In one line
Choosing how many modules to wire in series so that string voltage stays inside the inverter's operating window at both temperature extremes — the calculation that keeps a design legal and functional.
What it means
Two temperature-driven checks bound every string. The upper bound is the record-low site temperature applied to open-circuit voltage: cold raises Voc, and exceeding the inverter's maximum DC input or the system voltage rating (1,000 V or 1,500 V) is both a code violation and an equipment-destroying condition. The lower bound is the record-high cell temperature applied to maximum-power voltage: hot cells drop Vmp, and a string that falls below the inverter's MPPT window simply stops producing during the hottest, sunniest hours. Between those bounds, the designer also matches string current to the inverter input limit and keeps parallel strings identical in length so they share the maximum power point cleanly.
Worth knowing
- Cold sets the maximum string length; heat sets the minimum.
- Use record extremes, not seasonal averages.
- Parallel strings on one MPPT input should be equal in length and orientation.
- The 1,500 V system standard allows longer strings and lower balance-of-system cost than 1,000 V.
How it's calculated
N_max = V_max_inverter ÷ [Voc_STC × (1 + β × (T_min − 25))]- V_max_inverter — inverter maximum DC input voltage, or the 1,000/1,500 V system limit, whichever is lower
- Voc_STC — module open-circuit voltage at STC
- β — Voc temperature coefficient (negative, e.g. −0.0026 per °C)
- T_min — record low ambient temperature at the site
Field notes
- 1Pull the record low from a recognised meteorological dataset for the site, not from a nearby city's averages.
- 2Recheck string length whenever the module model changes mid-project — a substitution with different Voc invalidates the design.
- 3Leave headroom rather than designing to the exact limit; measurement and manufacturing tolerances are real.
Common mistakes
- Sizing on average winter temperature instead of the record minimum, then exceeding system voltage on the coldest morning of the decade.
- Mixing string lengths on a single MPPT input, guaranteeing permanent mismatch loss.
Frequently asked
How do you calculate maximum string length for solar?
Divide the inverter's maximum DC input voltage by the module's open-circuit voltage corrected to the site's record low temperature using the Voc temperature coefficient. Round down, and leave margin for tolerance.
What happens if a string is too long?
On a cold morning the string voltage can exceed the inverter's maximum input or the 1,000/1,500 V system rating — a code violation that risks destroying the inverter and voiding warranties.
Sources
Related terms
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