Solar PV Installation: Panel Types, Inverters, DNO & MCS Certification

Summary

Solar photovoltaic (PV) systems convert sunlight into DC electricity, which is then converted to 230V AC by an inverter for domestic use. Any surplus electricity not consumed in the property can be exported to the grid under the Smart Export Guarantee (SEG), for which energy suppliers pay an export tariff. This has replaced the older Feed-in Tariff (FiT) scheme which closed to new applicants in 2019.

For tradespeople, solar PV installation requires specific competencies beyond standard electrical installation. The MCS (Microgeneration Certification Scheme) certification is the industry benchmark, and MCS certification is required for SEG eligibility and most finance/grant schemes. MCS covers both the installer company and the specific products installed — all panels, inverters, and mounting systems must appear on the MCS Product Register.

The electrical elements of a solar PV installation fall under BS 7671:2018+A2:2022 Section 712 (Solar photovoltaic supply systems). The DC side of the installation — from panels through to the inverter — is particularly important: DC arc faults are much harder to extinguish than AC faults, and string voltage can be dangerously high (potentially 600V+ on a domestic system).

Key Facts

• MCS certification — Required for SEG eligibility; all installers must be MCS-certified; products must be on the MCS Product Register
• Smart Export Guarantee (SEG) — Energy suppliers with 150,000+ customers must offer an SEG tariff; rates vary by supplier
• System size (typical domestic) — 3-6kWp for most UK homes; larger systems may require DNO assessment
• Panel output — 350-450W per panel is typical for modern monocrystalline panels
• Number of panels (4kWp) — Approximately 10 x 400W panels
• Roof area required — Approximately 6-8m² per kWp (10m² per kW is a rough rule of thumb)
• Optimal orientation — South-facing at 30-40° pitch; east/west-facing will generate roughly 15-20% less
• Inverter types — String inverter (most common), microinverters (per panel), power optimisers
• Battery storage — Can be added (AC-coupled or DC-coupled); separate MCS product certification required for batteries
• DNO notification — G98 (systems ≤3.68kW/16A single phase, notification on day) or G99 (above 3.68kW or multi-phase, requires DNO approval first)
• DC isolation — A DC isolator must be fitted adjacent to the inverter; some products combine this
• Generation meter — Required for SEG; records actual generation for export metering
• Fire risk — Rooftop PV cannot be isolated in a fire until panels are physically covered; inform local fire service
• Structural survey — Confirm roof can bear load (typical system: 15-20kg/m²) before installation
• Planning permission — Most domestic roof-mounted PV is permitted development; flat roof and listed buildings may need consent

Quick Reference Table

Detailed Guidance

MCS Certification Process

To install solar PV and qualify installations for SEG, you must be MCS-certified:

1. Join a certification body — Bodies include NAPIT, NICEIC, Stroma, and others authorised by MCS
2. Demonstrate competency — Typically requires an approved training course (City & Guilds 2399, BPEC Solar PV, or equivalent) plus installation experience
3. Quality management system — Your business must have documented processes for installation, commissioning, and customer documentation
4. Insurance — Public liability insurance at appropriate level
5. Annual audit — MCS requires periodic quality audits of installation work

Each completed MCS installation generates an MCS certificate, which the customer needs to register for SEG with their energy supplier.

DNO Connection (G98/G99/G100)

The engineering recommendation that governs connection to the distribution network:

• G98 — Micro-generators up to 3.68kW (16A, single phase); can be installed and connected on the same day as notification
• G99 — Larger systems, three-phase, or where G98 conditions aren't met; requires DNO approval before energising
• G100 — Covers demand management and larger installations; less commonly relevant for domestic

In practice, most domestic solar PV (up to about 16 panels, single phase) qualifies for G98 and can proceed on notification without waiting for DNO approval. Systems over 3.68kW single phase require G99, and the DNO has 28 days to respond (though most respond faster).

The DNO may impose export limitation (often 3.68kW export limit even for larger systems) to protect local network capacity. An export limiting device may be required.

DC System Safety

The DC side (panels to inverter) is the most dangerous part of a solar installation:

• String voltage — A string of 10 panels at ~38V each generates around 380V DC open circuit; two strings in series could be 760V. Unlike AC, you cannot switch off the sun — panels continue generating until physically covered
• DC arc faults — DC arcs are sustained and don't self-extinguish at zero crossing as AC does; a DC arc can cause a fire
• DC isolators — Must be rated for DC voltages and currents; standard AC isolators are not suitable
• Fire risk labelling — Cables and isolators must be labelled "Warning: Solar PV System — DC supply cannot be isolated at inverter during daylight hours"

Battery Storage

Adding battery storage significantly increases installation complexity and cost. Two types:

DC-coupled battery:

• Battery connected directly to DC bus between panels and inverter
• Requires a hybrid inverter (or specific combined system)
• More efficient (less conversion losses)

AC-coupled battery:

• Battery connects to AC side of the system
• Separate battery inverter/charger
• Can be added to existing PV systems more easily

Battery storage adds roughly £3,000-£8,000 to system cost depending on capacity. The most common sizes are 5-10kWh. Batteries require their own MCS certification.

Planning and Permitted Development

Most domestic roof-mounted solar PV is permitted development and doesn't require planning permission, provided:

• The system doesn't protrude more than 200mm from the roof surface
• It's not on a listed building
• It's not within a World Heritage Site
• For flat roofs: doesn't protrude more than 1m above the roof line

Conservation areas and listed buildings require planning permission. Groundmounted systems generally require planning permission.

Frequently Asked Questions

Does my customer need to tell their energy supplier before installing solar PV?

They need to notify their DNO (not the energy supplier directly, though some suppliers ask separately). For systems up to 3.68kW, notification under G98 allows installation on the day of notification. For larger systems, G99 approval must be received first. After installation, the customer contacts energy suppliers to register for the Smart Export Guarantee to receive export payments.

What's the payback period for a typical domestic solar PV installation?

It varies significantly by location, electricity tariff, and self-consumption rate. A rough estimate: a 4kWp system generating 3,400kWh/year, with 50% self-consumed at 30p/kWh (saving £510) and 50% exported at 15p/kWh (earning £255), saves/earns approximately £765/year. At a system cost of £7,000, payback is around 9 years. With battery storage, self-consumption rises but so does the cost.

Can I install solar PV if I'm already a qualified electrician but not MCS-certified?

You can install the electrical works (connection to consumer unit, generation meter, grid connection) as a qualified electrician under Part P. However, without MCS certification, the installation doesn't qualify for the Smart Export Guarantee, and many customer finance products also require MCS. Most domestic customers expect MCS certification. In practice, it's not economical to offer solar PV installation without becoming MCS-certified.

What's the difference between a string inverter and microinverters?

A string inverter connects all panels in series (or parallel strings) to one central inverter. If one panel is shaded, it can drag down the output of the whole string. Microinverters fit one per panel and convert DC to AC at the panel — each panel operates independently, so shading of one doesn't affect others. Microinverters are typically 20-30% more expensive but can deliver 10-25% more generation on partially shaded roofs. They're also better for monitoring (you can see individual panel performance).

Regulations & Standards

• BS 7671:2018+A2:2022 Section 712 — Solar photovoltaic supply systems; specific wiring requirements

• MCS 001 — MCS installer standard; certification requirements for installers

• MCS 005 — MCS installation standard for PV; covers design, installation, and commissioning

• Engineering Recommendation G98/G99 — DNO connection requirements for micro-generators

• BS EN 62446 — Grid-connected photovoltaic systems; minimum requirements for system documentation, commissioning tests, and inspection

• Part P of the Building Regulations — New electrical circuits are notifiable work

• MCS (Microgeneration Certification Scheme) — Official MCS certification body

• OZEV Smart Export Guarantee — Government guidance on SEG

• Solar Trade Association — Industry body with installation guidance and standards

• ev charger — EV charger installation; often paired with solar

• consumer units — Consumer unit requirements for solar connection

• part p notifications — Part P notification for new circuits