PV module and string comparator

The modules to compare

Two at least, three at most. A series groups the power bins of a single datasheet: Vmp, Imp, Voc and Isc change from one bin to the next, and that is often where the difference is decided.

Your site

The lowest air temperature expected sets the maximum string length; the highest, together with the mounting, sets the minimum.

Minimum air temperature °C No thermal model: IEC 62548, NEC 690.7 Maximum air temperature °C Mounting Uc = 20 W/m²K Warranty horizon years

Your inverter

Take the eight values from your own inverter datasheet. Those shown when the page opens are those of a common three-phase inverter, to be replaced by yours.

Maximum input voltage V The absolute limit of the DC input Bottom of MPPT window V Top of MPPT window V Start-up voltage V Maximum current per MPPT A Maximum current per DC input A 0 if the datasheet does not give it Inputs per MPPT Number of MPPTs

Choose at least two modules to see the comparison.

Your string, module by module

Drag the slider: the three voltages that decide string length appear live, each against the limit of your inverter. Two at the coldest, one in high summer — a string can hold through winter and drop out in August.

Modules per string 20

The conventions behind the calculation

They are written here because a result without its conventions means nothing. On a study, each one is taken again from your site and your equipment.

Maximum string length — two limits, and neither replaces the other. Voc × number of modules, at the coldest, must stay below the inverter input voltage: that is the one that breaks equipment. Vmp × number of modules must stay below the top of the MPPT window: that one breaks nothing, it just produces less. We keep the smaller, and the table says which.
Cold: no thermal model. With no sun and no production, the cell is at air temperature. That is the convention of IEC 62548 and NEC 690.7.
Hot: the PVsyst model. Tc = Ta + G·α(1−η)/(Uc + Uv·wind), at 1000 W/m², with α = 0.9, Uv = 0, and Uc of 29, 20 or 15 W/m²K depending on the mounting. It is the market reference.
Vmp against temperature. Datasheets do not give this coefficient — none of the sixteen series in this database. We reconstruct it: kVmp = kPmax − kIsc. Checked to within 0.4 % against a PVsyst report from a real project, on the conservative side.
Strings per MPPT — the smaller of the two: maximum MPPT current divided by Imp, or the number of inputs. The MPPT short-circuit current does not enter the calculation: it is a withstand rating, not a working limit. No safety factor is applied; the remaining margin is shown.
String protection — IEC 62548, clause 7.5. A faulted string receives the current of the others: 1.25 × Kcorr × Isc, with Kcorr = 1.1 for bifacial. As long as that current stays below the module fuse rating, no protection is required. The rating does not limit the number of strings, it triggers an obligation.
Warranted power — 100 − first-year degradation − annual degradation × (term − 1). Manufacturers warrant a linear decline, not a compounded one.

What this table is not

It does not rank or recommend. Two modules that give the same architecture may be separated by price, lead time, after-sales service or an installer's experience — none of which is here.
It does not size your inverter. It compares modules and works out the string each one allows. How many strings to put on the inverter, and what array power to connect to it, is decided afterwards — that is another calculation, and it needs the clipping loss over a full year.
It says nothing about shading, the actual layout, or the space available. A string that passes electrically still has to fit on the roof.
It checks neither the strength of the structure, nor the allowable load of the frame. The kg/m² are given for the module alone, without its mounting system or its ballast.
Datasheet values are republished as they stand. A typo copied verbatim would pass the consistency checks.

You are a developer, EPC or installer, and you have to settle on a module before the purchase order.