Solar PV Systems — Panels, Inverters, Battery Storage, and DNO Notification for UK Electricians

Solar photovoltaic (PV) installation is one of the fastest-growing areas of domestic electrical work in the UK. With energy bills at record highs and the Smart Export Guarantee (SEG) paying customers for excess generation, demand from homeowners and landlords is strong. This guide covers everything a qualified electrician needs to know: system design, installation procedure, inverter selection, battery storage integration, G98/G99 DNO notification, and MCS certification requirements.

How Solar PV Systems Work

Solar panels generate direct current (DC) electricity when photons strike semiconductor cells — typically silicon. This DC output is converted to 230V 50Hz AC by an inverter, which synchronises with the grid and exports surplus generation or charges a battery storage system. UK residential systems typically range from 3.5kWp to 12kWp depending on roof space, orientation, and load profile.

System Components

• PV modules — monocrystalline (higher efficiency, 20–22%) or polycrystalline (lower cost, 16–18%). Most domestic installations now use monocrystalline half-cell panels.
• String inverter — converts DC from a series string of panels. Cost-effective for unshaded, single-aspect roofs.
• Microinverters — one per panel, ideal for roofs with partial shading or multiple aspects. Higher cost but superior yield under complex conditions.
• DC optimisers — panel-level MPPT (Maximum Power Point Tracking) with a central string inverter. A middle-ground solution for partial shade.
• Battery storage — lithium-ion battery systems (AC-coupled or DC-coupled) store excess daytime generation for evening use.
• Generation meter — records total kWh generated, required for SEG payments.
• Export limiter / smart meter — some DNOs require an export limiter if G99 approval is needed.

Regulations and Certification

Electrical Regulations

Solar PV installation is notifiable under Part P of the Building Regulations in England and Wales. All electrical work must comply with BS 7671:2018 (18th Edition), specifically:

• Section 712 — Solar photovoltaic power supply systems
• Regulation 443 — Protection against transient overvoltages (surge protection required)
• Chapter 54 — Earthing arrangements and protective conductors

The array wiring (DC side) is typically 4mm² or 6mm² single-core DC PV cable (double-insulated, UV-resistant). Never use standard AC cable on the DC side — it is not rated for the continuous high DC voltages involved (open-circuit voltage on a 10-panel string can exceed 400V DC).

MCS Certification

To qualify homeowners for the Smart Export Guarantee (SEG), the installation must be carried out by a MCS-certified installer using MCS-certified products. MCS (Microgeneration Certification Scheme) accreditation requires:

• Installer registration with an MCS-approved certification body (e.g., NAPIT, NICEIC)
• Completion of a recognised PV installation training course (e.g., City & Guilds 2399)
• Adherence to MCS installation standards (MIS 3002)

At completion, provide the homeowner with an MCS certificate, EPC (if applicable), and a Handover Pack including operating manuals and monitoring app setup.

G98 and G99 DNO Notification

Before connecting any generation plant to the grid, you must notify the Distribution Network Operator (DNO):

• G98 — For single-phase systems up to 3.68kW (16A per phase). Notification only — no pre-approval required, but must be submitted within 28 days of energisation.
• G99 — For systems above 3.68kW single-phase, or any three-phase system. Full application and DNO approval required before connection. Can take 4–11 weeks.

Most domestic systems (4–6kWp) on single-phase supplies require G99 approval. Submit the G99 application as early as possible in the project lifecycle. The DNO may require an export limiter or reduced export capacity.

System Design

Site Assessment

A proper site survey must establish:

• Roof orientation and pitch — South-facing at 30–40° is optimal. East/west split arrays are increasingly common with battery storage.
• Shading analysis — Use a Solar Pathfinder or app (e.g., SunEye, Solmetric) to quantify yield losses from chimneys, dormer windows, and neighbouring buildings.
• Structural assessment — Confirm rafter centres, rafter size, and roof condition. In-roof (BIPV) systems require rafter spacing of at least 600mm. Standard on-roof systems add approximately 15–20 kg/m² — most roofs can accommodate this but confirm with a structural engineer on older properties.
• Incoming supply assessment — Check fuse size (60A, 80A, or 100A), meter type (credit, prepayment, or smart), and available consumer unit capacity.

String Sizing

String design must keep the open-circuit voltage (Voc) within the inverter's maximum DC input voltage (typically 1000V DC for residential inverters) at the lowest expected temperature (-10°C in the UK). Use the panel manufacturer's temperature coefficient to calculate worst-case Voc. As a rule of thumb, limit UK residential strings to 8–12 panels per string on a 1000V inverter.

The string must also keep the inverter's MPPT range (typically 200–800V DC) during peak generation. Too few panels and the string will sit below the MPPT minimum; too many and you clip generation at high irradiance.

Installation Procedure

1. Fixing the Rail System

Most UK systems use aluminium rail mounting systems fixed through the roof covering into the rafters with stainless-steel hooks or brackets:

1. Mark rafter positions using a rafter finder. Confirm with a pilot hole.
2. Lift tiles and install waterproof hooks (lead-flashed or EPDM-sealed) into the rafter. Replace tiles over the hook shank.
3. Fit aluminium rails to the hooks using hook bolts. Rails run horizontal (landscape portrait) or vertical depending on panel orientation.
4. Ensure rails are level. Mid-clamps and end-clamps secure the panels to the rails.

2. DC Wiring

Run 4mm² or 6mm² DC PV cable from panels to the inverter location. Cables exit the roof through a purpose-made roof cable entry gland. On the roof, cable is clipped to the underside of the rail system. Protect against mechanical damage where cables pass through roof penetrations.

All DC cables must be clearly labelled with "DC PV Cable — DANGER: Live during daylight hours" at both ends and at intervals. Use MC4 connectors for panel-to-panel connections — crimp correctly (use the proper MC4 crimp tool) to prevent arc faults, which are a fire risk on DC circuits.

Install a DC isolator within 500mm of the inverter, accessible to the installer and emergency services. Some installers also fit a rooftop DC isolator to allow the string to be isolated at the array.

3. Inverter Installation

Inverters should be located in a cool, ventilated position — a garage or utility room is ideal. Avoid direct sunlight and locations above 40°C ambient. Wall-mount on a fire-resistant surface, leaving clearance as specified by the manufacturer for airflow.

Connect DC input from the array (observing polarity — DC reverse polarity will damage the inverter), and AC output to the consumer unit or dedicated AC combiner box. Fit surge protection device (SPD) on both DC and AC sides per BS 7671 Regulation 443.

4. AC Connection

The AC connection is typically a dedicated 20A or 32A circuit from the consumer unit using 2.5mm² or 4mm² twin-and-earth cable protected by an MCB of the correct rating. The generation meter is fitted in the AC line between the inverter and the consumer unit.

For a DNO-metered system, the generation meter is typically fitted between the inverter and consumer unit in a separate generation meter tails enclosure. Ensure the meter is accessible for reading.

Label the AC isolator, generation meter, and consumer unit MCB clearly: "SOLAR PV GENERATION — ISOLATE BEFORE WORKING ON MAINS". Fit a warning label at the consumer unit indicating PV generation is present.

5. Earthing and Bonding

The inverter chassis and metal mounting rails must be earthed. On TN-C-S (PME) supplies, check with the DNO before earthing the PV frame to the PME earth terminal — some DNOs prohibit this. Where prohibited, use a separate earth electrode for the PV frame.

The DC negative conductor must not be earthed (on transformerless inverters, the negative is floated at mid-potential to improve efficiency).

Battery Storage Integration

AC-Coupled vs DC-Coupled

• AC-coupled batteries (e.g., Tesla Powerwall, GivEnergy, SolarEdge Energy Bank) have their own built-in inverter/charger and connect to the AC side of the system. They can be retrofitted to any existing PV installation regardless of inverter brand.
• DC-coupled batteries connect to the DC side and share the main inverter. More efficient (no AC/DC/AC conversion losses) but require a hybrid inverter from the outset or a DC-coupled battery system that includes its own DC/DC converter.

Capacity Sizing

For most UK homes, 5–15kWh of usable battery capacity is typical. A 10kWh battery can shift most surplus daytime generation to evening use in summer, but will be exhausted by early morning in winter. Model expected self-consumption rates based on the household's evening load profile.

Battery Installation

Batteries must be installed as per manufacturer instructions, BS EN 62619 (safety requirements for lithium-ion batteries), and the relevant product safety guidance. Key requirements:

• Wall-mount indoors in a frost-free, ventilated location away from ignition sources
• Rated cable and fusing per manufacturer specifications
• DC isolation switch within reach of the battery
• Consumer unit changes to accommodate energy management system (may require additional RCBOs)

Commissioning

Before energising:

1. Inspect all MC4 connections for correct engagement (click indicates lock)
2. Measure open-circuit voltage on each string — confirm it matches the calculated Voc at ambient temperature
3. Insulation resistance test on DC cables (500V DC, should exceed 1MΩ per IEC 62446)
4. Verify inverter polarity connections (positive to positive, negative to negative)
5. Energise inverter AC supply first, then close DC isolator
6. Confirm inverter initialises, connects to grid, and begins generating
7. Check generation meter pulses or reads correctly
8. Set up monitoring portal/app and confirm remote monitoring is operational

Issue the completed test schedule (IEC 62446-1 commissioning report) along with the MCS certificate and handover pack.

Common Faults and Diagnostics

• Inverter showing "Grid Fault" or "Isolation Error" — Check AC supply, RCD not tripped, and confirm inverter is within voltage/frequency tolerance (230V ±10%, 50Hz ±1%).
• Low generation / MPPT below expected — Check string Voc and Imp. Compare measured vs expected values. A low Voc on a string indicates an open-circuit or bypassed panel. A low Imp indicates a shading or soiling problem.
• Arc fault detection alarm — Some modern inverters have built-in AFDD on DC circuits. Inspect all MC4 connections for poor contact or damage.
• Battery not charging — Check inverter export settings, battery charge/discharge schedules, and whether grid export limit is preventing charging.
• Generation meter not reading — Confirm wired in the correct direction (CT clamp orientation matters on pulse-type meters).

Smart Export Guarantee (SEG)

The SEG replaced the Feed-in Tariff in January 2020. Energy suppliers with more than 150,000 customers must offer SEG payments. Rates vary — typically 3p–15p/kWh — and are set by individual suppliers. To register:

1. Obtain MCS certificate from the installation
2. Apply to a SEG licensee of the homeowner's choice
3. A smart meter capable of half-hourly export readings is required for most tariffs

Advise customers to compare SEG rates before committing to a supplier. Octopus Energy and OVO typically offer competitive export rates.

Maintenance

Solar PV systems are generally low-maintenance, but annual checks are recommended:

• Visual inspection of panels for cracking, delamination, and soiling
• Check MC4 connectors and junction boxes for water ingress
• Verify rail fixings and hooks remain secure
• Check inverter event log for recurring faults
• Test DC and AC isolation switches operate correctly
• Clean panels if heavily soiled (deionised water, soft brush — avoid abrasives)

Products for Solar PV Installation