Solar PV string design matches modules in series to the inverter's voltage window, and the number that catches installers out is cold-weather voltage: a module with a -0.28%/C open-circuit coefficient can gain over 12% in Voc at -20C compared with its 25C rating (GreenLancer, 2025). Get that wrong and a January morning pushes the string past the inverter's DC input limit.
String design is governed for UK work by IEC 60364-7-712, which sits behind MCS installation practice (IEC, 2017). This guide walks through string voltage calculation, the MPPT window, the DC to AC ratio, cable sizing and the single line diagram, with a worked example an installer can reuse on the next quote.
Key Takeaways
- String Voc at the site's minimum temperature must never exceed the inverter's maximum DC input voltage, because a cold morning is when voltage peaks (GreenLancer, 2025).
- Voc rises as temperature falls: correct it with Voc at STC multiplied by 1 plus the coefficient times the temperature difference from 25C (Greentech Renewables, 2024).
- Power oversizing, a DC to AC ratio above 1, is a financial tool; voltage oversizing that breaches the inverter limit is a fault to avoid (Surge PV, 2025).
- IEC 60364-7-712 applies a safety factor to short-circuit current, and string current must stay within the inverter's per-MPPT input rating (IEC, 2017).
- Keep the string design, the single line diagram and the DNO paperwork consistent, because the grid connection is assessed against them.
What is solar PV string design?
A string is a set of modules wired in series, and string design is the process of choosing how many modules go in each string so the array stays inside the inverter's voltage and current limits across the whole year. The output feeds the inverter's maximum power point tracker, or MPPT (Greentech Renewables, 2024).
Three limits bound the answer at once. The maximum string voltage on the coldest day must stay under the inverter ceiling. The minimum string voltage on the hottest day must stay above the MPPT floor so tracking still works. And the string current must sit inside the MPPT input rating. A good design satisfies all three with margin, not just one at the expense of another.
Margin matters because datasheets describe ideal conditions and roofs do not. Module tolerances, ageing and a cold snap sharper than the design minimum all nudge the real numbers, so a string sitting one module below the calculated ceiling is safer than one squeezed to the exact limit. The reward for that discipline is an inverter that never trips on voltage and a system that tracks cleanly from the first frost to the hottest afternoon of the year.
How do you calculate string voltage?
Correct the module's open-circuit voltage for temperature. String Voc equals the module Voc at STC, multiplied by 1 plus the temperature coefficient times the difference between the design minimum temperature and 25C, then multiplied by the number of modules (Greentech Renewables, 2024). Voltage rises as temperature falls, so the cold case sets the maximum.
Take a module with a 41.5V Voc and a -0.28%/C coefficient, on a UK site with a -15C design minimum. The corrected Voc per module is 41.5 multiplied by 1 plus 0.0028 times 40, which is about 46.2V. Against a 1000V inverter ceiling, 1000 divided by 46.2 gives 21.6, so the string is capped at 21 modules. The table sets out that calculation:
- Module Voc at STC: 41.5 V, the datasheet figure at 25C.
- Voc temperature coefficient: -0.28%/C, from the module datasheet.
- Design minimum temperature: -15C, a conservative UK figure.
- Corrected Voc per module: about 46.2 V at -15C.
- Inverter maximum DC input: 1000 V, from the inverter datasheet.
- Maximum modules per string: 21, being 1000 divided by 46.2, rounded down.
Sizing strings to the inverter MPPT window
The upper limit is the cold Voc case above. The lower limit is the hot case: the module's maximum-power voltage falls in summer heat, and the string must still sit above the MPPT lower bound or the inverter stops tracking efficiently (Surge PV, 2025). Both ends need checking on every design.
In UK conditions the cold case is the tighter constraint, because winter mornings are cold and clear while roof temperatures rarely reach the extremes seen in hotter climates. A design that fits the coldest expected morning and the hottest expected roof temperature will track cleanly year round. The roof layout also feeds in, since splitting a mixed-orientation roof across two MPPT inputs avoids forcing one string to compromise. A south string and an east string on separate trackers each hold their own voltage window, rather than one dragging the other away from its best operating point.
What DC to AC ratio should you design to?
The DC to AC ratio, array kWp divided by inverter kW, is usually set above 1 because modules rarely hit their rated output in UK light. Ratios around 1.1 to 1.3 are common, capturing more morning and evening energy while accepting a little clipping at midday peaks (Surge PV, 2025).
This power oversizing is a financial decision and is safe. What is not safe is voltage oversizing, adding modules to a string until the cold Voc breaches the inverter ceiling. The two are separate: you can raise the DC to AC ratio by adding strings in parallel without touching the per-string voltage. Keep the two ideas apart on every design and the inverter stays protected. Confusing them is how a system that looked generous on paper ends up tripping offline on the coldest, brightest mornings of the year, exactly when it should be earning.
DC cable sizing for the string
String cable is sized for both current and voltage drop, and on typical PV runs voltage drop is the deciding factor rather than ampacity (OmniSol, 2025). MCS practice keeps the DC voltage drop from array to inverter under 3%, and many designers target closer to 1% because every 1% of drop costs roughly 1% of yield across the system life.
For a full treatment of conductor choice, the comparison between 4mm and 6mm PV cable covers the ampacity and voltage-drop numbers behind these limits. Size the cable early, because a long roof-to-inverter run can force a larger conductor than the string current alone would suggest, and discovering that after the array is up means re-pulling cable. Remember the run counts twice, out to the array and back, so a 25m roof route is 50m of conductor in the calculation.
String design and the single line diagram
Every string design ends in a single line diagram that shows modules, strings, protection, isolation and the inverter as one electrical schematic. The DNO reviews this alongside the connection application, so the string counts and ratings on the diagram must match the design exactly (single line diagram guide, 2026).
For anything above the notify-and-connect threshold the diagram supports a G99 application, and inconsistencies between the string design and the schematic are a common cause of DNO queries. Treat the single line diagram as the string design's certificate, not an afterthought drawn once the panels are up.
Which tools help with string calculations?
Most installers run the numbers in the inverter manufacturer's own string sizing tool, which already holds the DC limits and MPPT window, then sanity-check against a general solar string calculator or spreadsheet. The value of a calculator is speed on repeat quotes, not replacing the temperature-corrected check.
Whichever solar PV string design calculator you use, feed it the real datasheet coefficient and a conservative UK minimum temperature, because a tool defaulting to a mild climate will happily suggest a string that fails on the first cold morning. The calculation is only as honest as the temperature you give it.
What are the most common string design mistakes?
The classic error is sizing the string on the datasheet Voc at 25C and forgetting the cold correction, which leaves the string fine in the showroom and over voltage on the first frost (GreenLancer, 2025). A close second is mixing module types or orientations on one string so their currents never match.
Other recurring faults include ignoring the MPPT lower bound so a hot roof drops the string out of tracking, leaving unequal string lengths across one MPPT input, and drawing a single line diagram that no longer matches the strings actually installed. Each one passes a quick glance and fails on a specific weather day or at the DNO's desk. Checking both temperature extremes on every design catches most of them before they reach the roof.
Frequently Asked Questions
What temperature should UK string voltage be calculated at?
Use a conservative site minimum, commonly around -10C to -15C for UK locations, and correct Voc to that figure (Greentech Renewables, 2024). The colder the design temperature, the higher the calculated string voltage, so choosing a realistic low protects the inverter against the coldest clear morning the site will see.
Can you oversize the DC array beyond the inverter rating?
Yes, in power terms. A DC to AC ratio above 1 is standard and captures more energy in low UK light, with minor midday clipping (Surge PV, 2025). What you cannot do is let the cold-weather string voltage exceed the inverter's DC input ceiling, which is a separate limit from power oversizing.
What is a safe DC to AC ratio for UK solar?
Ratios of roughly 1.1 to 1.3 suit UK irradiance, trading a little clipping at peak for better yield across the shoulders of the day (Surge PV, 2025). The exact figure depends on orientation and the inverter's clipping behaviour, so check the specific inverter rather than applying one ratio everywhere. East-west roofs tolerate a higher ratio than a south roof, because their output is spread rather than peaked, so the midday clipping the ratio implies rarely actually happens.
Why does cold weather threaten the inverter, not hot weather?
Open-circuit voltage rises as temperature falls, so the coldest morning produces the highest string voltage, which is what can breach the inverter's DC input limit (GreenLancer, 2025). Hot weather lowers voltage and instead risks dropping the string below the MPPT floor, an efficiency issue rather than a fault.
Do I need a single line diagram for a string design?
For any grid-connected system the DNO expects a single line diagram, and it must match the string design (single line diagram guide, 2026). It documents string counts, protection and isolation, and forms part of the connection application, so it is a required deliverable rather than optional. If the diagram and the installed strings disagree, the connection can stall while the DNO queries which version is correct.
String design lives or dies on consistency between the calculation, the schematic and the DNO submission. Keeping those aligned is exactly where installer platforms like Reonic save time, by carrying the same system data from design through to the connection paperwork.
