String sizing calculator

For installers and designers

How many panels can this inverter take in a string?

Pick a panel and an inverter, or type the datasheet figures in. The string is checked on the coldest morning against the DC limit, on the hottest afternoon against the MPPT window, and for current on each tracker. Give it a panel count and it shows the ways of wiring them that work.

Panel

Power

Voc

Vmp

Isc

Imp

βVoc

%/°C

γPmax

%/°C

Inverter

AC output

Max DC voltage

MPPT min

MPPT max

Max input

MPPTs

Phases

Site

Coldest cell

°C

Hottest cell

°C

−10 °C and 70 °C are Installa’s UK defaults: a record-cold clear morning, and a panel surface in still summer air.

Panels to wire

Panels per string

–

Strings per MPPT

–max

–

DC design current

–A

Isc × 1.25, per string

Grid connection

–

The string window string voltage against panels in series

Voc at the coldest cell Vmp at STC Vmp at the hottest cell under the counts: ● works, ▲ warning, ✕ error

Why those limits

String voltage, panel by panel

Ways to wire them

Enter a number of panels in the rail to see the string layouts that work on this inverter, and the ones that do not.

Datasheet figures for library items come from the manufacturers’ published sheets, linked under each pick; check them against the sheet for the exact variant you are fitting. Cell temperatures, not air temperatures, drive the voltages. This tool checks string electrical limits only: it does not size cables, isolators or protection, and it does not replace the inverter manufacturer’s own configurator or the requirements of BS 7671 and the MCS installation standard.

The checks, and the codes

These checks are based on Installa’s electrical validation, and the codes are the ones it reports. Two of them work slightly differently here; they are listed below the checks.

PV003

Cold Voc over the DC limit

Voc rises as cells get colder: Voc(T) = Voc × (1 + βVoc × (T − 25)). At the coldest cell temperature, the string’s Voc must stay under the inverter’s maximum DC input voltage. This is the one that damages inverters.

PV001

Hot Vmp under the MPPT window

Vmp falls as cells get hotter: Vmp(T) = Vmp × (1 + γPmax × (T − 25)). At the hottest cell temperature the string must still sit above the MPPT minimum, or the inverter loses the maximum power point on the best afternoons.

PV002

STC Vmp over the MPPT window

At standard test conditions the string’s Vmp must be under the MPPT maximum. Between this and PV003 you get the maximum string length.

PV004

STC Vmp under the MPPT window

A warning, not an error: the string runs, but below the MPPT minimum in standard conditions, so it will not be tracked at its best.

PV005

Current per tracker

The strings on one MPPT add their current. Imp × strings must be under the tracker’s maximum input current or the inverter limits the array; Isc above that figure is a warning to check the tracker’s maximum short-circuit rating. Isc × 1.25 is reported as the DC design current for cables and fuses.

PV008 to PV010

DC to AC ratio

Array DC watts over inverter AC watts, judged once the strings themselves work. All three are warnings: above 1.5 heavy clipping is expected, above 1.3 some clipping, and below 0.8 the inverter is oversized for the array. Among layouts that work equally well, the suggestions prefer a ratio closest to 1.1, as Installa’s do.

Where this page differs from the product

PV001 uses the panel’s Pmax temperature coefficient (γPmax) for Vmp when the datasheet gives one. Installa’s design screen uses the Voc coefficient (βVoc). γPmax is the closer of the two, though still slightly optimistic, so leave a little room above the MPPT minimum.
PV005 compares Imp with the tracker’s maximum input current, and reports Isc × 1.25 as the DC design current for cables and fuses. Installa’s design screen compares Isc × 1.25 with the input current, so it can flag a string this page passes.

Questions installers ask

The check is for cell temperature on a clear, still, frosty morning just after sunrise, when the cells are as cold as the air and the sun is already on them. Minus 10 is the conventional UK design figure and errs on the safe side; a string that is one panel too long only shows up on that one morning a decade, and then it costs an inverter. Change it for other climates.

A dark panel in still air on a summer afternoon can run 30 to 40 degrees above the air. The check uses 70 as a hot-cell figure so the MPPT minimum is met when it matters most, which is exactly when the array is producing the most.

Use the maximum input current per MPPT, sometimes called maximum operating current. The higher figure, maximum short-circuit current, is what the tracker can withstand; if this tool warns that Isc is above the input current, that is the figure to check it against.

From the manufacturers' published datasheets, compiled for Installa's UK reference library; each pick links to the sheet it came from. The library is not exhaustive, and figures change between revisions, so type the values from the sheet in front of you when it matters.

No. Manufacturers' tools know things about their inverters that a datasheet does not say, and their sign-off may matter for the warranty. This tells you quickly whether a pairing is plausible and why a string is too long or too short.