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String Sizing 101 — How to Calculate How Many Panels Go in Series
String sizing is the calculation that determines how many panels connect in series in each string — and how many strings connect in parallel to each MPPT input. Get it right and your system runs at peak efficiency. Get it wrong and the inverter either clips production or throws fault codes.
Why String Sizing Matters
When panels connect in series, their voltages add while current stays the same. When strings connect in parallel, currents add while voltage stays the same.
The inverter's MPPT input has:
A maximum DC input voltage — exceed this and you destroy the inverter (instantly)
An MPPT operating range — the voltage window where the MPPT works efficiently
A maximum input current per MPPT — exceed this and production is clipped
The Three Key Panel Specs for String Sizing
Spec | Symbol | Condition | Use For |
|---|---|---|---|
Open Circuit Voltage | Voc | No load, STC | Maximum string voltage check |
Max Power Voltage | Vmp | At max power, STC | MPPT range check |
Short Circuit Current | Isc | Short circuit, STC | Maximum string current check |
Step-by-Step String Sizing Calculation
Example: JA Solar 550W panel (Voc = 41.3V, Vmp = 34.5V, Isc = 13.83A, Imp = 13.0A) with Deye 5kW inverter (Max DC: 550V, MPPT range: 80–450V, Max MPPT current: 13A)
Step 1: Maximum panels in series (voltage limit)
Max panels = Max DC input voltage ÷ Panel Voc
= 550V ÷ 41.3V = 13.3 → use 13 panels max
In practice, apply a temperature correction: Voc increases in cold weather. Add 10% safety margin → use 12 panels max for safety.
Step 2: Minimum panels in series (MPPT lower limit)
Min panels = MPPT Vmin ÷ Panel Vmp
= 80V ÷ 34.5V = 2.3 → minimum 3 panels
Step 3: Verify operating range
For 10 panels in series: String Vmp = 10 × 34.5V = 345V → within 80-450V MPPT range ✓
String Voc = 10 × 41.3V = 413V → below 550V max DC ✓
Step 4: Check current limit
String current = Panel Imp = 13.0A → within 13A MPPT limit ✓
If connecting 2 strings in parallel: 13.0A × 2 = 26A — exceeds the single MPPT 13A limit → need 2 separate MPPT inputs for 2 strings.
Result for This Example
1 string of 10 panels per MPPT input. The Deye 5kW has 2 MPPTs — so total: 2 strings × 10 panels = 20 panels = 11 kWp. For a 5kW inverter, use 1 string of 10 panels (5.5 kWp) — properly loaded at 110% DC:AC ratio.
Our SLD generator in the Installer Dashboard calculates this automatically from your selected panel and inverter specs.
Engr. Jason Morales — Founder, SolarEnergyPH