String Inverters, Optimisers or Microinverters?

There are three common ways to connect solar panels. A string inverter takes the combined output of a row of panels wired together. Optimisers are small boxes fitted behind each panel that adjust its output before it reaches a string inverter. Microinverters sit behind each panel and convert its output to AC on the roof. For an unshaded roof, a well-designed string system is usually the simplest and most reliable choice. Optimisers or microinverters earn their place where shading or several roof directions would otherwise drag the whole system down.

Key points

• A string inverter is the simplest setup, with the fewest parts on the roof.

• In a basic string, a shaded panel can hold back the others wired with it.

• Optimisers and microinverters let each panel work independently, which helps with shade and mixed roof directions.

• They also give panel-by-panel monitoring.

• They add electronics on the roof, which are harder to reach if they fail.

• Most modern string inverters have two or more inputs, so panels on different roof faces can already be kept apart.

• For a battery, a hybrid inverter or AC-coupled battery is needed alongside any of these.

How a string inverter works

Panels are wired in series into a string, so their voltages add up, and the string runs down to an inverter, usually in the loft or garage. The inverter converts the DC electricity into AC and continuously adjusts how it loads the string to get the most power from it (maximum power point tracking, or MPPT).

The weak point is that a string is only as strong as its weakest panel. If one panel is shaded by a chimney for part of the day, the current through the whole string can fall. Modern panels have bypass diodes that limit the damage, but some output is still lost.

Most inverters now have two or more MPPT inputs. That means panels on, say, an east roof and a west roof can be wired as separate strings, so each face works on its own.

How optimisers work

An optimiser is a small electronic box fitted behind each panel. It tracks the best operating point for that panel individually and adjusts its output so a shaded or weaker panel doesn’t pull down the rest of the string. The panels still feed a central string inverter, usually one designed to work with that brand of optimiser.

Optimisers also allow monitoring of each panel, so you can see if one is underperforming. Some optimiser systems are designed to reduce the DC voltage on the roof when the system is switched off, which is a safety feature specific to those systems. SolarEdge is the best-known optimiser system; we install and service SolarEdge, although for most homes we fit other equipment.

How microinverters work

A microinverter is a small inverter behind each panel. Each panel’s DC output is converted to AC on the roof, and the panels are connected together with AC cabling. Each panel works completely independently, and there is no large central inverter.

The trade-off is more electronics on the roof. If one fails, the panel has to be lifted to replace it, which may need access equipment. Adding a battery to a microinverter system usually means an AC-coupled battery, because there is no central DC inverter for the battery to share.

Which handles shade best?

Shade is the deciding factor. If your roof is clear for most of the day, the extra parts rarely pay their way. If a chimney, dormer, tree or neighbouring building shades some panels for part of the day, panel-level electronics can recover output that a basic string would lose. Our guide to the best direction and angle for solar panels explains how shade and orientation affect output.

Mixing approaches

It doesn’t have to be all or nothing. Optimisers can be fitted only to the panels that are shaded, with the rest wired normally, where the inverter supports it. That gets most of the benefit with fewer extra parts. Your installer should explain why they’ve chosen the design for your roof.

Batteries and future upgrades

If you might add a battery later, think about it now. A hybrid inverter can be ready for a battery from day one. An optimiser system may limit you to batteries designed for that brand. A microinverter system will normally need an AC-coupled battery. Our guides to what a hybrid inverter does and adding a battery to existing solar panels explain the options.

Frequently asked questions

Do I need optimisers on my solar panels?

Not usually on an unshaded roof. They help where some panels are shaded for part of the day or where panels face several directions.

Are microinverters better than a string inverter?

They handle shade better and give panel-level monitoring, but they put more electronics on the roof. On an unshaded roof, a string inverter is usually simpler and works very well.

Can I add optimisers to an existing system?

Sometimes, depending on the inverter. Ask your installer to check compatibility.

Can I add a battery to a microinverter system?

Usually, as an AC-coupled battery with its own inverter.

What happens if an optimiser or microinverter fails?

The rest of the system usually keeps working, but replacing the failed unit means lifting the panel, which may need access equipment.

Request a solar and battery quotation. We’ll assess the shading on your roof and explain which design gets the most from it.