The right size cannot be chosen from floor area or one electricity bill in euros. You need energy in kWh, consumption timing, usable roof area and electrical constraints. Compare alternatives using consistent assumptions.
Collect twelve months of data
Record monthly energy and interval readings where available. Identify seasonal heating, cooling, pumps and vehicle charging. Note absences and business shutdowns so exceptional periods do not distort the design.
A first numerical estimate
Array kWp ≈ target annual generation / specific yield. For a hypothetical 9,000 kWh target and 1,500 kWh/kWp specific yield, the first estimate is 6 kWp. This formula ignores demand timing, export limits, shading and connection conditions. It is not a final proposal.
Specific yield needs a real location assessment, for example using PVGIS, supplemented by inspection of installation conditions.
Check four constraints
| Constraint | Question |
|---|---|
| Space | Can panels, access routes and clearances fit? |
| Electrical infrastructure | What do the supply and design permit? |
| Consumption timing | How much generation is useful when it occurs? |
| Budget | Which option remains acceptable in a conservative scenario? |
Matching annual generation to annual demand does not eliminate electricity purchases every hour. Sizing only from annual totals can therefore oversize an installation for its actual self-consumption opportunity.
Future loads and storage
Add planned loads as separate scenarios. An electric vehicle charged mainly overnight differs from one available at midday. Keep the assumed annual mileage and charging schedule visible rather than hiding them in a general growth factor.
Select storage from surplus and later demand, not a fixed battery-to-array ratio. Also decide whether backup reserve should be retained, since that changes the energy available for daily savings.
What the assessment should deliver
A documented proposal compares at least two capacities with generation, direct use, imports, exports and financial assumptions. The method applies to both homes and businesses, but the load profiles differ. Browse real projects at different scales without copying another property's system size. A larger installation is useful only when its additional output has a justified purpose.
Compare three sizes consistently
After estimating annual demand and available space, request a smaller, base and larger option. For each, record module capacity, estimated generation, direct use, potential surplus and connection constraints. Using the same assumptions across options makes the differences understandable.
If an electric vehicle, heat pump or business expansion is planned, do not merely add an arbitrary annual kWh figure. Record when the new load will operate and whether it coincides with solar production. The right capacity is a technical and financial choice for a specific demand profile, not a universal rule per square metre.
Discuss your installation
Send your location, bills, operating hours and available roof space for an individual assessment.
Request an assessment