8 — Circuit Sizing and Current
- NEC Table 690.7(A) — Voltage Correction Factors