String (PV)
A group of PV modules wired in series so that their voltages add up while the current stays that of a single module.
Modules are connected in series to raise voltage to the range where inverters work efficiently and cable losses are low. In a string, voltages add and current is common, so a string of 20 modules with a maximum power voltage of 40 V operates near 800 V. Several strings can then be paralleled on one MPPT input or combiner box.
String length is bounded by two temperature-dependent limits:
β is the voltage temperature coefficient (about −0.25 to −0.30 %/°C). On the coldest morning, open-circuit voltage rises above its STC value and must stay below the system limit: 600 V (North American residential), 1,000 V (European residential and commercial) or 1,500 V (utility). On the hottest day, operating voltage must remain above the inverter minimum MPPT voltage.
Because current is shared, the weakest module limits the whole string: shading, soiling or a faulty cell on one module drags down the others unless bypass diodes activate. This mismatch sensitivity is why mixed orientations are split across separate MPPT inputs, or handled with power optimisers or microinverters.
Related terms
All terms →- DC/AC ratio
Ratio of a PV array DC nameplate capacity at STC to its inverter AC rating; also called inverter loading ratio (ILR).
- String inverter
An inverter that converts the DC output of one or more series strings of PV modules into grid-synchronised AC, typically with several independent MPPT inputs.
- Bypass diode
A diode wired in parallel with a group of cells in a PV module that lets string current flow around shaded or faulty cells, preventing hot spots.
- MPPT (Maximum Power Point Tracking)
Control algorithm in an inverter or charge controller that continuously adjusts operating voltage so a PV array delivers its maximum available power.
- Temperature coefficient
The percentage change in a PV module power, voltage or current per degree Celsius of cell temperature away from 25 °C.