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Courses/Solar PV & Renewables/System Design & Sizing

lesson · 9min · Lesson 8 of 29

Series vs parallel string configurations

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 · 9min
SERIESVR₁R₂Rₜ = R₁ + R₂same current everywherePARALLELVR₁R₂1/Rₜ = 1/R₁ + 1/R₂same voltage across each
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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

Series vs parallel string configurations

Solar Pv · Lesson 8 · Intermediate

IntermediateReview: professional review pending

Purpose

Combine modules in series and strings in parallel while preserving voltage/current, matching and protection rules.

Before you beginElectrical fundamentals · DC voltage and current

Learning objectives

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

Series connection adds voltage while current is limited by the series path. Parallel connection holds voltage approximately common and adds string currents.

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

Core theory

A series string should use compatible modules with similar orientation and irradiance on the same MPPT unless the equipment explicitly supports another arrangement. Mismatch and shading can move the combined I-V curve.

Parallel strings increase operating and short-circuit current. Check inverter per-MPPT current, cable/connectors, combiner, reverse-current exposure and whether string overcurrent protection is required.

Never mate connectors merely because they fit or share a trade nickname. Use compatible certified connector pairs, correct tools and manufacturer assembly/inspection instructions.

Terms, symbols, and units
TermMeaningSymbolUnit
Series connectionConnection that adds module voltages at common path currentNot applicableNot applicable
Parallel connectionConnection that adds string currents at common voltageNot applicableNot applicable
Reverse currentCurrent driven into a faulted string by other sourcesNot applicableNot applicable
Series: VΣ=ΣV; parallel: IΣ=ΣI

Only for compatible conditions; use Voc/Isc or Vmp/Imp consistently.

V and A
Worked exampleTwo identical 9-module strings each operate at Vmp=360 V and Imp=11 A. Find combined operating values in parallel.

Assumptions: Matched strings on a suitable common MPPT.

  1. Voltage: Parallel voltage remains about 360 V.
  2. Current: Current adds: 11+11=22 A.
  3. Power: 360×22=7.92 kW.

Combined MPP is approximately 360 V, 22 A and 7.92 kW.

Reasonableness check: It equals twice one string's 3.96 kW.

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

What increases when identical strings are paralleled?

Current. Series modules add voltage; parallel strings add current.

Answer: Current. Series modules add voltage; parallel strings add current.

Practical exercise

Draw and calculate two supplied series/parallel configurations, including reverse-current checks.

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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