Solar Panel Sizing Calculator — Design PV Systems for UK Roofs

Solar panel sizing is the process of determining the optimal system capacity (measured in kilowatts peak, or kWp) for a specific property. The goal is to match the solar generation to the property's electricity consumption, available roof space, and budget — while complying with grid connection regulations and building regulations.

In the UK, solar panel installations have grown significantly. With electricity prices remaining above 20p per kWh, a well-sized solar PV system can save a typical household between £500 and £1,200 per year on electricity bills, depending on the system size, self-consumption rate, and whether battery storage is included.

For electricians, solar PV represents one of the fastest-growing areas of work. The electrical installation must comply with BS 7671 Part 712 (Solar Photovoltaic Power Supply Systems), and the grid connection must meet Engineering Recommendation G98 or G99. The EIC certificate for the solar installation must record the system details, inverter specifications, and AC/DC isolation arrangements.

The Elec-Mate solar sizing calculator handles the design calculations, while the cable sizing calculator and voltage drop calculator verify the AC and DC cabling for the installation.

The system capacity in kilowatts peak (kWp) is determined by the number and wattage of the solar panels. Modern residential solar panels typically range from 370W to 430W per panel. A standard UK domestic system uses 8-12 panels, giving a total capacity of 3-5kWp.

The kWp calculation starts with either the available roof space or the desired annual generation. Working from roof space:

Working from electricity consumption: a typical UK household uses 3,000-4,000kWh per year. In southern England, each kWp generates approximately 900-1,000kWh per year on a south-facing roof. So a 3.5-4.0kWp system roughly matches the annual consumption of an average household — though not all generation will be consumed on site due to the mismatch between generation (daytime) and consumption (evening).

The orientation (compass direction) and tilt angle of the roof have a significant impact on the annual energy yield of a solar PV system. In the UK, the optimal orientation is due south (180° azimuth) at a tilt angle of 30-40 degrees. This maximises exposure to the sun throughout the year.

East and west-facing systems are increasingly popular because they spread generation across the morning and afternoon, improving self-consumption rates. A split system — panels on both east and west-facing roof slopes — can generate similar total energy to a south-facing system while providing a more useful generation profile for households that are occupied during the day.

Shading from trees, chimneys, dormers, and neighbouring buildings reduces yield. Even partial shading of a single panel can significantly reduce the output of the entire string (unless optimisers or microinverters are used). A thorough site survey is essential before finalising the panel layout.

The annual energy yield of a solar PV system depends on the system size (kWp), geographic location (solar irradiance), orientation, tilt, shading, and system losses. In the UK, the expected yield varies significantly by region:

System losses typically account for 10-20% reduction from the theoretical maximum yield. These include inverter efficiency (typically 96-98%), cable losses (1-2%), panel degradation over time (0.5% per year), soiling (1-3%), and temperature losses (panels are less efficient above 25°C). The Elec-Mate calculator applies standard loss factors to give a realistic yield estimate.

For financial calculations, multiply the annual yield by the current electricity rate (typically 20-30p per kWh) to estimate the annual saving. The power consumption calculator can help assess the household consumption profile to determine what proportion of solar generation will be consumed on site versus exported to the grid.

Battery storage allows surplus solar generation during the day to be stored and used in the evening when household consumption is highest. Without a battery, a typical household self-consumes only 30-40% of solar generation — the remainder is exported to the grid at a lower rate (typically 4-15p per kWh under the Smart Export Guarantee). With a correctly sized battery, self-consumption can increase to 60-80%.

The optimal battery size depends on the daily surplus generation and evening consumption pattern. For a typical 4kWp system generating 10-12kWh per day in summer, a 5-10kWh battery captures most of the surplus without being oversized for winter when generation is lower.

Battery installations carry specific regulatory obligations beyond standard Part 712 requirements. The IET Code of Practice for Electrical Energy Storage Systems is the required reference for determining testing regimes, acceptance criteria, and safety tests (including battery isolation, DC earthing arrangements, and battery management system considerations) — see GN3 Reg 1.7.

Where a stationary secondary battery is installed, mandatory warning notices must be fixed at the origin of the installation, at the metering position (if remote from the origin), and at each consumer unit or distribution board supplied from the battery — per GN3 Reg 2.37. Additionally, Reg 712.514.102 requires a permanent warning notice at every point of access to DC live parts (distribution boards, combiner boxes) stating that live parts may remain energised after isolation.

Isolation, ventilation, and fire safety must also be considered. The electrical testing calculators in Elec-Mate include tests specific to battery energy storage systems.

Any solar PV system connected to the electricity grid must comply with the relevant Engineering Recommendation for grid connection. The classification depends on the total generation capacity:

The G98 limit of 3.68kW per phase is based on 16A at 230V. This means most domestic single-phase systems of up to approximately 9 panels (9 x 410W = 3.69kWp) are close to the G98 boundary. The Elec-Mate calculator checks the inverter AC output against the G98/G99 threshold and flags when a G99 application is required.

It is important to note that the G98/G99 limit applies to the inverter AC output power, not the panel DC capacity. A 4kWp panel array with a 3.6kW inverter would qualify under G98 because the AC export is limited to 3.6kW. The BS 7671 eighteenth edition provides the regulatory framework for the electrical installation side.

This is one of the most critical safety distinctions between solar PV and conventional electrical work. Switching off an AC isolator or the inverter does not make the DC strings safe to work on. DC cable runs, combiner boxes, and string fusing remain at hazardous voltages (potentially 600V DC or higher on residential systems) whenever the panels are exposed to daylight.

To provide shock protection on the DC side, BS 7671 Reg 712.410.102 requires that one of the following protective measures is applied to all DC-side equipment:

In practice, the double insulation route (option a) is used for virtually all residential and commercial PV installations. This means specified double-insulated DC cable (typically to EN 50618 / H1Z2Z2-K), double-insulated connectors (such as MC4 or compatible), and double-insulated DC isolators throughout the DC string runs. Single-insulated cable is not acceptable for exposed DC string wiring.

Every PV system requires a commissioning test sequence before handover. BS 7671 Part 712 and BS EN 62446 (Grid-connected PV systems — minimum requirements for system documentation, commissioning tests, and inspection) define the acceptance test regime. The following tests must be completed and recorded in the commissioning report:

All test results must be recorded in the commissioning report. Failing to record results is a common error on PV installations and means the system cannot be formally signed off. The EIC certificate for the installation should reference the commissioning report and record the system details, inverter type, string configuration, and verification results.