Solar String Sizing Calculator

Calculate how many panels to wire in series per string to stay within inverter voltage limits. Accounts for temperature effects on open-circuit voltage.

Panel Specifications (from spec sheet)

Open-circuit voltage — Voc (V)

Labeled "Voc" on the panel spec sheet. Measured at STC (25°C).

Max power voltage — Vmp (V)

Labeled "Vmp" or "Vmpp" on the spec sheet. Always less than Voc.

Voc temperature coefficient (%/°C)

Negative value — labeled "Temp. Coeff. of Voc" on spec sheet. Typical: −0.29 to −0.35 %/°C.

Inverter Specifications (from inverter spec sheet)

Maximum DC input voltage (V)

Hard safety limit. Common values: 600V, 1000V, 1100V. Never exceed this on a cold day.

MPPT voltage range — minimum (V)

MPPT voltage range — maximum (V)

String Vmp must stay within MPPT range for the inverter to harvest power efficiently.

Site Temperature Range

Minimum ambient temperature (°C)

Use the coldest recorded temperature at your site. Cold = highest Voc = safety risk.

Maximum ambient temperature (°C)

Use hottest summer day. Hot = lowest Vmp = MPPT dropout risk.

Minimum panels/string

—

hot-day MPPT floor

Maximum panels/string

—

cold-day Voc ceiling

Recommended

—

midpoint of valid range

Cold-Day Voltages (min temp)

Voc per panel at min temp —

String Voc at min temp (recommended count) —

Inverter max input —

Hot-Day Voltages (max temp)

Vmp per panel at max temp —

String Vmp at max temp (recommended count) —

MPPT range —

Safety Checks

Cold-day Voc ≤ inverter max —

Hot-day Vmp ≥ MPPT minimum —

Nominal Vmp ≤ MPPT maximum —

Why String Sizing Matters

Solar panels connected in series form a "string." Their voltages add together. On a cold winter morning, Voc rises above the STC rated value — and if the string voltage exceeds your inverter's maximum DC input, you can damage or destroy the inverter on the very first cold day.

On a hot summer afternoon, Vmp drops. If it falls below the inverter's MPPT minimum, the inverter loses the ability to harvest power efficiently — or stops producing entirely.

String sizing is safety-critical and often skipped by installers using rule-of-thumb estimates. This calculator uses the exact formulas from IEC 60891 and standard inverter sizing practice.

Temperature Coefficient

The Voc temperature coefficient is always negative — voltage decreases as temperature increases. It's found on the panel spec sheet labeled "Temp. Coeff. of Voc" and expressed as %/°C. A typical value is −0.30 %/°C. Higher-efficiency panels often have coefficients closer to −0.29 %/°C.