Solar Inverters Explained: String, Hybrid, Optimisers or Micro?

Solar panels produce direct current (DC) electricity. Your home's appliances and the electricity grid use alternating current (AC). The inverter converts one into the other, and in doing so it also:

• Maximises output: by constantly adjusting the electrical operating point of the panels to extract the most power, a process known as maximum power point tracking
• Manages safety: disconnecting the system if the grid fails, so it cannot send power into the network during a power cut
• Records performance: logging generation and, in most modern inverters, sending data to an app or online portal
• Controls storage: in hybrid inverters, managing when a battery charges and discharges

Because it does so much, the inverter has a big influence on how well the whole system performs. A well-matched inverter keeps losses low and gives you clear information about generation. A poorly chosen one can limit output, make battery additions expensive and leave you guessing about whether the system is working properly.

Inverters also have a shorter life than panels, so the choice affects your costs later in the system's life as well as at installation.

A string inverter is the most common and usually the least expensive option. The panels are wired together in one or more series, called strings, which feed a single inverter, typically mounted in a loft, garage or utility room.

Advantages:

• Lower cost than most alternatives
• Simple, proven technology with a wide choice of brands
• One unit to monitor and replace

Limitations:

• Shading: panels in a string are linked, so shading on one panel can reduce the output of the whole string unless the design accounts for it.
• Different roof faces: panels facing different directions usually need separate inputs, known as MPPT inputs, on the inverter.
• Battery additions: a standard string inverter cannot connect a battery directly, so adding storage later usually means an AC-coupled battery with its own inverter.

For a simple, unshaded roof with all panels facing one direction and no plans for a battery, a good string inverter is often the best value. Widely used brands in UK homes include Solis, SolarEdge, SMA, Fronius and Growatt.

A hybrid inverter does everything a string inverter does, and can also connect a battery directly. It manages the flow of energy between panels, battery, home and grid.

Advantages:

• A single unit for solar and battery, saving space and cost if you install both
• Adding a battery later is usually straightforward, using compatible models
• Many support time-of-use tariffs, charging the battery cheaply overnight
• Some offer backup power during cuts, when configured for it

Limitations:

• Higher cost than a basic string inverter
• Battery compatibility is often limited to specific brands or models
• The same shading considerations as a string inverter apply

For many UK homes, a hybrid inverter is the best choice, because it keeps the option of adding a battery open without paying for one now. Brands commonly seen in UK quotes include GivEnergy, Fox ESS, SolaX, Solis, Sigenergy and SolarEdge.

If a battery is a realistic possibility within the next few years, the extra cost of a hybrid at installation is usually far less than the cost of adding a separate battery inverter later.

Power optimisers are small devices fitted behind each panel, or pairs of panels, that work with a compatible string inverter. Each optimiser adjusts its panel's output independently, so a shaded or underperforming panel does not drag down the rest of the string.

Advantages:

• Reduces losses from partial shading by chimneys, trees, dormers or neighbouring buildings
• Allows panels on different roof faces or angles to be combined more flexibly
• Panel-level monitoring, showing the output of each panel
• Some systems add safety features that reduce DC voltage on the roof when the system is shut down

Limitations:

• Additional cost for each panel
• More electronic components on the roof that could fail
• Usually unnecessary on a simple, unshaded roof

SolarEdge is the best-known optimiser system in the UK, and some other manufacturers offer optimisers that work with a wider range of inverters.

Optimisers earn their cost where shading or a complex roof would otherwise reduce output significantly. On a clear south-facing roof, they add cost without much extra generation.

Microinverters are small inverters fitted behind each panel, converting DC to AC at the panel itself. There is no central string inverter.

Advantages:

• Independent panels: each panel works on its own, so shading on one has little effect on the others
• Flexible layouts: panels can face different directions and be added in small numbers later
• Longer warranties: microinverters often carry warranties of up to 25 years, longer than most string inverters
• Lower DC voltages: the roof wiring carries AC rather than high-voltage DC

Limitations:

• Typically more expensive than string or hybrid systems
• Battery storage usually requires a separate AC-coupled battery
• If a microinverter fails, replacing it means work on the roof

Enphase is the best-known microinverter brand in the UK, with others such as APsystems also available.

Microinverters suit complex roofs with several faces, significant shading, or homes that want to start small and add panels over time. For a simple roof, a string or hybrid system usually offers better value.

Matching the inverter to your roof is mainly about shading and layout:

• Simple roof, one direction, little shading: a string or hybrid inverter is usually the best value.
• Two roof faces, such as east and west, little shading: a string or hybrid inverter with separate inputs for each face usually works well.
• Partial shading from a chimney, tree or dormer: optimisers on the affected panels, or a microinverter system, can recover lost output.
• Several roof faces, complex shapes or heavy shading: microinverters or a fully optimised system often perform best.
• Plans for a battery: a hybrid inverter, unless shading makes microinverters the better choice, in which case an AC-coupled battery can be added.

A good installer will explain why they have chosen a particular inverter for your roof, based on a shading assessment rather than habit or stock.

If two quotes propose different inverter types for the same roof, ask each installer to explain the expected generation with their design. The difference in output, compared with the difference in price, shows which is better value.

Inverters are rated by their maximum AC output, and that rating interacts with the grid connection rules. On a typical single-phase home supply, systems with an inverter output of 3.68kW or less connect under a simple notification process after installation. Larger inverters need prior approval from the network operator.

That is why many home systems pair 4 to 6kW of panels with a 3.68kW inverter, or with a larger inverter whose export is limited to 3.68kW. Panels rarely produce their full rated output in UK conditions, so the energy lost on the sunniest days, known as clipping, is usually small.

Points to check in a quote:

• The inverter's rated output and the total panel capacity
• Whether any export limit will be applied
• Whether a battery would increase the total capacity assessed by the network operator
• The expected clipping loss, if the panels significantly exceed the inverter rating

Our guide to how many solar panels you need explains why the 3.68kW figure does not cap the number of panels on your roof.

Panels typically last 25 years or more, but string and hybrid inverters usually last around 10 to 15 years. Most homeowners should therefore expect to replace the inverter once during the system's life.

When comparing quotes, check:

• Warranty length: 10 to 12 years is common for string and hybrid inverters, with some brands offering extensions. Microinverters often carry longer warranties, commonly up to 25 years.
• Warranty terms: whether the warranty covers the replacement unit only, or labour too, and who handles claims.
• Extensions: some manufacturers offer extended warranties if bought at installation.
• Replacement cost: a replacement string or hybrid inverter, including labour, typically costs several hundred pounds to over £1,000, depending on the model.

Budget for one inverter replacement when you work out your payback, because it is a predictable cost rather than an unlikely one.

Our guide to home solar panel costs explains how inverter choice affects the overall price.

Almost every modern inverter includes monitoring through a smartphone app or online portal, showing how much your system generates and, with suitable metering, how much you use and export.

Monitoring matters for two reasons:

• Spotting faults: a sudden drop in output, or zero generation on a bright day, usually points to an inverter fault, a tripped isolator or a communication problem. Without monitoring, a fault can go unnoticed for months.
• Using more of your own power: seeing when your panels generate most helps you schedule washing machines, dishwashers and car charging to make the most of your solar output.

Optimiser and microinverter systems add panel-level monitoring, showing each panel's output, which makes faults and shading easier to diagnose.

When comparing quotes, ask what the monitoring shows, whether it measures consumption and export as well as generation, whether there is any subscription cost and whether it works with smart tariffs or home energy systems. Most mainstream brands include free monitoring for the life of the inverter.

When comparing quotes, ask each installer:

1. What make and model of inverter is proposed, and why that type suits your roof
2. Whether it is battery-ready or a hybrid, and what adding a battery later would involve
3. How shading on your roof has been assessed, and whether optimisers or microinverters are needed
4. What the inverter's rating is, and whether any export limit will be applied
5. What the warranty term is, whether it can be extended and who handles claims
6. What a replacement would typically cost later in the system's life
7. What monitoring app or portal is included, and what it shows

Compare quotes on the whole system, not just the inverter, but do not overlook it. The inverter decides how easily the system can grow with your needs.

Our guides to how many panels you need and solar and battery costs cover sizing and storage. When you are ready, compare quotes from up to three MCS-certified installers.