How to size PV strings, step by step

Sizing a PV string is a voltage problem: enough modules in series to keep the inverter tracking on a hot day, but not so many that a cold morning pushes the string past the inverter's maximum input. NEC 690.7 governs the high end, and the inverter's MPPT window governs the low end. Here is the worked method.

Gather the four numbers

Before any math, pull these from the module datasheet and inverter spec sheet:

• Module Voc and its temperature coefficient of Voc (usually %/°C or mV/°C).
• Module Vmp and its temperature coefficient of Vmp.
• Inverter maximum DC input voltage (a hard ceiling — never exceed).
• Inverter MPPT voltage range (minimum and maximum tracking voltage).

You also need two site temperatures: the record low (from ASHRAE extreme-annual-mean-minimum tables, per 690.7) and a high cell temperature (commonly 70 °C is used as a design worst case for hot rooftops).

Step 1 — Correct Voc to the cold extreme

Voc climbs as temperature drops. Correct from the 25 °C rating down to your record low:

• Temperature delta = 25 °C minus your record low (a positive number).
• Voltage rise = module Voc times the Voc coefficient times that delta.
• Cold Voc = rated Voc + voltage rise.

Example: a 41.0 V Voc module, coefficient -0.27 %/°C, record low of -10 °C. Delta is 35 °C. Rise is 41.0 x 0.0027 x 35 = 3.87 V, so cold Voc is about 44.9 V.

Step 2 — Maximum modules per string

Divide the inverter ceiling by the cold Voc and round down:

• Max modules = inverter max input voltage / cold Voc.
• With a 600 V inverter: 600 / 44.9 = 13.4, so 13 modules maximum.

Rounding down is not optional — 14 modules would hit 629 V on the coldest morning and can trip or damage the inverter. This is the calculation a reviewer checks first. See our PV string sizing 690.7 reference for the code text.

Step 3 — Correct Vmp to the hot extreme

Vmp falls as the array heats up. Correct from 25 °C to your high cell temperature:

• Delta = high cell temp minus 25 °C.
• Voltage drop = module Vmp times the Vmp coefficient times that delta.
• Hot Vmp = rated Vmp - voltage drop.

Example: 34.0 V Vmp, coefficient -0.35 %/°C, 70 °C cell. Delta is 45 °C. Drop is 34.0 x 0.0035 x 45 = 5.36 V, so hot Vmp is about 28.6 V.

Step 4 — Minimum modules per string

Divide the inverter's minimum MPPT voltage by hot Vmp and round up:

• Min modules = inverter minimum MPPT voltage / hot Vmp.
• With a 60 V minimum: 60 / 28.6 = 2.1, so 3 modules minimum.

Round up so the string still clears the tracking floor when the roof is hottest and production would otherwise be lost.

Step 5 — Pick a string length in range

Any count from the minimum through the maximum is electrically valid. In our example, 3 to 13 modules per string works; the final choice balances roof layout, inverter channel count, and keeping strings matched in length.

Always confirm your record-low temperature and NEC edition with the local AHJ, and remember a licensed electrician or PE must review and stamp the final design. For the bigger picture, see how to size a residential solar system.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.