C-003·Components / Solar generation and conversion
String inverter
Central inverter design, string limits and failure implications.
A string inverter converts DC from one or more series-connected PV strings into AC for the home and grid. One central inverter normally serves the whole array.
It is a common, well-understood architecture, but the array must be designed around its voltage, current and MPPT limits. A panel count chosen only from roof area is not an electrical design.
What a string does
Modules connected in series carry the same current and their voltages add. A string of modules therefore has:
- an operating voltage based on the sum of module operating voltages
- an open-circuit voltage based on the sum of module open-circuit voltages
- a current set by the string and its modules rather than added across the series connection
Strings connected in parallel keep broadly the same voltage while their currents add. That distinction determines whether an arrangement fits an inverter input.
The inverter’s MPPT adjusts operating voltage to find the best available power under the current irradiance and module temperature.
MPPT operation
An inverter can have one or more independent trackers. Separate trackers can serve roof planes with different orientation, pitch or shading, provided each connected string remains inside its tracker’s voltage and current limits.
Inputs labelled separately are not always independent trackers. Some are parallel inputs to one tracker and share its electrical limit. Read the circuit diagram and data sheet rather than counting sockets.
Strings sharing one tracker should be electrically suitable to operate together. Combining substantially different orientations or string lengths on one tracker can prevent either string operating at its own optimum.
Voltage design
Module voltage changes with cell temperature. Open-circuit voltage rises in cold conditions, so the design must prove that the complete string remains below the inverter’s absolute DC maximum at the site’s minimum design temperature.
Operating voltage must also remain inside the tracker’s working range in the relevant hot and low-light conditions. A string can be safe below the absolute maximum yet too short to start or track reliably when modules are hot.
The design record should show the module data, number in series, temperature coefficients and calculated cold and hot cases. It should use the exact module variant because similarly named panels can have different electrical values.
Current design
The inverter data sheet normally gives a maximum operating current and a maximum short-circuit current for each input or tracker. These are different limits.
Parallel strings add current. Newer high-current modules can exceed the input capability of an older inverter even where the string voltage is correct. A connector that physically fits does not prove electrical compatibility.
Current, voltage and array-power limits all have to pass. Compliance with one does not compensate for exceeding another.
Array power and clipping
The array’s peak DC rating can be higher than the inverter’s rated AC output where the manufacturer permits it. This can improve utilisation in weak light because modules rarely deliver nameplate power for long.
When available DC power exceeds the inverter’s AC ceiling, the inverter clips output. Some clipping can be an intentional design result. Excessive clipping can waste generation or indicate that an arbitrary sizing ratio was used.
There is no universal DC-to-AC ratio for a UK home. Orientation, pitch, shading, location, export limit, module temperature and inverter limits all affect the choice. The manufacturer maximum is a boundary, not an automatic design target.
Shading and mismatch
All modules in a series string carry the same current, so persistent shade or a weak module can reduce string performance. Bypass diodes can protect shaded cell groups and limit some loss, but they do not make the string immune to mismatch.
Separate trackers, careful string layout or compatible module-level power electronics can help where roof conditions justify them. They should follow the inverter and optimiser manufacturer’s approved design. Mixing unapproved electronics into a string can create voltage, shutdown and warranty problems.
The best architecture depends on the actual shade pattern. A simple unshaded roof does not automatically need module-level equipment, while a complex roof should not be forced onto one unsuitable string for convenience.
Isolation and connectors
PV strings remain live in daylight whenever modules are connected, even if the AC supply is off. DC switching and connectors must be correctly rated for the voltage, current, polarity and environment.
Mating connectors from different manufacturers because they look alike can produce poor contact and heating. Use compatible, documented connector pairs and the correct tooling.
The installation design should identify DC isolation, cable routing, polarity, overcurrent protection where required and firefighter considerations. A switch marked “solar” does not prove every DC conductor between roof and inverter is de-energised.
Failure implications
One central inverter is a shared point of conversion. If it fails, generation from every connected string normally stops. The benefit is that diagnosis and replacement occur at an accessible central unit rather than across many roof-mounted devices.
Monitoring may report total array power, individual tracker power and string current depending on the model and metering. A total-power graph alone can hide one underperforming string, so commissioning records should preserve tracker and string values where available.
Repeated insulation, arc, grid-voltage or input faults need diagnosis. Resetting the inverter does not identify damaged DC cable, water ingress, connector heating or a network-voltage problem.
Replacement and expansion
A replacement inverter must match the existing string voltage, current, array arrangement, earthing and grid-connection requirements. The same kW rating is not sufficient.
Adding modules can alter:
- cold open-circuit voltage
- hot operating voltage
- tracker current
- string matching
- registered capacity
- export limitation
- Feed-in Tariff records where the installation is accredited
The revised design and schematic must account for the changed equipment and applicable DNO process.
As at 22 July 2026, EREC G98 Issue 2 covers qualifying small-scale generation up to 16 A per phase. Connections outside its conditions use the applicable G99 process. The registered capacity and type-test evidence, not an export tariff or inverter marketing name, determine the route.
Key product differences
String inverter designs differ in:
- number of genuinely independent trackers
- voltage and current limits per input and tracker
- approved array-power range
- single or three-phase connection
- declared efficiency across load, not only the peak
- noise and installation environment
- monitoring granularity and local access
- grid type-test evidence
- warranty provider and support route
Related entries
Applies to
Solar
Last reviewed
22 Jul 2026