Both solve the same problem: one panel performing badly dragging down the others. On British roofs, which are often small, complicated and partly shaded, that problem is more common than in sunnier countries with big simple roofs.
The problem they solve
On a conventional system, panels are wired in series into a string. The string is limited by its weakest panel. A chimney shadow across one panel, or leaves, or a panel that has developed a fault, pulls down the output of every panel on that string.
The two solutions
Optimisers are small devices fitted to each panel, working alongside a central string inverter. Each panel’s output is conditioned individually before it joins the string, so a shaded panel no longer holds back the others.
Microinverters replace the central inverter entirely with a small inverter on each panel. Each panel is independent and converts to AC on the roof.
Comparing them
| String inverter alone | Optimisers | Microinverters | |
|---|---|---|---|
| Cost | Lowest | Middle | Highest |
| Handles shading | Poorly | Well | Well |
| Panel-level monitoring | No | Yes | Yes |
| Mixed roof directions | Poorly | Well | Well |
| Equipment on the roof | Minimal | Some | Most |
| Single point of failure | The inverter | The inverter | None |
| Replacing a failed unit | Ground level | On the roof | On the roof |
The last two rows are the real trade-off. Microinverters remove the single point of failure, so one failing does not stop the system. But when one does fail, somebody has to get on the roof to reach it, which means scaffolding again.
When they are worth it
- Genuine shading from a chimney, dormer, aerial, neighbouring building or trees
- Panels on more than one roof face, particularly facing different directions. See multiple roofs
- An awkward layout where panels are split around obstructions
- You want panel-level monitoring, which is genuinely useful for spotting a fault years later
When they are not
If you have a single unshaded roof face, a plain string inverter is the right answer and the money is better spent on more panels or a battery.
An installer who proposes optimisers or microinverters on an unshaded south roof should be able to say why. Sometimes there is a good reason. Sometimes it is simply what they always fit.
A note on fire safety
Some optimiser and microinverter systems include rapid shutdown, bringing DC voltage on the roof down quickly in an emergency. That is a genuine benefit and worth asking about, particularly on a property where access is difficult.
What to ask
- Do I actually have shading that justifies this?
- What does the system produce with and without them, on your estimate?
- What is the warranty on the optimisers or microinverters specifically?
- If one fails in year eight, what does replacing it involve and cost?
- Is scaffolding needed to reach them?