String design
Cold open-circuit string voltage and maximum modules per string
- where
- module open-circuit voltage at minimum temperature (V)
- module open-circuit voltage at STC (V)
- temperature coefficient of Voc (%/°C)
- minimum cell temperature (°C)
- string open-circuit voltage at minimum temperature (V)
- modules in series
- maximum modules per string
- inverter maximum DC input voltage (V)
- string Voc by the 1.2 multiplier (V)
Explain this equation
Works out the highest voltage a PV string can reach on a cold, bright morning and how many modules can safely go in series. Use it whenever you design a string for an inverter.
Silicon panels make more voltage when cold. β_V is negative, so a temperature below 25 °C makes the bracket bigger than one and raises Voc. Multiplying by the modules in series gives the string voltage. Dividing the inverter maximum by the cold module voltage and rounding down gives the most modules allowed. The 1.2 × Voc figure is the simple rule of thumb some guidance uses when no coefficient is known.
The module voltage changes linearly with temperature over the working range, at the datasheet rate.
Sense check: a 40 V module at −0.3 %/°C reaches about 44.2 V at −10 °C, so ten in series gives about 442 V; a 600 V inverter allows 13.
Watch out: exceeding the inverter's maximum DC voltage can destroy it. Use the lowest likely temperature for the site, not an average winter one.
Open-circuit voltage rises as modules get colder. This finds the string voltage on a cold, bright morning and the most modules you can put in series without exceeding the inverter limit.
Enter your values to see the result.
Good to know
Use the Voc coefficient from the module datasheet (typically −0.25 to −0.35 %/°C for crystalline silicon). At dawn in winter the cells sit at about air temperature, so use the lowest expected ambient for the site; −10 °C is a common UK design figure, colder for exposed or upland sites. The IET method of multiplying Voc STC by 1.2 is shown for comparison; both must stay below the inverter maximum, the module maximum system voltage and the voltage rating of the d.c. switch-disconnector and cable. The same equation, with the measured cell temperature, gives the expected string Voc for BS EN 62446-1 commissioning tests.
Common mistake
Checking the string against the inverter's MPPT range or nominal voltage instead of its absolute maximum input voltage, or using the Pmax coefficient instead of the Voc coefficient.
Limits
Crystalline silicon modules without optimisers. Thin-film modules can show higher initial Voc; follow the manufacturer. Design must meet BS 7671 Section 712 and the IET Code of Practice.
Sources
- IET Wiring Matters, April 2026: Solar PV d.c. switch-disconnector selection (UOC MAX = modules × UOC STC × 1.2; ISC MAX = strings × ISC STC × 1.25; alternative method using site minimum temperature and module coefficient)
- BS 7671:2018+A4:2026 Requirements for Electrical Installations, Section 712 Solar photovoltaic (PV) power supply systems
- IET Code of Practice for Grid-connected Solar Photovoltaic Systems, 2nd edition (2022)
- BS EN IEC 62446-1:2016+A1:2018 Photovoltaic (PV) systems – Requirements for testing, documentation and maintenance – Part 1: Grid connected systems
- Module manufacturer datasheet: STC values (Voc, Vmp, Isc, Imp, Pmax), temperature coefficients of Voc, Pmax and Isc, NOCT/NMOT and dimensions
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.