SOLAR PV GUIDE

Solar PV System Design UK: Complete Design Guide

Everything you need to design a solar PV system in the UK — from kWp sizing and string design to inverter selection, DC cable sizing, G99/G98 DNO notification, and MCS 001 compliance.

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13 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

  • 1System size (kWp) is calculated from available roof area, panel wattage, and annual consumption — a typical UK home uses 3,500 kWh/year and a 3–4 kWp system will offset the majority of daytime demand.
  • 2String design must account for UK irradiance data, minimum and maximum inverter input voltages (Vmp × panels must fall within the MPPT window), and shading analysis.
  • 3Three inverter topologies are available: string inverters (lowest cost), microinverters (per-panel optimisation), and hybrid inverters (battery-ready). Each has different design implications.
  • 4DC cable sizing under BS 7671 Section 712 must account for temperature correction, voltage drop (max 3% recommended), and short-circuit current from multiple strings.
  • 5G99 notification is required for systems ≥ 16 A per phase; G98 self-certification covers systems below this threshold. Both require DNO approval before export.
  • 6MCS 001 governs all MCS-certified installations and mandates a detailed design assessment, shading analysis, and energy yield estimate using PVGIS or equivalent software.
01 · Solar PV Guide

System Sizing: kWp Calculation for UK Homes

The first step in any solar PV design is determining the appropriate system size, expressed in kilowatt-peak (kWp). This is the rated output of the array under Standard Test Conditions (STC: 1,000 W/m² irradiance, 25°C cell temperature, AM1.5 spectrum).

  • Step 1 — Establish annual consumption: Check the customer's electricity bills. A typical UK three-bedroom home uses 3,500 kWh/year; a four-bedroom home with an EV charger may use 6,000–8,000 kWh/year. Higher consumption justifies a larger system.
  • Step 2 — Apply UK yield factor: In the UK, 1 kWp of south-facing panels at 35° tilt generates approximately 850–950 kWh/year in southern England and 750–850 kWh/year in Scotland. East/west splits yield 15–20% less than south-facing. Use PVGIS for a location-specific figure.
  • Step 3 — Check roof area: A 400 Wp panel is approximately 1.7 m². A 4 kWp system requires 10 panels and approximately 17 m² of usable roof area. Exclude areas affected by shading, vents, skylights, and the required 300 mm perimeter clearance under most MCS guidelines.
  • Step 4 — Consider self-consumption: Larger systems export more and self-consume proportionally less. For a home without battery storage, a system sized to meet 80–100% of annual consumption is typically optimal. Adding battery storage improves self-consumption and justifies a larger array.

Current panel technology offers 400–430 Wp per panel for standard 60/66-cell modules, with premium panels reaching 450–500 Wp. Higher wattage panels reduce the number required and are increasingly cost-effective for UK installations.

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02 · Solar PV Guide

String Design for UK PV Arrays

String design determines how panels are electrically connected to the inverter. Each string is a series connection of panels — voltage adds, current stays constant. Getting string design right is critical to system performance and inverter longevity.

  • Maximum string voltage: Multiply the panel Voc by the number of panels, then apply a temperature correction for UK winter conditions. Use a temperature coefficient of approximately −0.29%/°C and assume a minimum ambient temperature of −10°C (giving a correction factor of approximately ×1.15). The resulting voltage must not exceed the inverter's maximum DC input voltage (typically 600 V for residential, 1,000 V for commercial).
  • MPPT window: Multiply the panel Vmp by the number of panels and check that the resulting voltage falls within the inverter's MPPT (Maximum Power Point Tracking) voltage range at the expected operating temperature range. Both cold-weather (low irradiance, high voltage) and hot-weather (high irradiance, lower voltage) conditions must be checked.
  • Shading and orientation: Panels on the same string must be on the same roof plane with the same orientation and tilt. Mixing south-facing and east-facing panels on one string dramatically reduces yield. Where shading is unavoidable, use DC power optimisers (e.g., SolarEdge, Tigo) or microinverters to mitigate mismatch losses.
  • String fusing: BS 7671 Section 712 requires consideration of reverse current protection. Where more than two strings are connected in parallel, string fuses or combiner boxes must be used to protect against reverse current damage. Single-string systems generally do not require fusing.
03 · Solar PV Guide

Inverter Sizing: String vs Micro vs Hybrid

The inverter converts DC power from the array into AC power for the property and grid. Three main topologies are used in UK residential and small commercial installations, each with distinct design implications.

  • String inverters — a single inverter handles one or more strings. The most cost-effective solution for unshaded, south-facing roofs. Inverter rating is typically sized at 80–100% of the array kWp (slight undersizing, known as DC clipping, is acceptable and often improves energy harvest in the UK's low-irradiance climate). Leading brands include SolarEdge, SMA, Fronius, and Growatt.
  • Microinverters — one small inverter per panel, eliminating string mismatch losses and providing per-panel monitoring. Ideal for complex roofs, partial shading, and east/west splits. Higher upfront cost but improved yield in challenging installations. Enphase is the dominant brand in the UK market.
  • Hybrid inverters — combines a solar inverter with a battery charge controller. Required for DC-coupled battery storage (e.g., Givenergy, Solis, Growatt SPF). If battery storage is planned now or in the future, specifying a hybrid inverter avoids the cost of replacing the inverter when storage is added. Most hybrid inverters also include backup power functionality.

For UK residential installations, a 3.6 kW single-phase inverter is the most common choice for systems up to 4 kWp. Systems above 3.68 kW (16 A) require G99 notification to the DNO. Three-phase inverters are used where the property has a three-phase supply, balancing load across phases.

04 · Solar PV Guide

DC Cable Sizing Under BS 7671 Section 712

DC wiring between the PV array and the inverter operates under continuous load conditions and must be sized in accordance with BS 7671:2018+A4:2026 Section 712, using specialist PV cable rated for outdoor and UV-exposed environments.

  • Cable type: Use EN 50618 (H1Z2Z2-K) double-insulated, halogen-free, UV-resistant solar DC cable. Standard twin-and-earth is not suitable. Minimum cross-section is typically 4 mm² for residential strings; 6 mm² for longer runs or higher current strings.
  • Current rating: DC cable must be rated for 1.25 × Isc (short circuit current) of the string to allow for irradiance exceeding STC. Apply temperature derating for roof-mounted cables (installation method B or clipped direct in high-temperature environments).
  • Voltage drop: Limit DC voltage drop to 3% (MCS 001 recommendation) between the array and the inverter DC input. Longer cable runs may require 6 mm² cable to keep voltage drop within limits.
  • Routing and protection: Positive and negative conductors must be run together (or in the same conduit) to minimise loop inductance. Cables must be mechanically protected where accessible and labelled at both ends. Arc fault detection devices (AFCI) are now available and recommended where cables pass through occupied spaces.
05 · Solar PV Guide

AC Connection Requirements

The AC output of the inverter must be connected to the property's consumer unit via a dedicated AC isolator and appropriately sized AC cable. The connection point and protection arrangements must comply with BS 7671 Section 712 and the inverter manufacturer's installation requirements.

  • AC isolator: A clearly labelled, lockable AC isolator must be installed adjacent to the inverter. This allows safe isolation of the inverter from the AC side without switching off the consumer unit. The isolator must be rated for the inverter output current.
  • Generation meter: A generation meter records total electricity produced by the system. Required for SEG export tariff applications. Many modern inverters include an integrated generation meter via their monitoring portal.
  • Consumer unit connection: The inverter AC output connects to a spare way in the existing consumer unit (or a new sub-board). The circuit must be protected by an appropriately rated MCB. RCD protection is provided by the inverter's built-in RCD function (most modern inverters include Type B RCD detection) and the consumer unit's existing RCD.
  • Export limiting: Where the DNO requires export limiting (common for G99 connections or where the local network is constrained), the inverter must be configured with a CT clamp or smart meter connection to limit export to the agreed level.

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06 · Solar PV Guide

G99/G98 DNO Notification

All grid-connected solar PV systems must be notified to the Distribution Network Operator (DNO) before or after commissioning, depending on the system size. The relevant Engineering Recommendations are G98 (small systems) and G99 (larger systems).

  • G98 — systems up to 16 A per phase (≤ 3.68 kW single-phase): The installer self-certifies and notifies the DNO within 28 days of commissioning. No prior approval is required. The DNO may inspect or request information but cannot normally object. The vast majority of UK residential systems fall under G98.
  • G99 — systems above 16 A per phase: Prior DNO application and approval is required before the system can be connected. The DNO has up to 45 working days to respond. G99 applications require detailed design documentation including single-line diagrams, protection settings, and anti-islanding details. Budget for potential reinforcement costs if the local network is constrained.
  • Smart Export Guarantee (SEG) metering: Following DNO approval, the customer must apply to an SEG licensee (energy supplier) for an export tariff. A smart meter or export meter is required. DNO notification numbers should be included in the SEG application.

DNO notification is separate from building regulations notification (which requires a Microgeneration Installation Certificate via a competent person scheme) and MCS certification. All three are required for a compliant, grant-eligible installation.

07 · Solar PV Guide

MCS Design Requirements (MCS 001)

MCS 001 (Microgeneration Installation Standard: Issue 3.0) is the technical standard governing MCS-certified solar PV installations in the UK. Compliance is mandatory for installations to qualify for the Smart Export Guarantee and most grant schemes.

  • Design assessment: The installer must carry out a formal design assessment documenting system size, panel and inverter specifications, string configuration, shading analysis, roof orientation and tilt, and an energy yield estimate using PVGIS or equivalent software.
  • Shading analysis: Horizon shading, near shading (chimneys, dormer windows, trees), and self-shading must all be assessed. Where shading losses exceed acceptable thresholds, the design must use optimisers or microinverters to mitigate the impact on yield.
  • MCS Product Directory: All panels, inverters, and batteries must be listed on the MCS Product Directory to be used in an MCS-certified installation. Check the directory before specifying products — not all panels available in the UK are listed.
  • Installation certificate: On completion, the installer must issue an MCS Installation Certificate (MIC) and Handover Pack to the customer. The MIC is the document the customer needs for SEG applications, insurance, and property sales. It is registered on the MCS database.
08 · Solar PV Guide

For Electricians: Solar PV Design Documentation

MCS-certified solar PV installations require thorough design documentation before work begins and a complete certification package on completion. Organised electricians who produce professional design packages win more work and complete MCS paperwork faster.

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