How to Size a Solar String — Getting Panels per String Right the First Time

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SolarEnergyPH

Solar Guide — Philippines

Article 68 — Day 68

Subject: String Sizing: How Many Panels Per String? (And Why Temperature Matters)
Preview: Connect too many panels in series and you blow the inverter. Too few and it never turns on. Here's the exact calculation — with Philippine temperatures.

How to Size a Solar String — Getting Panels per String Right the First Time

A "string" is a series connection of solar panels. Each panel's voltage adds up — and if the total string voltage exceeds your inverter's maximum input voltage, you permanently damage the inverter. If it falls below the minimum MPPT voltage, the inverter won't produce power. String sizing is one of the most technically critical steps in solar design.

Why Temperature Changes Everything

Panel voltage is not fixed. It changes with temperature — this is described by the temperature coefficient of Voc on the panel datasheet (typically −0.28% to −0.35%/°C). In the Philippines:

Condition	Temperature	Effect on Voc	Risk
Cold morning (Benguet/Tagaytay)	~10–15°C	Voc rises ~3–5%	May exceed inverter max V
STC (standard test)	25°C	Nominal Voc	—
Hot summer afternoon (Metro Manila)	~40–50°C	Vmp drops ~7–10%	May fall below MPPT min V

Condition

Temperature

Effect on Voc

Risk

Cold morning (Benguet/Tagaytay)

~10–15°C

Voc rises ~3–5%

May exceed inverter max V

STC (standard test)

25°C

Nominal Voc

—

Hot summer afternoon (Metro Manila)

~40–50°C

Vmp drops ~7–10%

May fall below MPPT min V

The String Sizing Calculation

Using a 550W panel with Voc = 41.5V, Vmp = 34.6V, Tc = −0.30%/°C, and a Deye 5kW inverter (max 500V input, min MPPT 80V):

Limit	Formula	Result
Max panels (cold limit)	500V ÷ (41.5V × (1 + (−0.30/100) × (15−25)))	500 ÷ 42.7V = 11 panels max
Min panels (hot limit)	80V ÷ (34.6V × (1 + (−0.30/100) × (45−25)))	80 ÷ 32.5V = 3 panels min
Safe string range	—	3 to 11 panels per string

Limit

Formula

Result

Max panels (cold limit)

500V ÷ (41.5V × (1 + (−0.30/100) × (15−25)))

500 ÷ 42.7V = 11 panels max

Min panels (hot limit)

80V ÷ (34.6V × (1 + (−0.30/100) × (45−25)))

80 ÷ 32.5V = 3 panels min

Safe string range

—

3 to 11 panels per string

Practical Rules for Philippine Installers

Always apply temperature correction — don't just divide inverter max voltage by nominal Voc

For rooftop residential in Metro Manila, use 15°C as minimum temp (cold season) and 50°C as maximum (roof surface adds 5–10°C above ambient)

Never exceed 80% of inverter max input voltage as a safety margin

Most 5kW residential inverters: 8–11 panels per string works well with 550W mono panels

Use the free String Sizing calculator at solarenergyph.shop — plug in your panel and inverter specs directly

Engr. Jason Morales — Founder, SolarEnergyPH