The string inverter.

01How it works

Panels are joined one after another, like batteries in a torch, so their voltages add up. That line of panels is a string. One or two strings run down from the roof to a single inverter on the wall, which turns their direct current into the alternating current your home uses.

The inverter does one other job continuously. It hunts for the precise electrical load at which each string gives the most power, and holds it there as the light changes. That part of the inverter is called a tracker, and most have two or more, so each string can be managed on its own.

02Why simple is a virtue

On the roof there are panels, cable and nothing else. The one piece of working electronics is at eye level, in the shade, where it can be read, serviced and one day replaced without anyone climbing a ladder.

Fewer parts in the hardest place to reach is a real advantage over twenty years.

On the roof: panels and cable. Everything that thinks is on the wall.

03Where it struggles

Because the panels in a string are joined in a line, the same current flows through all of them. A panel in shade can't pass as much, and the rest of the string is held back with it.

Modern panels soften this. Each is divided into sections that can be bypassed, so a shadow across one corner takes out a section, not the whole panel. But regular shade on a string, from a chimney or a tree, costs more than the shaded area suggests.

04Faces and trackers

Panels facing different ways see different light, so they shouldn't share a string. Each face wants a tracker to itself. A roof with a north face and a west face suits a two-tracker inverter perfectly.

A roof broken into four or five small faces runs out of trackers. That's the point at which we'd look at panel-level electronics instead.

05With a battery

A hybrid string inverter runs the panels and a battery through one unit, with the battery charged directly from the panels. It's a tidy arrangement: one box on the wall and the battery beside it.

If a battery might come later, choosing a hybrid inverter now means the battery can simply be added.

06What you'll see in the app

Monitoring on a string system shows the system as a whole, and usually each string. It doesn't show each panel. For most homes that's all the detail anyone needs: the day's curve tells you whether the system is working as it should.