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Courses/Solar PV & Renewables/System Design & Sizing

lesson · 10min · Lesson 7 of 29

String sizing: voltage and current matching

Course syllabusCourse overview
01PV Physics & Panel Technology
  1. ReadPhotovoltaic effect: how a cell works
  2. ReadMonocrystalline vs polycrystalline vs thin-film
  3. ReadPanel specifications: Voc, Vmp, Isc, Imp
  4. ReadTemperature and irradiance effects
  5. quizPV physics quiz
02System Design & Sizing
  1. ReadSite survey: roof orientation, shading, pitch
  2. ReadString sizing: voltage and current matching
  3. ReadSeries vs parallel string configurations
  4. ReadYield estimation: kWh/kWp calculations
  5. exerciseSystem sizing design exercise
03Inverters
  1. ReadString inverters: topology and MPPT
  2. ReadMicroinverters and power optimisers
  3. ReadHybrid inverters for battery systems
  4. ReadInverter selection criteria
  5. quizInverter quiz
04Battery Storage
  1. ReadBattery chemistries: LFP vs NMC
  2. ReadBattery sizing for self-consumption
  3. ReadAC vs DC coupled systems
  4. ReadBattery safety and installation requirements
  5. exerciseBattery sizing exercise
05Grid Connection
  1. ReadG98: systems up to 3.68kW per phase
  2. ReadG99: larger systems, DNO approval
  3. ReadExport limitation and smart export tariffs
  4. ReadGeneration and export metering
  5. quizGrid connection quiz
06Installation & Commissioning
  1. ReadRoof mounting systems: rail and clamp
  2. ReadDC cable sizing and routing
  3. ReadCommissioning and functional testing
  4. quizFinal assessment
Lesson · 10min
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In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
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ElectraCore lesson handout

String sizing: voltage and current matching

Solar Pv · Lesson 7 · Intermediate

IntermediateReview: professional review pending

Purpose

Choose modules per string by proving cold maximum voltage, hot MPPT/start voltage and all current/input limits.

Before you beginElectrical fundamentals · DC voltage and current

Learning objectives

  • Interpret the governing data
  • Calculate on a declared basis
  • Identify design limits
  • State uncertainty

String length has an upper bound from cold Voc and a lower/operating bound from hot Vmp. Both must be checked against exact inverter data under compatible worst cases.

ARRAY STRING AND YIELD DESIGNARRAY STRING AND YIELD DESIGNMODULEMPPTcold Voc · hot Vmpirradiance · temperature · shading · exact product limits

Core theory

Maximum corrected string Voc, including adverse module tolerance and minimum cell temperature, must not exceed inverter absolute DC input voltage, module maximum system voltage or other component ratings.

Minimum corrected string Vmp at maximum credible cell temperature must remain within the relevant MPPT/start operating requirements. Nominal STC Vmp is not the hot boundary.

Check per-input Isc and operating-current limits, number of strings per MPPT, compatible orientations, connector/cable/protection ratings and inverter manufacturer string-design rules.

Terms, symbols, and units
TermMeaningSymbolUnit
MPPT windowInput-voltage range over which tracking is declaredNot applicableV
Absolute maximum DC voltageInput voltage that must never be exceededNot applicableV
StringModules connected in seriesNot applicableNot applicable
Worked exampleCorrected cold Voc is 54.9 V/module and inverter absolute maximum is 600 V. Find the voltage-only upper integer string length.

Assumptions: No additional design margin/tolerance remains to add.

  1. Divide: 600/54.9=10.93.
  2. Integer: Maximum cannot round up: N≤10.
  3. Continue: Check hot Vmp, current and every other component before accepting 10.

Voltage-only upper bound is 10 modules per string.

Reasonableness check: 11×54.9=603.9 V would exceed 600 V.

Common mistakes
  • Treating STC as guaranteed field output
  • Using nominal values instead of exact datasheets
  • Ignoring temperature, tolerance or mismatch

Where this appears in practice

Traceable PV design uses exact product data, site evidence and compatible worst-case assumptions.

SafetyPV arrays can sustain hazardous DC voltage and arcs whenever illuminated. Use competent PV-specific isolation, connectors, PPE and access controls.
Local code checkUse the exact module, inverter, mounting and protection manufacturer data; current IEC/BS EN/BS 7671 requirements; MCS standards where applicable; site-specific climate, structure, fire, wind, shading and DNO constraints. PV conductors remain live in daylight. Classroom calculations do not authorise roof access, DC connection, isolation or energisation.

Knowledge check

Why must 10.93 become 10, not 11?

Maximum-voltage limits cannot be rounded upward. The integer string must remain at or below the boundary.

Answer: Maximum-voltage limits cannot be rounded upward. The integer string must remain at or below the boundary.

Practical exercise

Find the feasible integer range from supplied cold Voc, hot Vmp and inverter limits.

Summary

  • Conditions define the value
  • Worst cases must be compatible
  • Record sources and assumptions

Sources and review

  • IEC 61215-2:2021: Terrestrial photovoltaic modules: test procedures: IEC; 2021; International
  • HEM-TP-18 PV generation and self-consumption: UK Government; Current published methodology; United Kingdom
  • PV research data and modelling tools: National Renewable Energy Laboratory; Current online resources; International research

Editorial review date: 2026-08-22. Professional electrical review is pending.

Educational material for learning and preliminary checks. Verify current local requirements and exact equipment instructions. This lesson does not replace competent professional work.

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