String length is decided by the coldest morning of the year

Voltage and temperature move in opposite directions

Most people learn that solar panels perform worse when hot, and that is true of power output. What is less widely known is the mechanism, which is that the voltage falls as temperature rises while the current barely changes. Power is the product of the two, so power follows voltage downwards.

Run that relationship the other way and it produces the constraint that governs system design. On a cold morning, panel voltage rises above the rated figure, and the colder it is the higher it goes. Open-circuit voltage — the voltage across a panel with nothing drawing current from it — is the highest voltage the panel will ever present, and it occurs at the coldest moment when the sun first reaches the array and no load is connected.

That combination, cold and bright and unloaded, happens on a clear winter morning shortly after sunrise. It lasts moments, and everything downstream has to survive it.

The upper limit, and why it is a hard one

Two limits apply at the top. The inverter has a maximum permitted direct-current input voltage, above which it may be damaged. And the installation as a whole has a maximum system voltage set by the panel rating and by the wiring standard, which governs insulation and isolation requirements throughout.

Neither of these is a limit to be approached casually, because exceeding the inverter maximum can destroy it and exceeding the system voltage puts the installation outside the standard it was certified against. So the design calculation takes the panel’s open-circuit voltage, applies its temperature coefficient of voltage, and extrapolates to the lowest temperature the site can reasonably be expected to reach.

Which temperature to use is a design judgement informed by local climate records, and it should be a genuine local extreme rather than a typical winter figure. This is one of the places where a system designed by someone unfamiliar with the location can be quietly wrong: the same array and inverter that work perfectly in a mild coastal climate can exceed the voltage limit inland where winters are colder.

The lower limit, which is more often the annoying one

A string can also be too short. An inverter needs a minimum input voltage before it will start, and a somewhat higher one before its tracker can operate across a useful range. If a string is too short, the inverter starts late in the morning, stops early in the evening and may drop out entirely on a dull day.

The worst case for the lower limit is the mirror of the upper one: a hot afternoon, when panel voltage is at its lowest, with the array producing at its operating voltage rather than open-circuit. Cell temperatures well above air temperature are ordinary in summer, and the calculation has to use the cell figure.

So the designer is working inside a window with a cold-morning ceiling and a hot-afternoon floor, and the number of panels per string has to satisfy both. On a small roof with an awkward number of panels, that window is sometimes uncomfortably narrow, and it is a common reason a proposed layout has to change.

Why strings should be equal, and what to do when they cannot be

Where an inverter has more than one tracker input, each can run a string of a different length, which is exactly how a roof with two orientations should be wired. Where two strings share a single tracker input, they must be the same length, because they are effectively in parallel and a voltage mismatch means one string drags the other away from its optimum.

This is why the shape of a roof so often decides the shape of an electrical design. A dormer, a chimney or a hip that reduces one section to five panels while another holds nine creates a mismatch that cannot be resolved by wiring alone.

Microinverters dissolve the problem entirely, since each panel has its own converter and there is no series string to balance. Optimisers loosen it substantially, because they can adjust their output voltages to make an uneven string behave. Both are legitimate answers, and both cost more than a string layout that happened to fit.

What to look for in a design document

A properly documented design states the panel model and its temperature coefficients, the design minimum and maximum temperatures used, the resulting worst-case string voltages, and the inverter limits those must sit within. That is a page of arithmetic and it should be available on request.

Its absence is not proof of anything, since experienced installers use software that performs the calculation as part of producing the layout. But if a proposal specifies a string length without any reference to the temperature extremes at the site, it is worth asking what figures were assumed. The answer takes a minute and the consequence of getting it wrong is measured in equipment.

It is also worth noting when the design leaves no margin. A string sized to sit just under the inverter maximum at the assumed low temperature is a design that depends on the assumption being right, and climate records occasionally get exceeded.