Solar PV system size calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Start from | My roof area |
| Roof faces | South face: usable area 20 m² |
| Area of one panel | 1.95 m² |
| Panel power | 430 W |
| kk: kWh per kWp for your roof | 850 kWh/kWp |
| Shading factor | 1 × |
10 panels, 4.30 kWp: about 3,655 kWh a year (indicative)
| Result | Value |
|---|---|
| Panels that fit | 10 |
| System size | 4.3 kWp |
| Estimated generation a year (indicative) | 3,655 kWh |
| kk used | 850 kWh/kWp |
| Roof face | Usable area (m²) | Panels |
|---|---|---|
| South face | 20 | 10 |
Show the working
- South face: 20 m² ÷ 1.95 m² = 10.26, rounded down = 10 panels
- System size: 10 panels × 430 W ÷ 1000 = 4.3 kWp
- Generation: 4.3 kWp × 850 kWh/kWp × 1 = 3,655 kWh a year (your kk)
How it works
- Panels per face = usable area ÷ panel area, rounded down (only whole panels fit)
- kWp = panels × panel power (W) ÷ 1000
- Generation (kWh a year) = kWp × kk × shading factor (MCS MIS 3002 method)
We don't estimate savings, payback or export payments: they depend on your tariff and how you use the power.
The inverter is sized by the installer to the array and the maker's data; an MCS installer also handles the DNO notification.
Indicative only: an MCS installer surveys the roof and gives the generation estimate.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Shading factor | 1 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 E7 (PV1): shading factor 1.0 = no shading. Researcher to cite the MCS shading method. |
Regulations and standards
- MCS MIS 3002 and MGD003 (estimating PV generation)
- BS 7671:2018+A4:2026 Section 712 (solar photovoltaic power supply systems)
Explained: BS 7671 (the IET Wiring Regulations), Approved Document P (electrical safety).
The MCS method and BS 7671 are used across the UK. Planning rules for panels differ between England, Wales, Scotland and Northern Ireland.
| Table | Clause | Nations | Status |
|---|---|---|---|
| kWh per kWp (kk) by postcode zone, orientation and pitch | MCS MIS 3002 / MGD003 irradiance data set (Researcher to confirm whether the tables may be reproduced, brief v2 E7 and §16 Q1) | UK-wide | 0/0 values entered |