PV string sizing note
Checks that the array stays inside the drive input window at both temperature extremes, and writes out the calculation so you can paste it into a bid or a handover pack.
Formal procurements often ask for exactly this, signed, for every pump and module combination. Most calculators do the arithmetic and give you nothing to hand over.
The module
Negative. Typically −0.25 to −0.29.
Negative. Typically −0.30 to −0.40.
The site
Record, not average. Cell taken as equal to air.
Cell taken as air plus 30 K.
The drive
The result
14 modules per string fits inside the window
The cold check is the one that usually decides the answer. The hot check matters on short strings, where the array can fall below the drive start voltage on a hot afternoon.
Your sizing note
Copy this into a bid, a handover pack or an email. It states the method and the assumptions so a reviewer can check it rather than take it on trust.
Method and assumptions
V = Vstc × ( 1 + coefficient ÷ 100 × ( Tcell − 25 ) ) × N
Cold case uses Voc with cell temperature equal to the record low ambient. Hot case uses Vmp with cell temperature equal to the record high ambient plus 30 K.
The 30 K cell rise is a normal working assumption for a free-standing array in still air. A roof-mounted or closely packed array runs hotter; adjust the high ambient figure upward if that is your case.
This page checks voltage only. It does not check current against the drive input rating, string fusing, or cable voltage drop between array and cabinet, all of which are separate calculations.
It is a working tool, not a signed design document. A calculation that forms part of a contract should be issued and signed by whoever is taking responsibility for the design.
Want this checked against a real drive?
Send the module datasheet and the site and we will come back with the string arrangement and this calculation done against the drive we are actually offering.
String sizing FAQ
What is a PV string sizing note?
A short calculation showing that the array voltage stays inside the pump drive or controller input window at both the lowest and the highest design temperature. Formal procurements often ask for one per pump, controller and module combination, signed, because it is the check that stops a correctly chosen pump being fed by an array that trips the drive.
Why is the cold case the one that fails?
Because open-circuit voltage rises as modules cool, and the drive has a maximum input voltage. A string sized on the datasheet number at 25 °C can exceed that limit on the first clear cold morning, which is exactly when nobody is watching.
What cell temperature should I use?
For the cold case, the record low ambient, because a module that has been dark all night sits close to air temperature. For the hot case, ambient plus a cell rise, commonly 25 to 35 K in still air. This page uses 30 K, which is a normal working assumption.
Does this replace a signed calculation from the supplier?
No. It produces the same arithmetic in a form you can check and hand over, which is useful for comparing offers and for preparing a bid. A calculation that forms part of a contract should be issued and signed by whoever is taking responsibility for the design.
Which input window should I use?
Take it from the drive datasheet for the model being offered. The defaults here are the higher of the two windows on the cabinets we supply, 350 to 780 V; the lower window runs 150 to 450 V. Using the wrong window is the quickest way to get a confident, wrong answer.
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