Wickfords Solar · Design Tool

PV String Sizing Calculator

Work out how many panels can safely be wired in series for a given inverter, based on your local temperature extremes. Enter your panel and inverter datasheet values below — the result updates as you type.

Panel specifications

From the panel's datasheet, at Standard Test Conditions (STC).

Voc volts

Vmp volts

Isc amps

Temp. coefficient of Voc %/°C

Usually −0.30 to −0.40 %/°C

Temp. coefficient of Vmp %/°C

Usually −0.40 to −0.50 %/°C

Pmax watts

Inverter specifications

From the inverter's datasheet.

Max input voltage volts

MPPT min voltage volts

MPPT max voltage volts

MPPT max current amps

Site temperature conditions

Local design temperatures — the two extremes that stress the string voltage.

Min ambient temp °C

Coldest expected morning — use record low or ASHRAE 99.6% design temp

Max ambient temp °C

Hottest expected day

Cell temperature rise above ambient °C

Typically 25°C for roof mount, 20°C for ground mount / open rack

Min panels

–

Recommended

–

Max panels

–

VOLTAGE WINDOW 0V – 0V

Voc cold

Vmp hot

0V 600V

Safe MPPT window Cold Voc (@ recommended) Hot Vmp (@ recommended)

Voc at min temperature–

Vmp at max cell temperature–

String current (Isc)–

Recommended string power–

How this calculator works

1. Cold weather check — sets the maximum string length

On a cold morning, open-circuit voltage rises. If too many panels are wired in series, the string can exceed the inverter's maximum input voltage and cause damage.

Voc(cold) = Voc × (1 + (T_min − 25) × βVoc)

Max panels = floor(Inverter Vmax ÷ Voc(cold))

2. Hot weather check — sets the minimum string length

On a hot day, operating voltage drops. If too few panels are wired in series, the string can fall below the inverter's MPPT window and stop tracking the maximum power point efficiently.

Vmp(hot) = Vmp × (1 + (T_cell − 25) × γVmp)

Min panels = ceil(Inverter MPPT min ÷ Vmp(hot))

3. Current check

The string's short-circuit current (Isc) is compared against the inverter's MPPT maximum current rating, which matters most once strings are combined in parallel.

Typical panel values

60-cell panel: Voc ~37V, Vmp ~31V
72-cell panel: Voc ~45V, Vmp ~37V
120-cell half-cut: Voc ~49V, Vmp ~41V
144-cell half-cut: Voc ~58V, Vmp ~48V

Typical inverter values

Residential string inverter: Vmax 600V, MPPT 150–500V
Commercial string inverter: Vmax 1000V, MPPT 250–850V

This tool gives an indicative string sizing range based on the datasheet values you enter. It does not account for wiring voltage drop, shading, or site-specific derating. Always verify against your inverter manufacturer's guidelines, and have the final electrical design reviewed by a qualified professional before installation.