String Inverters for Home Solar

A string inverter is the single box that takes the direct current (DC) made by a row of solar panels wired in series and turns it into alternating current (AC) that the home and the grid can use. Electrical Safety First describes it as the most common and cheapest type of solar inverter, because the panels are simply connected in series and share one inverter¹. The inverter handles the combined DC output of one or more of these strings and converts it to grid-compatible AC².

The figures that matter follow from that design. SolaX, an inverter maker, states that a quality string inverter runs at 95 to 98% efficiency and can perform as well as costlier options on a simple, unshaded roof³. SolaX also gives a typical working life of around 10 to 15 years⁴. The Energy Saving Trust says most inverters carry at least five years' warranty, often extendable by up to 15 years⁵. It puts the cost of replacing an inverter at around £800⁶. That replacement cost is the main running cost a solar owner should expect over the life of the panels.

For a household's independence, a string inverter is what makes roof generation usable at all. It is also a single point of dependence. When it stops, the whole string stops. A plain string inverter serves solar only, relies on the grid connection to operate, and is backed by one manufacturer's warranty.

What a string inverter does: DC from a series of panels into grid-compatible AC

Solar panels generate direct current, while household appliances and the mains run on alternating current. HMRC's VAT guidance describes the inverter as the part that "transforms direct current (DC) electricity generated from the solar panels into alternating current (AC) for mains" use⁷. A string inverter does this centrally. Sungrow calls it "a centralized unit that handles the conversion for an entire group or string of solar panels"⁸. The name comes from the wiring: the panels are joined in series to form a string, and the inverter is built to take that string's output⁹. SolaX sets out the path simply: solar panels, then the DC string, then the central inverter, then the electrical panel¹⁰.

String design and the voltage window

Wiring panels in series adds their voltages together, so the number of panels in each string decides whether the inverter sees a voltage it can work with. Tigo describes string design as defining "the electrical operating envelope of modules, strings, conductors, inverters, MLPE, and protection devices". It adds that the result depends on temperature, irradiance, configuration and the ratings of every connected component. It advises checking against the current datasheet and installation manual¹¹.

Solis gives a practical example of what happens when a string is too short. If too few modules are in series, the string voltage stays low in weak early-morning light and does not reach the inverter's starting voltage, so the inverter starts late. Shading, vegetation, dirt or damage can have the same effect, lowering string voltage and delaying start-up¹². String sizing is therefore an installer's calculation matched to the chosen inverter, not a fixed rule. A larger array may use two or more strings, each feeding its own input. The wider picture of how inverters are chosen and rated is covered in solar inverters explained, and the cabling that carries the DC is covered in solar DC cabling and connectors.

The most common and cheapest type, suited to unshaded roofs

Independent and maker sources agree on where the string inverter sits in the market. Electrical Safety First calls it the most common and cheapest option¹. Bluetti describes it as "the most common and affordable type, suited to roofs with no shading issues"¹³. Good Energy says string inverter systems are "some of the cheapest options available"¹⁴. Sungrow adds that a string inverter usually has a lower initial price than the combined cost of microinverters for the same capacity⁸.

The low cost comes from the design. One unit does the conversion for many panels, and the wiring is simpler: SolaX notes that series wiring to a single inverter "reduces setup time and labor costs"¹⁵.

The condition attached to that low cost is the roof. The string approach works best where all the panels face the same way and receive the same light through the day. On a single south, east or west slope with no chimneys, dormers, trees or neighbouring buildings casting shadow, every panel in a string produces a similar current, and the drawback of series wiring barely shows. Roofs with more than one orientation, or with shade at some hours, change the picture; this is set out below in the section on limits. How orientation and shade affect output generally is covered in roof orientation, pitch and shading and how shade affects solar panel output.

For most householders the choice of inverter is made by the installer as part of the system design. The string inverter tends to be the default, with other types proposed where the roof calls for them.

Efficiency: 95 to 98% peak, but real-world output depends on conditions

All published efficiency figures for string inverters come from makers, and they measure peak conversion under test conditions. SolaX gives 95 to 98% for a quality string inverter³. Sungrow says string inverter efficiency is "often above 95%"¹⁶. SolaX elsewhere states modern string inverters exceed 97% and that its own models often achieve up to 99.03%¹⁵. A further SolaX guide says string inverters typically offer "the highest lab-rated efficiency (97 to 99%)", with the most efficient inverters on the market reaching 98% to 99% peak¹⁷. By comparison, SolaX puts hybrid inverters at around 95 to 97% peak³.

Canadian Solar publishes maximum efficiency figures across its string inverter ranges¹⁸:

The spread between 95% and 99% reflects different measures and different products. Some figures are a maker's typical range, some a best model's peak, and some the top of a commercial range. A home is unlikely to see the lab peak all day. Tigo notes that real performance depends on temperature, irradiance and configuration¹¹. SolaX links an inverter's working life to operating temperature and installation conditions⁴.

In practice the inverter is rarely the largest source of loss in a string system. Shade, dirt or a mismatched panel pulls the whole string down, as described below. An inverter a percentage point more efficient on paper will not recover output lost that way. Panel efficiency itself is covered in reading a solar panel datasheet.

Cost: the cheapest inverter type, with replacement around £800

There is no single published UK price for a string inverter on its own. Inverters are normally supplied as part of an installer's quotation. The figures that do exist come from two kinds of source, and none of them states its VAT position.

The independent figures cluster around replacement, which is the cost an owner is most likely to meet directly. The range reflects system size and brand: a larger inverter or a premium maker sits at the top. As a component of a new installation, the string inverter's cost is part of the overall system price. It is usually lower than the combined cost of microinverters for the same capacity⁸. Whole-system prices are covered in how much solar panels cost.

Replacement matters for the long-run sums because panels outlast inverters. Around £800 at roughly the 10 to 15 year mark is a known future cost that belongs in any savings and payback estimate, since the inverter is usually the first major part to need replacing.

Warranty: 5 to 12 years, depending on the maker

String inverter warranties are shorter than panel warranties, and the sources give a spread of terms. The Energy Saving Trust says most inverters have "warranties of five years as a minimum", which can often be extended by up to 15 years⁵.

Several maker guides give 5 to 12 years, while another gives 10 to 12 years, and no ruling settles which is typical. The difference reflects brands and extension options rather than an error. Canadian Solar's single-phase string inverter range, for example, carried a 5 year warranty extendable up to 20 years (datasheet dated March 2019)²⁹. A solar battery retailer advises at least a 10 year warranty on the inverter and UK-based technical support³⁰.

Expected life sits close to the warranty. Maker guides put string inverters at around 10 to 15 years⁴. Good Energy says they "typically only last 10 to 15" years³¹. Jackery notes microinverters may come with longer warranties depending on the brand³⁰. SolaX gives microinverters 20 to 25 years²⁴. Alternergy gives optimisers a 25-year warranty²⁸.

Any warranty is only as good as the company behind it and the conditions attached, such as registration and installation by a qualified installer. See solar panel warranties and lifespan.

Single-phase or three-phase: which fits which home

A string inverter is built to feed either one phase of the supply or all three. Makers offer both for homes. Canadian Solar's residential string inverters are single phase, while its commercial and utility ranges are three phase¹⁸. Deye states that single-phase string inverters require minimal maintenance⁹.

Three-phase designs are not confined to commercial roofs. The following residential products illustrate the range on the UK market:

The inverter must match the property's supply and the connection the network operator approves. Every UK grid-connected solar inverter must comply with G98 or G99²⁶. Larger systems, and three-phase ones in particular, are more likely to come under the fuller G99 process. That process is set out in connecting solar to the grid, with export caps covered in export limitation. Maker ranges for single and three phase are covered on brand pages such as Sungrow inverters, SMA solar inverters and Fronius Symo inverters.

Where a string inverter falls short: shading, single-point failure and no panel-level shutdown

The series wiring that makes a string inverter cheap is also its main weakness. Because panels in a string share the same current, shading on one panel can reduce the output of the whole string¹⁵. Good Energy puts it plainly: panels in a string system "are only as efficient as the lowest performing panel"¹⁴.

Electrical Safety First makes the same point: "if one panel fails, it could impact the whole system"¹. SolaX rates shade performance for a string-based hybrid as "moderate", since the entire string can be affected, and notes that adding DC optimisers can help reduce string shading losses³⁷. Low string voltage from shading, dirt or damage can also delay the inverter's start each morning¹².

Single point of failure

Sungrow says string inverters are reliable, but "a failure in the inverter will halt the entire string's operation". Where a system has more than one string inverter, the others keep running⁸. Midsummer Wholesale, a distributor that sells microinverters, reports that about 1 in 100 string inverters fail. It also claims an average 4 kW UK installation using microinverters is about twice as reliable as a string inverter installation³⁸. That is a seller's claim and is not independently confirmed.

No panel-level shutdown

A string system keeps DC at string voltage on the roof and in the cable down to the inverter. SolaX notes that string systems may need additional shutdown equipment¹⁰. Panel-level shutdown usually comes from optimisers or microinverters, not the inverter alone. DC isolation and fire safety are covered in solar PV safety.

Installation, servicing and monitoring

A string inverter is typically installed in a garage, a utility room or on an exterior wall¹⁰. Sungrow places it at a central location, typically near the main electricity panel or at a ground site⁸. Marley notes the aim is for the inverter to be out of the way but "easily accessible for maintenance or monitoring"³⁹. Some units are rated for either setting: the SolarEdge Home Short String Inverter is sold for indoor or outdoor mounting³². Sungrow describes string inverters as used on rooftops, carports, ground mounts and tracker systems⁴⁰. It adds that they have a lightweight, compact design and integrated mounting hardware¹⁶.

Siting, ventilation and ingress protection

Sungrow lists a high IP (Ingress Protection) rating among string inverter features, alongside surge protection and anti-islanding². The IP rating tells an installer whether a unit can go outdoors or needs shelter. Operating temperature bears on inverter life⁴. A unit mounted where air can move around it, and out of direct sun, is working within the conditions its maker assumes. The maker's installation manual sets the clearances¹¹.

Servicing at ground level

Ground-level siting is the practical advantage over panel-level electronics. SolaX notes that string inverters are easier to service at ground level, while repairing a microinverter may require roof access and panel removal¹⁰. Replacing a failed string inverter is a wall job, not a scaffold job.

Monitoring: string-level data, not panel-level

Sungrow says string inverters provide monitoring at the string level⁸, and advanced models include remote monitoring and diagnostics¹⁶. SolaX says string inverters typically report system, string or tracking-input (MPPT) data "unless power optimizers are added"¹⁰. The owner sees that a string is underperforming but not which panel is responsible. Most remote monitoring runs through the maker's app or cloud service, which is another dependence on that company. See monitoring a solar PV system and solar panel maintenance.

String inverter, optimisers or microinverters: how they compare

There are three common designs. A string inverter can be used on its own. It can be paired with power optimisers, which Marley describes as working "similarly to microinverters but are used with a string inverter", enhancing individual panel performance⁴¹. Or each panel can have its own microinverter.

Optimisers also let an installer oversize the array. Alternergy says optimisers enable oversizing to double the inverter power, with longer strings reducing cable and installation costs²⁸. SolaX states its residential string inverters allow up to 200% DC to AC oversizing⁴³. The SolarEdge Home Short String Inverter is described as optimised for complex roofs with several facets and orientations³². E.ON Next notes that microinverter systems can be easier to expand than systems using optimisers and a central string inverter⁴².

On a plain unshaded roof, the string inverter alone does the job at the lowest cost³. On a roof with shade or several orientations, optimisers or microinverters address the weakness at extra cost. The trade-offs are covered in more depth in microinverters vs string inverters, power optimisers, microinverters and east or west facing panels.

Safety, standards and certification: IEC 62109, CE marking and G98

The core product safety standard for PV inverters is IEC 62109.

The same source says that in Europe string inverters must carry CE marking, showing compliance with European safety, health and environmental protection standards⁴⁴. Connection to the network is a separate requirement. All UK grid-connected solar inverters must comply with the G98 or G99 engineering recommendations, which govern how equipment connects to the distribution network²⁶. Anti-islanding protection, which Sungrow lists with surge protection as a standard feature, is what makes the inverter shut down when the grid fails. This stops it energising a network that engineers believe is dead².

The fixed wiring is covered by a third layer. Electrical Safety First notes that registered UK electricians work to BS 7671, the national safety standard, and provide a safety certificate⁴⁵. For plug-in solar, government says "the UK safety specification is more stringent than that of Germany"⁴⁶. That statement concerns plug-in kits, not fixed string systems. It is covered in plug-in solar kits.

The full set of standards is covered in solar panel standards and testing, and installer certification in MCS certification for solar.

What a string inverter means for energy independence

A string inverter turns the roof into a source the home can use, but on its own it offers only partial independence. Three dependences remain.

• The grid. A grid-tied string inverter relies on the grid to operate. Its anti-islanding protection shuts it down when the grid fails², so without further equipment the panels do not power the home in a power cut. See solar panels in a power cut.
• Solar only, daytime only. Units such as Sunsynk's three-phase string inverter are sold for solar-only installations³⁴. Surplus that is not used as it is generated goes to the grid, paid at whatever export rate applies. See getting paid for solar export.
• The maker. Warranty, spare parts and app monitoring all depend on one company staying in business and supporting the product.

Battery readiness

A string system can gain storage in two ways. A battery can be added alongside with its own inverter, or the string inverter can be replaced with a hybrid. SolaX notes that hybrid string inverters can support DC-coupled batteries¹⁰. It says its residential string inverters integrate with EV chargers, heat pumps and storage⁴³. Typical UK battery installation costs are £4,000 to £8,000, depending on capacity, inverter brand and electrical work³⁰.

Storage changes where the power goes. Home Energy Scotland says a battery lowers the amount exported and so reduces export payments, but the overall savings are greater than relying on export⁴⁷.

When a string inverter reaches the end of its 10 to 15 year life⁴, replacement is a natural point to reconsider the setup. See adding battery storage, string inverter vs hybrid inverter, hybrid inverters and solar and household energy independence. The broader guide is at solar PV for UK homes.