Types of solar inverters: which one to choose

Panels generate direct current and the grid and your appliances run on alternating current. Converting one into the other is the inverter's obvious job, but not the most interesting one.

What decides how much energy you get is maximum power point tracking, known by its acronym MPPT. The combination of voltage and current at which a group of panels delivers the most power changes continuously with radiation and temperature, and the inverter searches for it many times a second. How many independent trackers it has determines how many groups it can divide your installation into to optimise them separately.

And it does a third job almost nobody talks about: anti-islanding protection. If the grid goes down, the inverter disconnects automatically so as not to energise sections where technicians may be working. It is why a grid-connected system shuts down during a blackout, and one of the things the operator verifies during legalization.

If your roof has no shading and all the panels face the same direction, a string inverter is usually the most cost-effective option.

If there is partial shading, chimneys, aerials, a tank, or panels on planes with different orientations, microinverters or optimizers stop an affected panel dragging the rest down.

The reason is electrical: in a string the panels are in series, so the same current flows through all of them. The module that can contribute least sets the pace, just like the narrowest link in a pipe. That is why shade over a single panel does not cost you one panel's output, but a fraction of the whole string.

Before deciding on an architecture, it is worth looking at the roof at different times and in different seasons. Shade that does not exist at midday in July may be obvious at four in the afternoon, and trees grow.

It is the classic architecture: panels are grouped into one or more strings and all of them reach a single unit, usually installed on an accessible wall near the panelboard.

Its strength is simplicity. A single unit to maintain, a single point to diagnose, the lowest cost per watt and an enormous catalogue. Being at head height and in the shade, it runs cooler and is easy to inspect or replace.

Its limits are those imposed by series wiring: sensitivity to shading, difficulty mixing orientations within the same string, and a single point of failure — if the inverter goes down, all output goes down. Models with several MPPTs considerably ease the orientation problem, because they let two roof planes be treated as independent groups.

Here each panel has its own inverter underneath it, and the conversion to alternating current happens on the roof. Each module works at its optimum point unaffected by what its neighbours are doing.

That solves the shading and mixed-orientation problem at its root, allows growth module by module and provides individual monitoring, which makes diagnosis almost trivial: you can see exactly which panel is underperforming. With no high DC voltages running across the roof, the safety profile is also different.

In exchange, it costs more per watt and multiplies the number of electronic units installed outdoors, exposed to the heat under the panel. The practical trade-off comes the day one fails: the fault affects only one module, but repairing it means climbing onto the roof and removing the panel, not walking up to a wall.

An optimizer is a small unit per panel that adjusts its operating point, but does not convert to alternating current: a central inverter still does that.

It recovers much of what is lost to shading and mismatch between modules, and offers individual monitoring, at an intermediate cost. It also keeps the central inverter somewhere accessible.

Its trade-off is that it retains the single point of failure of the central inverter and adds electronics on the roof. In practice it is a sensible option when there is moderate shading and you want per-panel monitoring without taking on the cost of microinverters.

Microinverters and optimizers let you see performance panel by panel, which makes it easier to detect faults. A string inverter monitors the system as a whole: you will notice output falling, but not where the drop comes from without an inspection.

The central inverter is also the component you will most likely replace during the life of the system, with a typical service life of 10 to 15 years against 25 or more for the panels. It is worth budgeting for from the start rather than as an unforeseen expense.

Microinverters spread that risk across many units: it is unlikely they will all fail at once, but each intervention means working at height. When comparing proposals, ask about the warranty for each architecture and, above all, who covers the labour for a replacement, which is not always included.

A detail almost nobody checks that explains price differences between proposals: how much panel capacity is connected to how much inverter capacity.

It is normal and correct to install somewhat more panel capacity than the inverter's nominal rating, because modules rarely reach their peak output. That oversizing makes better use of the unit and usually improves the economics of the whole.

Taken to an extreme, however, it cuts output precisely during the hours of highest generation, because the inverter clips the output. If a proposal is noticeably cheaper with the same panels, check whether the inverter is smaller: it may be a legitimate design decision, but it should be explained and reflected in the estimated output.

The ones that most often force a redesign or leave you living with less output than expected.