About the tool and its data
The Solar Sizer gives a quick, indicative answer to “what size of solar and battery suits this property?” for homes and businesses. It’s built for the UK but works worldwide.
How it works
- Sunshine. It looks up the solar output at your location, for your roof’s direction and angle, from PVGIS.
- Your electricity use. It uses your annual kWh if you know it. If not, it estimates it from CIBSE typical-practice energy benchmarks for your building type and floor area, adjusted for heating, an electric car, opening hours and your local climate.
- A typical day, every month. It simulates a bright, an average and a dull day for each month, hour by hour, with and without a battery. On a time-of-use tariff the battery is assumed to be scheduled to cut your bill.
- Many sizes, three options. It tries many solar and battery sizes, then shows Budget, Balanced and Max independence options, with costs and savings over 25 years.
What to bear in mind
- It’s an estimate. It uses a typical year of weather, not your actual roof, shading or usage pattern, so the more detail you give the better it is.
- Prices, tariffs and rates are indicative and change. You can edit all of them on the results page.
- It’s designed for systems up to about 250 kWp of solar and 500 kWh of battery. Larger or more unusual sites need a proper design.
- The roof size it estimates is a rough guess from the floor area unless you enter it.
- Always get a survey and quote from an accredited installer before buying.
Data sources and credits
- Solar and climate data: PVGIS, © European Union, 2001–2026 (Joint Research Centre).
- Energy use benchmarks: CIBSE (Chartered Institution of Building Services Engineers), typical-practice figures.
- Electricity prices: Ofgem’s energy price cap (October to December 2026), Smart Export Guarantee tariffs and supplier time-of-use tariffs, rounded.
- System costs: MCS and DESNZ installation cost data, and UK installer price guides, rounded.
- Carbon: UK government greenhouse gas conversion factors for electricity, with an assumed yearly fall in grid carbon.
- Maps and places: © OpenStreetMap contributors; map software by Leaflet; UK postcode data from postcodes.io.
- Typefaces: Eczar and Mukta, via Google Fonts.
Questions or corrections? Email TODO-FILL-IN: contact email address. See also privacy and terms.