Solar PV String Sizing – Detailed Sample Calculation (550W Module)

String sizing must satisfy both:

• ✅ Maximum inverter DC voltage (at cold temperature → Voc highest)

• ✅ Minimum MPPT voltage requirement (at hot temperature → Vmp lowest)

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📌 Given Data

PV Module (550W Mono PERC)

• Rated Power = 550 W

• Voc (STC) = 49.5 V

• Vmp (STC) = 41.2 V

• Temperature Coefficient of Voc = –0.28 % / °C

• STC Temperature = 25°C

Inverter DC Input Range

• MPPT Operating Range = 200V – 1000V

Site Temperature Data

• Minimum Temperature = –5°C

• Maximum Temperature = 45°C

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🧮 Step 1 – Correct Voc at Minimum Temperature (Worst Case Overvoltage)

At colder temperatures, Voc increases.
We must ensure string voltage does not exceed inverter’s 1000V limit.

🔹 Temperature Difference (Cold Condition)

$$\Delta T = 25 - (-5) = 30°C$$

🔹 Voc Correction Formula

$$Voc_{corrected} = Voc_{STC} + (|Temp\ Coeff| × \Delta T × Voc_{STC})$$ $$= 49.5 + (0.0028 × 30 × 49.5)$$ $$= 49.5 + 4.158$$ $$Voc_{corrected} ≈ 53.66V$$

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🧮 Step 2 – Maximum Modules per String (Voltage Limit Check)

$$Max\ Modules = \frac{Max\ Inverter\ Voltage}{Corrected\ Voc}$$ $$= \frac{1000}{53.66}$$ $$≈ 18.63$$

Since we cannot exceed inverter rating:

✅ Maximum = 18 Modules per String

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🧮 Step 3 – Correct Vmp at Maximum Temperature (Worst Case Undervoltage)

At high temperatures, Vmp decreases.
We must ensure string voltage remains above MPPT minimum (200V).

🔹 Temperature Rise

$$\Delta T = 45 - 25 = 20°C$$

🔹 Vmp Correction

$$Vmp_{corrected} = 41.2 - (0.0028 × 20 × 41.2)$$ $$= 41.2 - 2.31$$ $$≈ 38.9V$$

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🧮 Step 4 – Minimum String Voltage Check

$$Total\ Vmp = 38.9V × 18$$ $$≈ 700V$$

Compare with inverter MPPT minimum:

$$700V > 200V$$

✅ Condition Satisfied

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📊 Final Technical Summary

Parameter	Value
Corrected Voc (–5°C)	53.66 V
Max Modules per String	18
Corrected Vmp (45°C)	38.9 V
Total String Vmp	~700 V
Inverter Range	200–1000 V
Design Status	SAFE

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✅ Engineering Conclusion

You can safely design the system with:

👉 18 Modules per String

This configuration:

• Prevents inverter overvoltage during winter mornings

• Maintains stable MPPT operation during peak summer

• Ensures compliance with IEC-based design practice

• Provides voltage margin for long-term reliability

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