Solar String Sizing Calculator

A solar module produces its highest voltage when it is cold, and the nameplate is measured at twenty-five degrees Celsius. On a clear freezing morning the open-circuit voltage of every panel in the string is around twelve percent higher than the label, and a string sized on the label is over the inverter’s maximum before the sun has properly risen. That is what NEC 690.7 exists to prevent. This calculator applies the temperature correction at both ends — the cold ceiling that protects the inverter, and the hot floor below which it stops tracking — and gives the range of string lengths that satisfies both.

Maximum modules per string: 12 600 V limit ÷ 46.31 V of cold Voc per module. This is the NEC 690.7 calculation — corrected open-circuit voltage at the lowest expected temperature, not the nameplate figure.
Voc at -15 °C: 46.31 V Nameplate Voc is 41.5 V at 25 °C. A -0.29 %/°C coefficient over 40 degrees of cooling raises it 11.6%. Panels produce their highest voltage on a bright, freezing morning before the array has warmed up — the worst case is a clear winter sunrise, not a summer afternoon.
If you used the nameplate Voc: 14 modules Sizing on 41.5 V gives 14 modules, and that string reaches 648 V on the coldest morning — 48 V over the limit. Inverters do not survive this, and the failure happens on a clear cold day months after commissioning, when nobody connects it to the string length.
Minimum modules per string: 8 At 70 °C cell temperature, Vmp falls to 28.29 V per module. The string has to stay above the inverter's 200 V MPPT threshold on the hottest day, or the inverter drops out of tracking and the array stops producing exactly when the sun is strongest.
Valid string length: 8 to 12 modules Any count in this range satisfies both the cold voltage ceiling and the hot voltage floor. Longer strings within the range mean lower current, smaller conductors and less voltage drop, so the top of the range is usually the better choice.
Your 12 module string: Within limits 556 V at the coldest expected temperature, against a 600 V limit, and 339 V at the hottest cell temperature, against a 200 V minimum. Both ends clear.
Cell temperature, not air temperature: 70 °C A module in full sun runs 25 to 35 degrees Celsius above the surrounding air, and more when it is mounted flat against a roof with little airflow behind it. Using the air temperature for the hot case makes the low-voltage limit look comfortable when it is not.
Which low temperature to use: -15 °C The convention is the extreme minimum design dry bulb temperature for the site — the coldest it is expected to get, not the average winter low. Using a mild figure is the same error as using the nameplate Voc, just smaller.

The two limits a string has to sit between

Cold weather is the design case

Photovoltaic modules have a negative temperature coefficient for voltage: as they get colder, their voltage rises. The coefficient is published on the datasheet, typically around minus three tenths of a percent per degree Celsius for open-circuit voltage.

Nameplate figures are quoted at standard test conditions, which include a cell temperature of twenty-five degrees. Take the same module to minus fifteen and its open-circuit voltage is about twelve percent higher than the label. On a string of fifteen modules, that is the difference between five hundred and thirty volts and just under six hundred.

The moment of maximum risk is a clear, cold morning immediately after sunrise, before the array has warmed up in its own sunlight. Open circuit voltage also appears whenever the inverter is not drawing current — at start-up, during a fault, or after a grid outage.

This is why the calculation uses the extreme minimum design temperature for the site rather than an average winter low. The array only has to see the condition once.

The other end of the window

The opposite limit is quieter and costs energy rather than equipment. An inverter tracks the maximum power point of the array only within a voltage window, and below the bottom of that window it stops tracking and the array stops producing.

Voltage at maximum power falls with heat, and it falls faster than open-circuit voltage does — coefficients around minus four tenths of a percent per degree are common. A module at seventy degrees cell temperature is producing meaningfully less voltage than its label.

Seventy degrees is not unusual. A module in full sun runs twenty-five to thirty-five degrees above the surrounding air, and a roof-mounted array with little space behind it runs hotter still. Using air temperature for this calculation makes a marginal string look comfortable.

A string that is too short spends the best hours of the hottest days outside the tracking window, which shows up as an array that produces less in July than in May and looks like a shading or soiling problem.

Longer strings, within the range, are better

Any string length between the two limits works. Within that range there are reasons to prefer the longer end.

A longer string delivers the same power at a higher voltage and a lower current, and conductor sizing is driven by current. That means smaller cable, fewer strings for the same array, fewer combiner terminations and less voltage drop over the run from the roof to the inverter.

It also means the array reaches the inverter’s tracking window earlier in the morning and stays in it later in the evening, which adds a small but real amount of annual production.

The reason not to go to the very top of the range is margin. Datasheet coefficients are typical values with tolerance, and site temperature records get broken. Leaving one module of headroom below the calculated maximum costs a little production and removes the failure mode entirely.

What this is based on

NEC 690.7 — maximum photovoltaic system voltage from temperature-corrected Voc
Module datasheet temperature coefficients for Voc and Vmp, referenced to 25 °C STC
Inverter datasheet — maximum input voltage and MPPT operating window

A string voltage check using datasheet coefficients and the temperatures you enter. It does not size conductors, overcurrent protection, rapid shutdown equipment or grounding, and it does not account for module tolerance or degradation. Use the specific module and inverter datasheets, and the extreme minimum design temperature published for your site.

Frequently asked questions

Why does cold weather raise panel voltage?

Photovoltaic cells have a negative temperature coefficient for voltage. Nameplate figures are quoted at 25 °C, and at minus fifteen a typical module produces around twelve percent more open-circuit voltage. The worst case for an inverter is a clear freezing morning, not a hot afternoon.

What does NEC 690.7 require?

That maximum photovoltaic system voltage is calculated using the sum of module open-circuit voltages corrected for the lowest expected ambient temperature, rather than the nameplate value. It is the code requirement behind the cold-Voc calculation.

What happens if a string is too long?

The array exceeds the inverter’s maximum input voltage on a cold morning. Inverters do not tolerate this, and the failure typically appears months after commissioning on the first clear cold day, which makes it hard to connect back to the string length.

What happens if a string is too short?

On the hottest days the string voltage falls below the inverter’s MPPT threshold and it stops tracking, so the array produces nothing during the strongest sun. It looks like a shading or soiling fault rather than a design one.

Should I use ambient or cell temperature?

Cell temperature for the hot case, which runs 25 to 35 °C above ambient in full sun and more on a tight roof mount. For the cold case, ambient is appropriate — a module in the dark or at sunrise is at air temperature.