String Inverters

What a string inverter is

One box, usually the size of a small briefcase, mounted on a wall. The panels are wired in series into a string — positive of one to negative of the next — and the string arrives at the inverter as a single DC circuit at a few hundred volts. The inverter converts it to 230V AC and feeds it into the consumer unit.

This is the original architecture and still the most common. It remains the default for good reasons rather than habit.

How a string is designed

Series wiring adds voltage. Ten panels at roughly 40V open circuit produce roughly 400V; twenty produce 800V. Two constraints bound the design.

The upper limit is the inverter’s maximum DC input voltage. Panel voltage rises as temperature falls, so the string must stay within that limit on the coldest bright morning of the year, not at the temperature on survey day. This is why string lengths are calculated against a design minimum temperature rather than estimated.

The lower limit is the inverter’s start-up and MPPT operating voltage. Too short a string and the inverter will not begin generating until later in the morning, and will drop out earlier on dull days.

Between those, the designer chooses panel counts per string that fit the roof geometry and the tracker allocation.

MPPT and orientation

Each MPPT input holds everything connected to it at one operating voltage, chosen to maximise power from that group. That is fine when the group behaves as one. It is not fine when it does not.

Put an east-facing string and a west-facing string on the same tracker and the tracker settles on a compromise. In the morning the east array wants a different voltage from the west array, and neither gets it. Split them onto separate trackers and each finds its own peak independently. So the practical rule is one tracker per orientation, and an inverter with two or three MPPT inputs is what makes a split-roof design work properly.

Where it is the right choice

An unshaded roof, one or two orientations, no more strings than the inverter has trackers. That describes the majority of UK domestic installations, and on those roofs a string inverter produces essentially the same annual yield as any other architecture for less money and with fewer components on the roof.

It is also the right choice when serviceability matters. Everything is in one accessible unit. A fault is diagnosed from the display or the app, and a replacement takes a morning without scaffolding.

Where it is the wrong choice

Where shading falls across part of the array for a meaningful part of the day. Where the roof breaks into several small arrays at different angles and the tracker count cannot cover them. Where the owner does not want high-voltage DC running from the roof through the loft. In those cases optimisers or microinverters are worth their cost, and we will say so.

Installation implications

Siting is the main decision. Ventilated, accessible, protected from direct sun, and close enough to both the array and the consumer unit to keep cable runs sensible. Lofts work but run hot in summer, and heat shortens electronic life. DC isolation is required at the inverter and, depending on the design, at the array. Everything gets labelled so the next electrician can see what is in front of them.

Cost and warranty

A string inverter is the cheapest way to convert an array, and on a straightforward roof the saving against per-panel electronics is significant enough to fund other parts of the project. Warranty terms differ substantially between manufacturers, and paid extensions are commonly available at the point of purchase — worth considering, because this is the component most likely to be replaced first. The exact term, conditions and extension pricing appear on the datasheet and in your written quotation.