Solar inverters explained: string, hybrid and micro

Panels get the attention. The inverter decides more.

It converts direct current from the roof into alternating current your house can use, it is the component most likely to need replacing during the system’s life, it sets the grid paperwork your installation needs, and its type constrains what you can add later. Choosing it is one of the two or three decisions in a solar installation that actually matter.

What it does

Panels produce direct current. Your house runs on alternating current. The inverter converts between them, losing a small percentage as heat in the process.

It also does something less obvious and more important: it continuously adjusts the electrical operating point of the array to extract as much power as conditions allow. How well it does that, and at what granularity, is the real difference between inverter types.

One UK specific fact worth knowing before the types: the inverter’s rating, not the panels’, is what the grid connection rules measure. Up to 16 amps per phase, which is 3.68kW single phase, the installer notifies the network operator within 28 days under G98; above that a G99 application needs approval before installation. This is why so many UK systems carry a 3.6kW inverter under a larger array, and why “what inverter size, and which connection route” is a question your quote should already have answered.

String inverters

A series of panels, a string, is wired to a single inverter.

Advantages: simplest, cheapest, fewest components, well proven. One unit to replace when it eventually fails, in an accessible location.

The limitation: the string is optimised as a whole. If one panel is shaded, soiled or underperforming, it constrains the string it belongs to. On an unshaded roof with all panels on one plane, this costs you nothing. On a shaded roof it can cost a lot.

For a straightforward roof, this is the right answer and paying more is paying for a problem you do not have.

Optimisers

Small units behind each panel that handle the per panel electrical optimisation, while a single string inverter still does the conversion.

What they solve: a shaded or underperforming panel no longer drags its string. Panels on different orientations can sit on one string.

The cost: more hardware, more money, and more components on the roof where replacing one means access, potentially scaffolding.

Worth it when you have genuine shading or a split array. Not worth it on a clean south facing pitch, which is why an optimiser line on a quote for an unshaded roof is a question to ask, not a premium to accept.

Microinverters

A complete small inverter behind each panel, converting at the panel.

Advantages: full per panel independence, no high voltage direct current cabling on the roof, and easy expansion later since adding a panel means adding its inverter. Some manufacturers warrant microinverters for as long as the panels themselves, which removes the mid-life replacement from the projection for that component.

Trade offs: highest cost per watt, and many units on the roof. Individual failures are less consequential than a central inverter failing, but access for replacement is harder and involves the scaffolding the central unit never needs.

Suited to complicated roofs: multiple orientations, awkward shapes, unavoidable shading.

Hybrid inverters

A hybrid handles both the array and a battery.

This is the one to think about even if you are not buying storage now. If a battery is likely within a few years, specifying a hybrid at installation avoids either replacing the inverter or adding a second one when the time comes, which is a large part of why retrofitting storage costs more than fitting it at the same time.

The trade off is straightforward: pay more now for an option, or pay less now and more later if you exercise it. Whether it is worth it depends entirely on how likely the battery actually is. Price both paths: the hybrid premium today against a second visit and a second inverter later, weighted by your honest probability of buying the battery at all.

The question that decides it

Does my roof have shading, or panels on more than one orientation?

No: a string inverter is right. Consider a hybrid if storage is likely.

Yes: per panel electronics probably earn their cost. Get the shading properly assessed rather than eyeballed, the MCS estimation method requires a shading assessment anyway, and the assessment is what justifies the extra spend.

Sizing and clipping

Inverters are rated in kilowatts, and it is normal for that rating to sit somewhat below the array’s peak rating. UK arrays rarely reach their rated output, our model assumes 850 kilowatt hours per kilowatt per year precisely because real conditions sit far below laboratory ones, so a slightly smaller inverter runs closer to its efficient operating range most of the time.

Taken too far, it clips output on the brightest days: a 5kW array behind a 3.68kW inverter will flat-top on clear summer middays and the energy above the line is simply not generated. Ask what array to inverter ratio has been specified and what annual loss is expected from clipping. A competent installer will have a figure, and “none” from a heavily overpanelled quote is not a figure.

Lifespan and warranty

Inverters do not last as long as panels, and warranty terms say so: central inverter warranties are typically far shorter than the 25 year performance warranties on the modules above them, though some manufacturers now cover residential inverters for longer, GivEnergy’s twelve year residential warranty being one published example, with conditions attached.

The practical point for your projection: a replacement belongs in it, priced and placed mid-life. Our calculator includes one rather than assuming the cheapest component of the system outlives the dearest, and a twenty year savings table with no inverter replacement is understating costs at exactly the point the numbers get large.