String design voltage windows

A string is a group of solar panels wired in series, and its voltage window is the DC voltage range the string must stay inside for the inverter it feeds to start, track its output and operate without over-voltage. String design sets how many panels go in series so the string voltage stays within the inverter’s limits across the full temperature range the site sees.

The four limits every string must satisfy

Panel datasheets give two voltages. The open-circuit voltage (Voc) is measured with no load connected. The maximum-power voltage (Vmp) is the voltage while the panel delivers power. Voc is always higher than Vmp. A series string’s voltage rises as the panels get colder and falls as they get hotter, so the design has to hold at both temperature extremes at once. Four inverter limits bound it:

At the coldest expected temperature, the string’s open-circuit voltage must not exceed the inverter’s absolute maximum DC input voltage.
At the coldest temperature, the string’s operating voltage must stay at or below the top of the inverter’s MPPT window, the voltage range across which the inverter can hold the panels at their most productive point.
At the hottest cell temperature, the string’s operating voltage must stay above the bottom of the MPPT window.
At the hottest temperature, the string’s open-circuit voltage must stay above the inverter’s start-up voltage, the level it needs to reach before it will begin to run.

The first limit carries the most at stake. Crossing the MPPT limits costs yield; crossing the absolute maximum can damage the inverter. A reference case shows the shape of it: an inverter with a 250 to 480 V MPPT window, a 600 V absolute maximum and a 300 V start-up voltage needs its cold open-circuit voltage held at or below 600 V, its cold operating voltage at or below 480 V, its hot operating voltage at or above 250 V and its hot open-circuit voltage at or above 300 V.

Why the coldest bright day sets the voltage ceiling

For silicon panels, voltage and temperature move in opposite directions. Below the 25 °C of standard test conditions (STC), both Voc and Vmp rise; above 25 °C, both fall. The worst case for the upper voltage limit is a bright, cold day, when full sunlight drives the current while the low cell temperature pushes open-circuit voltage to its peak.

STC (1,000 W/m² irradiance, 25 °C cell temperature, air mass 1.5) is a benchmark for comparing panels, not a worst case. Real output can exceed the STC rating under strong irradiance. Some datasheets also quote a nominal operating cell temperature (NOCT) figure. NOCT does not represent the cold extreme and must not be used for worst-case voltage sizing.

The UK method: a 1.2 multiplier on open-circuit voltage

UK design uses a single blanket multiplier rather than a site-by-site temperature calculation. The IET (Institution of Engineering and Technology) sets the maximum open-circuit voltage of a string as the number of modules, multiplied by the module’s open-circuit voltage at STC, multiplied by 1.2. The 1.2 accounts for the voltage rise at temperatures below STC. That figure sizes equipment as well as checking the inverter ceiling: it is used to select DC switch-disconnectors and other components rated to the string’s worst-case voltage. How many modules a string can hold is bounded by the module’s Voc, the available roof area, the inverter’s start-up voltage and the inverter’s maximum voltage.

The 1.2 multiplier is deliberately conservative. For a typical module losing about 0.3 % of its voltage per degree, a 1.2 factor implies a temperature roughly 65 °C below STC, far colder than any realistic UK low. It is a safety-margin convention, not the voltage rise at a specific UK temperature.

The temperature-coefficient method

A designer who does not use the blanket 1.2 corrects the string against the actual site temperature extremes, using the module’s Voc temperature coefficient, the negative percentage change in voltage per degree printed on the datasheet. The cold open-circuit voltage is the STC value scaled up by that coefficient across the gap between 25 °C and the site’s lowest temperature. Dividing the inverter’s maximum voltage by the cold open-circuit voltage, and rounding down, gives the longest permissible string. Dividing the start-up voltage by the hot operating voltage, and rounding up, gives the shortest.

For a 600 V inverter with typical crystalline panels, worked examples give roughly 6 to 13 modules per string, the exact count set by the module’s Voc and the chosen cold temperature. The 1.2 method and the coefficient method are not in tension: the coefficient method is exact for a given site, the 1.2 is a conservative shortcut that reads across every UK site without a temperature lookup.

Temperature figures for UK sites

The coefficient method needs two temperature inputs. For the upper limit it needs the site’s record low ambient temperature, ideally the low that coincides with enough sunlight to raise the voltage. For the lower limit it needs the average high ambient temperature plus a cell-temperature rise that depends on how the panels are mounted. UK designers are pointed at Met Office climate extremes records for the low figure.

The standards do not fix a single UK design-low temperature; the designer selects it. In practice a figure around −10 to −15 °C is commonly used.

Current limits and parallel strings

Voltage sets the string length; current sets how many strings can share an inverter input. A string carries the current of a single panel, set by the panel’s short-circuit current (Isc). Wiring strings in parallel adds their currents together, so more parallel strings means more current into the inverter input. Each MPPT input has a current rating, and exceeding it makes the inverter overheat and run inefficiently, so current has to be checked alongside voltage. Datasheets usually give two current figures per input, a maximum operating input current and a higher maximum short-circuit current.

A UK inverter’s input envelope: the GivEnergy Hybrid LV Gen 3

The GivEnergy Hybrid LV Gen 3, a formerly sold UK domestic hybrid inverter, shows the envelope in numbers. Its PV input allows a maximum voltage of 580 V, needs 150 V to start, tracks across a 120 to 550 V MPPT range and takes up to 15 A operating current and 20 A short-circuit current per MPPT. It has two MPPTs, one string each, rated to 7.5 kWp per string. These archived figures remain relevant to installed units, but GivEnergy Ltd ceased trading in April 2026.

A string on this inverter has to reach at least 150 V when warm to start, sit inside 120 to 550 V to track and never exceed 580 V open-circuit when cold. The start-up voltage sits 30 V above the MPPT floor, so a string sized only to the 120 V MPPT minimum can still fail to wake up on a cold morning.

What happens when a string sits outside the window

Over-voltage on a cold, bright day trips the inverter on an over-voltage fault, and in the worst case stresses or destroys it. One documented case involved an older inverter rated to 580 V fed by two strings of eight 250 W panels: on bright cold days the string open-circuit voltage rose past 580 V and the inverter shut down repeatedly. The fix is more, shorter strings so the cold open-circuit voltage stays under the inverter limit.

Under-voltage is the opposite fault. A string that is too short, or one whose voltage falls below the MPPT floor or the start-up level on a hot day, leaves the inverter under-producing or unable to start.

Optimisers and module-level electronics

Systems that put power electronics on each panel change this picture. SolarEdge power optimisers hold the string at a fixed working voltage regardless of panel count or temperature, which takes cold open-circuit voltage out of the string-length calculation. Confirm the exact behaviour against SolarEdge’s design guidance before relying on it.

The UK standards that govern string design

In the UK, MCS standard MIS 3002:2025 covers grid-parallel PV installations up to 50 kWp. It does not restate the string arithmetic. It defers the detailed electrical design and inspection to the IET Code of Practice for Grid-Connected Solar PV and to BS 7671 (the UK wiring regulations) and BS EN 62446-1. On commissioning, MCS records capture the measured open-circuit voltage and short-circuit current of each string at STC, so the string design is verified at install.