Power Optimisers: What They Do and When They Are Used

A power optimiser is a small electronic unit fitted to the back of a single solar panel. Each one is connected to one panel and acts as a DC-to-DC converter, monitoring that panel's performance and adjusting its output so it generates as much energy, as efficiently as possible¹. The panel still feeds into a string inverter, which converts the combined DC power to household AC, but the optimiser gives each panel its own point of control instead of leaving it at the mercy of the weakest panel on the string.

The reason optimisers exist is mismatch. On a plain string system, panels are connected in series, so shading, dirt or a fault on one panel drags down the output of every panel wired after it. Optimisers work similarly to microinverters but are used with a string inverter, enhancing the performance of individual panels by mitigating the effects of shading². In the UK they cost about £40 to £150 per panel¹, and they matter most on roofs that are partially shaded: a roof that is more than 80% shaded can reduce output by as much as 50%, while a roof with 20% shading or less is best³.

For a household's energy independence, optimisers are an enabler rather than a foundation. They squeeze more out of the panels you have where shade would otherwise waste it, and they give panel-by-panel visibility of the system. They do not remove any dependence: the home still needs the grid, a string inverter that will one day need replacing, and, on some systems, an internet connection for the monitoring app. The rest of this page explains what optimisers do, what they cost, when they earn their place and when they do not.

What an optimiser does, in plain terms

Put simply, solar optimisers work by monitoring the performance of each panel and adjusting its output to generate as much energy, as efficiently as possible¹. Without them, a string of panels behaves like a team paced by its slowest member: the inverter sees the string as a whole and settles on one operating point for all of it. With optimisers, each panel is held at its own best operating point whatever its neighbours are doing.

That has two practical consequences:

• Yield. A panel in shade, or one running hotter or dirtier than the rest, no longer caps the output of the whole string. Some manufacturers estimate that an optimised system can produce up to 25% more generation than a non-optimised system, though that is a maker's estimate for shaded conditions rather than a figure to expect on a clear roof¹.
• Visibility. Because each optimiser reports on its own panel, a fault shows up as one panel underperforming rather than as a vague drop in total output.

One MPPT per panel, on the back of the module

The technology inside an optimiser is Maximum Power Point Tracking, or MPPT, for each panel¹. A string inverter normally performs MPPT for whole strings of panels at once, which works well when every panel faces the same way and receives the same light. An optimiser brings that tracking down to module level: it constantly measures the panel it is attached to and holds it at the voltage and current where that individual panel delivers the most power.

Physically, each optimiser is a small unit connected to a single solar panel, acting as a DC-to-DC converter¹. It takes whatever the panel produces, conditions it, and passes it on to the string in a form the inverter can use. Some panels now arrive with the optimiser already in place: UKSOL MCS certified monocrystalline panels have been supplied with Tigo optimiser units factory fitted during the manufacturer's assembly process⁴, which removes a separate installation step and one set of roof-level connections.

Shading and mismatch: the losses optimisers are there to prevent

Shading is the problem optimisers were designed to solve. String inverters are the most common and cheapest option as they connect solar panels in series⁵, and a series connection means the current through every panel is the same. If one panel is shaded by a chimney, a dormer, a satellite dish or a neighbour's extension, its reduced output constrains the string. Micro-inverters and power-optimisers separate panels so a single panel failure does not impact the whole system, though both may be more expensive than a plain string design⁵.

The scale of the loss depends on how much of the roof is affected. Which? reports that a roof with 20% shading or less is best, and that a roof that is more than 80% shaded can reduce output by as much as 50%³. Energy Saving Trust is blunter about the worst cases: if shading is unavoidable, an installer might recommend microinverters or power optimisers, because these let each panel work independently⁶. Bromley Council's energy advice makes the same point from the other direction, warning that if a roof is heavily shaded, solar panels may not be the most suitable option at all⁷. Optimisers soften shading; they do not abolish it.

Shading is also a formal part of an installation assessment, not just a sales conversation. The MCS scheme's standard for solar PV systems includes a Shade Factor (SF) as a field in its assessment documentation⁸, and MCS states that its shade assessment method is estimated to yield results within 10% of the actual annual energy yield for most systems⁹. So the shading that decides whether optimisers are worth having is something a competent installer measures and records, and the estimate of what optimisers will recover should be grounded in that same survey. Our page on how shade affects solar panel output covers the underlying mechanism in more detail.

What optimisers actually add to your yield

The honest answer is: it depends on the shading, and the biggest claims deserve caution. The up-to-25% figure quoted by some manufacturers is an estimate for optimised versus non-optimised systems in conditions where shading bites¹; on an unshaded south-facing roof there is far less mismatch for an optimiser to recover, and the gain shrinks towards the cost of the hardware.

Two independent qualifications matter here. Renewables First, writing about what makes a good solar PV roof, notes that the use of optimisers will reduce the losses of the overall system but individual panel losses will still affect performance¹⁰. In other words, an optimiser stops one panel dragging down its neighbours, but it cannot make a shaded panel produce power that the light simply is not delivering. And the ideal case for solar remains a large, un-shaded, predominantly south-facing roof space, as the Renewable Energy Consumer Code puts it¹¹, with an unshaded south-facing roof described as ideal for maximum performance by Energy Saving Trust¹². On such a roof, optimisers are largely insurance against future shading, for instance from a tree that has not yet grown.

What a household should expect in practice is a system that performs closer to its potential through the parts of the day when shade crosses the roof, and a clearer picture of where losses occur. What optimisers do not do is change the fundamental arithmetic of the site: if the survey says heavy shading, the question is whether solar is worthwhile at all, not which electronics to hang on it.

Optimiser or microinverter: the choice between module-level options

Optimisers and microinverters both move the intelligence of the system down to panel level, and for many homes the choice between them is more consequential than the choice to go module-level at all. The difference is where the conversion to AC happens. An optimiser conditions DC power at the panel and still relies on a string inverter to do the conversion²; a microinverter converts at the panel itself, so there is no high-voltage DC string at all.

Both approaches separate panels so a single panel failure does not impact the whole system, though both may be more expensive than a plain string setup⁵.

Our comparison pages go deeper on this choice: see microinverters vs string inverters, Enphase vs SolarEdge and SolarEdge vs Tigo optimisers.

The cost: around £40 a unit, and the gap to microinverters

In the UK, solar optimisers range from about £40 to £150 per panel¹. The lower end of that range is what gives optimisers their place in the market: they allow for monitoring without the higher cost of microinverters², and microinverters are generally more expensive than solar optimisers¹. For a typical array of, say, ten panels, the hardware difference between the two approaches is therefore substantial before any installation labour is counted.

To put the per-unit cost in context, a small home solar installation of 2 to 3kW costs roughly £4,000 to £6,000 for solar alone¹³, and a string inverter that fails will cost about £500 to £1,000 to replace according to the Centre for Sustainable Energy¹⁴, with Energy Saving Trust putting the usual figure at around £800¹⁵. Optimisers are an add-on to that budget, not a substitute for any part of it: the string inverter is still required.

String inverter plus optimisers: where the combination fits

The standard arrangement is a string inverter with an optimiser behind each panel. String inverters connect multiple solar panels in a series, making them cost-effective and easy to install², and the optimisers preserve that simple, cheap central conversion while removing the series connection's main weakness, its sensitivity to the weakest panel. Power optimisers work similarly to microinverters but are used with a string inverter, enhancing the performance of individual panels by mitigating the effects of shading².

This combination fits best where the roof is mostly good but not perfect:

• one chimney, a velux window or a dormer casting a patch of shade;
• a patch of morning or evening shade from a tree;
• roofs with multiple orientations, since panels facing different directions produce at different voltages and currents, which is exactly the mismatch optimisers smooth out.

Some installers now treat module-level electronics as the default rather than the upgrade: Renewables First includes optimisers as standard in its residential solar PV service, to further maximise output¹⁶.

What the household gains is the low cost and simplicity of a string inverter with much of the resilience of panel-level architecture. What remains is the string inverter itself, a single box whose failure takes the whole system offline until it is replaced, at a cost of roughly £500 to £1,000¹⁴. Our pages on string inverters and solar inverters cover that side of the system.

Adding panels later: expansion without rewiring

Optimisers also change the arithmetic of extending a system. If you decide to add more solar panels later, additional optimisers can be added without complex rewiring or replacing your inverter¹³. Because each panel is conditioned individually, the string inverter sees a managed input rather than a rigid chain, which gives more freedom in how many panels are added and how the array is arranged.

That said, the independent evidence points the other way for the microinverter route: systems with microinverters can be easier to expand than systems using optimisers and a central string inverter¹, because each microinverter is self-contained and the central box does not need to accommodate a longer string. If a household's plans involve phased expansion, say panels now and more after a loft conversion, that difference is worth raising with the installer at survey stage.

For energy independence, expandability matters more than it first appears. A household that can add panels without replacing its inverter can grow its self-generation as its needs grow, and an optimiser-based design is one of the routes that makes that possible, within the limits of the inverter's capacity.

Safety: shutdown and arc fault protection on the DC side

Module-level electronics bring safety features that a plain string system cannot offer. Some optimiser models offer:

• monitoring for overheating;
• real-time alerts for issues;
• easy ways to shut down the whole solar system at the flick of a switch¹.

The more significant feature is on the DC side: systems built on SolarEdge's integrated S series optimisers have an automatic safety shutdown, called Safe DC, with AFCI (Arc Fault Circuit Interrupter) and heat detection at the module level⁴, a specification repeated in the official innovation measure documentation for these systems¹⁷.

What this means in practice is that the DC cabling on the roof, which on a plain string system sits at high voltage whenever the panels are lit, can be reduced to a safe state when the system shuts down. Arc fault detection addresses one of the specific fire risks of DC circuits: a poor connection that draws an arc. Electrical Safety First's guidance on solar panels notes that micro-inverters and power-optimisers separate panels so a single panel failure does not impact the whole system, though both may be more expensive⁵, and module-level heat detection extends that protection to spotting a problem at the panel itself.

These are real safety gains, and they are the reason some households choose module-level electronics even on an unshaded roof. They do not make a solar system hazard-free, and they sit alongside, not instead of, the standard protections covered on our solar PV safety page.

Monitoring and remote diagnostics, panel by panel

With the latest solar panel optimisers, you can monitor your solar setup's performance via an app¹, and this is where much of the day-to-day value sits. Instead of a single generation figure for the whole array, the household sees each panel individually, so a soiled panel, a failed optimiser or a new shading problem shows up as one line dropping rather than as a mystery shortfall in the monthly total.

The official specifications for SolarEdge's integrated optimiser modules describe the app features in more detail: a virtual layout of the system to aid fault detection and remediation, remote system monitoring and automatic fault notifications⁴. That virtual layout matters for maintenance economics: a fault can often be diagnosed and sometimes remedied remotely, and where a roof visit is needed, the installer knows which panel to go to rather than inspecting the array panel by panel.

Two cautions belong here. First, monitoring is only as good as its connection: some optimiser systems depend on an internet link for the app features, though not all do, and the Anglo Solar PV system with built-in Tigo optimisers is specified to function without any connection to the internet¹⁸. Second, panel-level monitoring is available from microinverters too, so it is a reason to choose module-level electronics over a plain string, not a reason to choose optimisers over microinverters. Our solar PV monitoring page covers the wider monitoring picture.

Making sure optimisers work with your panels and inverter

Optimisers are not universal parts. They are used with a string inverter², and they must be matched to the panels they serve. On retrofit, the answer to whether existing panels can take optimisers is yes, as long as you choose optimisers that are compatible with your panels¹, which makes the panel's electrical characteristics, not just its physical mounting, part of the specification.

For new installations the matching is handled at design stage, and the factory-fitted route removes it altogether: UKSOL panels with Tigo optimisers have the units fitted during the manufacturer's assembly process⁴, so panel and optimiser arrive as a tested combination. Aira's residential systems take a similar integrated approach, with panels connected to Solar String Optimisers that regulate voltage and current across the system¹³.

One expectation to hold firmly: optimisers reduce the losses of the overall system, but individual panel losses will still affect performance¹⁰. Compatibility and quality electronics recover what is recoverable; they do not repeal the underlying physics. A household choosing optimisers should expect a system that copes better with imperfection, not a system immune to it, and should treat any installer claim of a specific yield gain as something to be checked against the shade assessment recorded in the survey⁹.