System objectives and electricity use
Use a full year of electricity data where it is available. If it is not available, record the gap and any planned loads. The annual total is useful, but the timing also changes the design. A home that is occupied through the day can use more solar directly than a home with most demand in the evening. An EV, heat pump or planned battery changes the load profile again.
Write down the intended outcome:
- reduce daytime grid import
- generate as much annual electricity as the usable roof reasonably allows
- add a battery now or leave a defined route for one later
- use spare solar for an EV or hot-water diverter
- limit export because of a DNO condition
- provide backup power through equipment designed for that purpose
A larger array may increase annual generation while also increasing export. A battery may raise solar self-consumption, but its usable capacity, charge and discharge power, losses and control settings determine the result. Backup also needs suitable switching, earthing and circuit design. The presence of a battery does not mean the house will stay powered during a cut.
Three quantities describe different parts of the result:
- annual generation is the estimated AC electricity produced by the PV system
- self-consumption is the share of that generation used on site
- self-sufficiency is the share of the home’s electricity demand supplied by the system
A savings figure adds import prices, export prices and operating assumptions to those technical estimates. It can change without any change to the hardware.
Roof condition, structure and usable area
A roof survey needs to establish condition and structural capacity. A neat covering is not proof that the rafters, trusses, fixings or deck can carry the proposed loads. The assessment also needs to include the rails, brackets, ballast where applicable, wind uplift and snow rather than panel weight alone.
MIS 3002 requires a suitably competent person to check a pitched roof. Unusual construction, visible distress, altered timbers or doubt about the structure require a structural engineer. A ballasted flat-roof design expressly requires an engineer because the ballast and its concentrated loads form part of the structural calculation. The roof loading survey entry explains the evidence to expect.
Roof condition is a separate decision. If the covering is likely to need replacement during the life of the array, doing that work first avoids paying to remove and refit the system later. Ask the surveyor to record leaks, damaged covering, previous alterations and any inaccessible details that were assumed rather than inspected.
The usable area then depends on the real geometry:
- orientation and pitch of each roof plane
- chimneys, dormers, rooflights and parapets
- shade from nearby objects and the horizon
- safe edge distances and access
- fixing positions and cable routes
- planning or listed-building constraints
Orientation and pitch affect both annual yield and the time of day at which the array generates. East and west planes should not be averaged into a fictitious south-facing plane. A shading analysis should show which part of the array is affected, when it is affected and how the modules share strings or MPPT. Optimisers and microinverters can change the electrical response to partial shade, but they cannot recover sunlight that does not reach the modules.
Planning rules differ across the UK. Roof-mounted domestic solar may fall within permitted development, but the conditions and exceptions vary by nation, property and location. Listed buildings, conservation areas, flats, flat roofs and local restrictions need a specific check before work starts. In Northern Ireland, nidirect advises owners to check planning with the relevant council before installation, particularly for listed buildings and conservation areas. Building rules are separate from planning permission.
Array, inverter and yield design
The panel schedule should name the exact module, quantity and total array rating in kWp. Panel watts are measured at standard test conditions. They do not state how much electricity the system will produce on that roof.
The design also needs the inverter model, continuous AC rating, MPPT allocation and a clear single-line diagram. “5 kW inverter” or “battery ready” does not identify the compatibility or connection design.
Array sizing and electrical design use:
| Design area | Required information |
|---|---|
| Roof layout | Module positions, roof planes, orientation, pitch, obstacles and access |
| Shade | Survey method, affected areas, shade factor and any mitigation |
| Generation | Annual AC estimate, stated method and separate results for materially different planes |
| Inverter | Exact model, AC rating, MPPT use, string arrangement and monitoring |
| Grid connection | Proposed G98 or G99 route, existing generation or storage included and any export limit |
| Future additions | The actual inverter, battery or EV integration route rather than a general “ready” label |
The standard MCS PV performance estimate uses the array rating, location, orientation, pitch and shade. It is an annual estimate, not a generation guarantee. Hourly modelling, battery operation and financial forecasts use additional assumptions and should remain identifiable rather than being folded into one unexplained result.
Compare panel count and module wattage alongside the roof-plane layout, MPPT allocation, shade, expected generation by time of day and whether the layout can be built as drawn. Higher-rated modules may fit more kWp into limited space, while a different layout may produce a broader daily profile or avoid a badly shaded area. Compare the annual AC result, the timing of generation and the buildable layout together.
Inverter architecture affects later changes. A string inverter keeps most conversion equipment off the roof. Microinverters convert at module level. A hybrid inverter can connect a compatible battery on its DC side. None is a universal best choice. Shade, roof access, battery plans, replacement options, monitoring and grid capacity all influence the decision.
G98, G99 and export limitation
The DNO needs a record of generation connected in parallel with its network. The DNO is not the electricity supplier and the connection record is not an export tariff.
In Great Britain, G98 can apply where the generating equipment is Fully Type Tested and its aggregate Registered Capacity is no greater than 16 A per phase. That corresponds to 3.68 kW on single phase and 11.04 kW on three phase at the nominal voltage used by G98. The capacity test concerns the continuous AC generating equipment, not the panel array’s DC kWp or the battery’s kWh capacity.
A qualifying single-premises G98 installation can be commissioned before notification, with the required installation document sent to the DNO within 28 days. G99 applies above the G98 limit or where the G98 conditions are not met. Most domestic G99 routes require an application before installation. Existing solar, storage and other generation must be included when the route is assessed.
A G100 export limitation scheme can cap the power leaving the connection point, but it does not automatically turn larger generating equipment into a G98 installation. The DNO notification for PV overview separates these capacity terms and links to the detailed G98 and G99 routes.
Northern Ireland uses the separate G98/NI and G99/NI framework through NIE Networks. Do not assume a Great Britain form or deadline applies there.
The written design records the intended connection route. Where G99 approval is required, it precedes installation. The owner remains responsible for making sure the DNO has received the notification or granted the required pre-installation authorisation, even where the installer submits the documents.
Certification and electrical condition
MCS certification, electrical competent-person registration and consumer-code membership are separate. Each applies to a different part of the work or commercial relationship. Where they apply, the records identify the responsible legal entity and the relevant technology or registration scope. A trading name or certification logo does not establish either point. The RECC and HIES entry explains the consumer-code boundary.
Deposit protection, workmanship guarantees, insurance-backed guarantees and manufacturer warranties also cover different events. Scheme membership alone does not establish that a particular payment or installation has been registered for protection. Contract terms and procurement evidence are covered separately in Comparing solar quotes.
The electrical survey should establish whether the supply, earthing, bonding, consumer unit and protective devices are suitable for the proposed addition. A spare way in the consumer unit does not prove capacity. A full EICR is not automatically required before every solar installation, although the condition or complexity of the existing installation may justify one.
PV design changes during installation
The visible work normally starts with safe roof access and the mounting system, followed by the modules, DC cabling, inverter, AC connection, protection, metering and monitoring equipment. The exact order depends on the property and design. Electrical and roof work must be carried out by people competent for their part of the installation.
Site conditions can force a change. Hidden roof defects or an obstruction may alter the fixing positions or reduce the array, while a different cable route or substituted inverter can change the strings, monitoring, battery compatibility or DNO submission.
Material changes need to go back through the design rather than being treated as an informal site adjustment. The final layout, schematic, equipment schedule, generation estimate and connection paperwork describe what was installed rather than a superseded design.
Before the scaffold comes down, the finished roof can be viewed from safe ground against the final layout. Obvious damage, displaced tiles, untidy cable routes or unexplained layout changes are easiest to resolve while access is still available. The owner should not climb onto the roof to inspect the work.
PV commissioning and handover records
Commissioning checks the installed system against its design and records the test results. Handover gives the owner the information needed to operate it, identify the equipment and support a later warranty, sale or alteration.
For a domestic MCS solar installation, the final file should include as applicable:
- final array layout, equipment schedule, serial numbers and single-line diagram
- roof and mounting assessment, including an engineer’s report where required
- final performance estimate and shade information
- commissioning and PV inspection records
- Electrical Installation Certificate for the new electrical work
- Building Regulations compliance evidence for notifiable work
- MCS certificate
- DNO G98 acknowledgement or G99 acceptance and completion evidence
- operating, shutdown and fault information
- manufacturer manuals and monitoring access
- maintenance requirements and servicing schedule
- workmanship guarantee, financial-protection documents and manufacturer warranties
- final contract, variations and paid invoice
The PV commissioning pack reference page covers the records in more detail. The MCS certificate, electrical certificate, Building Regulations certificate and DNO evidence are separate documents. Do not accept one as a substitute for the others.
Set up the monitoring account while the installer can demonstrate it. Confirm who owns the account, which email address controls it, how another owner or maintainer can be added and what happens if the installer stops trading. Record the normal readings shown on a clear day so later changes have some context.
SEG and export payments
SEG applies in Great Britain. Participating suppliers set their own rates, contract terms and evidence requirements. Export payments use actual readings from qualifying export metering, not the generation total shown by the inverter.
A domestic SEG application has three core evidence requirements:
- suitable certification for the installation and installer
- an export meter capable of half-hourly measurement
- an export MPAN
The supplier’s application may also ask for the DNO connection record and evidence that the applicant owns the system or is entitled to its export. An installation receiving FIT generation payments can receive SEG after opting out of FIT export payments, but SEG and FIT export payments cannot both be claimed for the same electricity.
The import supplier and SEG supplier can be different companies, although some commercial tariffs tie a higher export rate to an import tariff or installation route. Check both contracts before switching. Where a battery can export electricity charged from the grid, the supplier may apply additional metering or eligibility conditions.
The SEG eligibility entry explains the scheme rules and the evidence a supplier may request. Commissioning the array does not start an export contract automatically. Keep the supplier’s acceptance, opening reading and effective date with the handover records.
Northern Ireland does not use SEG. The Utility Regulator says a microgenerator can sell metered export directly to an electricity supplier or through an agent. That requires a separate Northern Ireland export arrangement, with its own contract and evidence, rather than a SEG application.
Solar generation records and monitoring
Store the final documents in two places and use filenames that identify the system and date. Keep original certificates rather than screenshots of installer portals. Record account-transfer instructions for monitoring, warranties and export payments.
Review generation over comparable periods rather than treating one cloudy day or one app figure as a fault. Weather, shade and household use all change the visible numbers. Persistent warnings, repeated shutdowns, damaged equipment or a clear output change need investigation using the manufacturer instructions and the installer’s fault route.
Routine checks should stay at ground level unless a competent contractor provides safe access. The solar maintenance intervals entry separates normal owner checks from electrical testing and roof work.