Solar PV Installation Guide
MCS pathway, panel technology, inverter selection, DC cable sizing (H1Z2Z2-K), isolation, AC integration, SPDs, metering (SEG), and battery integration
MCS certificationDC cablingInverter typesBattery integration
MCS Certification Pathway
MCS (Microgeneration Certification Scheme) certification is required for installations claiming Smart Export Guarantee payments.
MCS certification requirements
MCS Standards, MIS 3002| Requirement | Detail | Notes |
|---|---|---|
| Electrical qualification | NVQ Level 3 (or equivalent) plus AM2 practical assessment | Must be a fully qualified electrician before applying |
| BS 7671 (18th Edition) | Current Wiring Regulations qualification | Must be in date — typically valid for 5 years |
| MCS-approved training | Complete an MCS-recognised PV installation course | Typically 3-5 days covering design, installation, and commissioning |
| MCS certification | Apply through an accredited certification body (e.g. NICEIC, NAPIT, MCS direct) | Includes desk-based assessment and on-site audit of completed installations |
| Part P registration | Competent person scheme for domestic electrical work | Required for self-certification of notifiable work |
| Consumer Code membership | RECC, HIES, or equivalent consumer protection scheme | Mandatory for domestic MCS installations — protects homeowners |
| Public liability insurance | Minimum 2M cover, 5M recommended | Must cover solar PV installation work specifically |
MCS certification must be renewed annually with ongoing audits and CPD.
Panel Technology
Understanding panel types helps you recommend the right product for each installation.
Solar panel technology comparison
MCS, manufacturer datasheets| Parameter | Monocrystalline | Polycrystalline |
|---|---|---|
| Cell structure | Single silicon crystal | Multiple silicon crystals |
| Efficiency | 20-23% | 15-18% |
| Appearance | Uniform black | Blue speckled |
| Temperature coefficient | -0.3 to -0.4%/°C | -0.4 to -0.5%/°C |
| Low-light performance | Better | Moderate |
| Cost per watt | Higher | Lower |
| Lifespan | 25-30 years (warranted) | 25-30 years (warranted) |
| UK market share | Dominant (90%+) | Declining |
Temperature Coefficient Matters
The temperature coefficient indicates how much power output decreases as the panel heats up above 25 degrees C (STC). A coefficient of -0.35%/degree C means the panel loses 0.35% of its rated output for every degree above 25 degrees C. On a hot UK summer day with a panel temperature of 55 degrees C, that is a 10.5% reduction. Monocrystalline panels typically have a better (lower) temperature coefficient.
Inverter Selection
The inverter converts DC from the panels to AC for the building and grid. Selecting the correct type and size is critical.
Inverter type comparison
MCS, IEC 62109| Type | How It Works | Pros | Cons |
|---|---|---|---|
| String inverter | Single inverter converts DC from one or more strings | Lower cost, proven technology, easy to maintain | Entire string affected by one shaded panel; single point of failure |
| Microinverter | Individual inverter on each panel converts DC to AC at roof level | Panel-level optimisation; no single point of failure; safer (no high-voltage DC) | Higher upfront cost; more components on roof; harder to maintain |
| Hybrid inverter | String inverter with integrated battery charger and management | Single unit for PV and battery; future-ready; simplified installation | Higher cost; battery compatibility may be limited to specific brands |
DC:AC Ratio = Total Panel DC Capacity / Inverter AC Rating- DC:AC Ratio
- = Typically 1.0 to 1.25 for UK installations
- Total Panel DC Capacity
- = Sum of all panel Wp ratings
- Inverter AC Rating
- = Maximum continuous AC output of inverter
MCS MIS 3002
MPPT Voltage Window
The inverter MPPT (Maximum Power Point Tracking) input has a minimum and maximum voltage. The string voltage must stay within this window across all operating temperatures. Calculate string Voc at -15 degrees C (UK minimum) to check it does not exceed the inverter maximum. Calculate string Vmp at the highest expected cell temperature (typically 70 degrees C) to check it does not fall below the MPPT minimum. Use the panel temperature coefficients for Voc and Vmp from the datasheet.
DC Cable Requirements
DC cables for PV must be specifically rated for the application. Standard building wiring cables must not be used.
DC cable specification
BS EN 50618, IEC 62548| Parameter | Requirement | Notes |
|---|---|---|
| Cable type | H1Z2Z2-K solar cable | Double-insulated, halogen-free, UV-resistant |
| Standard | BS EN 50618 | Replaces older TUV 2Pfg 1169 standard |
| Typical sizes | 4mm² or 6mm² | Sized for Isc and voltage drop requirements |
| Voltage rating | Must exceed maximum string Voc at -15°C | Typically 1000V or 1500V DC rated |
| Current rating | Must exceed 1.25 x Isc of the string | Safety factor accounts for irradiance above STC |
| UV resistance | Mandatory for roof-level exposed sections | H1Z2Z2-K is inherently UV-resistant |
| Connectors | MC4 or equivalent locking DC connectors | Must be same manufacturer on both sides — mixed connectors void warranty |
V_d (DC) = 2 x I_mpp x R_cable x L- V_d (DC)
- = DC voltage drop in volts
- I_mpp
- = String current at maximum power point (A)
- R_cable
- = Cable resistance per metre (Ω/m)
- L
- = One-way cable length in metres
IEC 62548
Maximum DC Voltage Drop
Total DC cable voltage drop should not exceed 3% of the string voltage at maximum power point. Excessive voltage drop reduces system efficiency and can push the operating voltage below the inverter MPPT minimum window. For long cable runs from roof to inverter, consider upsizing from 4mm\u00B2 to 6mm\u00B2.
DC Isolation
Safe DC isolation is critical for maintenance and emergency response. DC circuits cannot be de-energised while panels are exposed to light.
DC isolation requirements
BS 7671, IEC 62548, BS EN 62109| Isolator | Location | Purpose | Rating |
|---|---|---|---|
| Inverter DC isolator | Adjacent to inverter (within 1m) | Maintenance isolation of inverter from DC supply | Must exceed max string Voc at -15°C and Isc |
| Rooftop DC isolator (firefighter switch) | At or near the panel array | Allows first responders to de-energise DC cabling between roof and inverter | Must exceed max string Voc at -15°C and Isc |
| AC isolator | Adjacent to consumer unit | Isolate inverter AC output for CU maintenance | Rated for inverter maximum AC output current |
DC Danger: Panels Cannot Be Switched Off
Solar panels generate DC voltage whenever exposed to light — they cannot be switched off. Even on overcast days, a string can produce hazardous voltages (300-600V DC). DC arcs are more dangerous than AC because there is no zero-crossing point to extinguish the arc. All DC isolation must be done with load-rated DC switch-disconnectors (not AC isolators). Clearly label all DC equipment with warning notices per BS 7671 Section 514.
AC Integration
The AC connection from the inverter to the consumer unit requires careful design for protection and compliance.
AC-side requirements
BS 7671:2018+A2:2022, IEC 62548| Component | Specification | Notes |
|---|---|---|
| MCB | Type B, 16A or 20A (matched to inverter output) | Dedicated MCB in consumer unit — do not share with other circuits |
| RCD protection | 30mA — Type B if inverter lacks integral RCMU; Type A if RCMU present | Type B RCDs detect DC fault current from the inverter |
| SPD (AC side) | Type 1+2 or Type 2 as determined by risk assessment | Mandatory where risk assessment requires — PV systems are high risk for transients |
| SPD (DC side) | Type 2 minimum, DC-rated | Installed between strings and inverter DC input |
| AC cable | T&E or SWA, sized to inverter maximum AC output | Typically 4mm² T&E for domestic up to 3.68 kW |
| Earthing | All metallic frames and mounting bonded with minimum 4mm² | Main earthing terminal connection required |
Type B RCD Cost and Availability
Type B RCDs and RCBOs are significantly more expensive than Type A (typically 150-300 pounds vs 30-60 pounds). Where possible, specify an inverter with an integral RCMU to allow the use of a standard Type A device. Check the inverter datasheet for RCMU compliance with IEC 62109-2 Annex E before relying on this.
Metering and Smart Export Guarantee
Correct metering enables homeowners to claim payments for exported electricity under the Smart Export Guarantee (SEG).
Metering requirements
Ofgem SEG guidance, MCS requirements| Meter | Purpose | Who Provides |
|---|---|---|
| Generation meter | Records total kWh generated by the PV system | Installed by MCS-certified installer — required for SEG |
| Smart meter (import/export) | Records electricity imported from and exported to the grid | Energy supplier — must be configured for export measurement |
| CT clamp (optional) | Provides real-time generation and consumption data to monitoring system | Installed with inverter monitoring system (e.g. SolarEdge, Enphase) |
Smart Export Guarantee Explained
The SEG requires licensed energy suppliers with 150,000+ customers to offer a tariff for exported electricity. Rates vary between suppliers and are typically 3-15p per kWh exported. To qualify: the installation must be MCS-certified, the system must be 5 MW or less, and a smart meter capable of recording export must be installed. The homeowner chooses their SEG supplier — it does not have to be the same as their import supplier.
Battery Integration
Battery storage increases self-consumption and provides backup capability. The integration method depends on the inverter type.
Battery integration methods
BS EN 62619, MCS battery storage standard| Method | How It Works | Pros | Cons |
|---|---|---|---|
| AC-coupled | Separate battery inverter connects to the AC side of the consumer unit | Retrofit-friendly — works with any existing PV inverter; modular | Double conversion loss (DC-AC-DC); additional inverter cost; more components |
| DC-coupled | Battery connects to the DC bus of a hybrid inverter | Higher efficiency (single conversion); fewer components; lower cost | Requires hybrid inverter; less flexible for retrofitting to existing PV |
| Hybrid inverter | Single inverter manages PV strings, battery, and grid connection | All-in-one solution; simplest installation; lowest component count | Battery compatibility may be limited to same brand; single point of failure |
Battery Safety and Regulations
Battery energy storage systems must comply with BS EN 62619 (safety) and the relevant parts of BS 7671. Key requirements include: ventilation for battery gases (particularly lithium-ion thermal management), fire separation from occupied spaces, accessible isolation for maintenance, and clear warning labels. The battery must be installed in a location where temperature remains within the manufacturer's operating range (typically 5-35 degrees C). For systems above 1 kWh, Building Regulations may apply. Consult the MCS battery storage installation standard for detailed requirements.