Will this string stay inside the limits?
Cold-morning Voc is the check that damages inverters, and it is the one people skip. Every figure comes from your datasheets — nothing here is assumed.
Will this string stay inside the limits?
Cold-morning Voc, hot-afternoon Vmp, and the current checks — against figures from your own datasheets.
The module
The inverter
Design temperatures
These are not the same figure and the difference matters. Cold is the record low ambient at your site — at dawn there is no sun to warm the module, so cell and air temperature are effectively equal. Hot is a cell temperature, which on a roof runs 25–35 °C above the air.
The configuration
Every limit, checked separately
Working
String lengths that work
How this is calculated
coldVoc per module = Voc(STC) × [1 + coefficient × (coldTemp − 25)]
hotVmp per module = Vmp(STC) × [1 + coefficient × (hotTemp − 25)]
string voltage = modules in series × per-module voltage
Because the coefficient is negative on essentially every silicon module, a temperature below 25 °C makes the bracket larger than one and the voltage rises. That is why the cold check is the one that destroys inverters, and why it happens on a clear winter morning rather than in a heatwave.
Series adds voltage; parallel adds current. They are kept strictly apart above.
Limit logic: a datasheet maximum is a value that must not be exceeded, so landing exactly on it passes — but with a warning, because a design with no margin fails on the first morning colder than the one you designed for.
A check that could not be run because a figure is missing is reported as not checked, never as passed.
Why cold is the dangerous direction
Nearly everyone's intuition is that heat is the enemy of electronics, and for output that is true — a hot module produces less. But open-circuit voltage moves the other way. A module with a −0.27%/°C coefficient sitting at −10 °C produces about 9.5% more Voc than its nameplate. Ten of them in series at 49.5 V nominal is 495 V on paper and roughly 542 V on a January morning. Against a 600 V inverter that is fine; against the same inverter with twelve modules it is not, and the damage is done before anyone is awake to see it.
Ambient or cell temperature?
Getting these round the wrong way is the second most common error here. The cold figure is ambient, because at dawn there is no sunlight to warm the module above the surrounding air — using a cell temperature would understate the voltage and the whole point of the check. The hot figure is a cell temperature, because that is what governs Vmp, and a module in full sun on a roof runs well above the air around it. If you have only air temperatures, add roughly 25–35 °C to the summer figure and say so in your notes.
What this tool will not do
It will not tell you a conductor size, a fuse rating, a breaker, a disconnect arrangement, a grounding method or a rapid-shutdown configuration. It will not tell you a design is code-compliant, because compliance is determined by your jurisdiction and inspector, not by arithmetic. And it will not fill in a missing datasheet figure — if a coefficient is absent, the calculation stops rather than producing an authoritative-looking number built on a guess.
What it is for: catching an obviously wrong string length before anyone orders equipment, and giving you a sheet of figures to take to the person who will sign off the design.