Why Two 4 kWp Solar Systems Can Produce Different Amounts of Electricity
What 4 kWp Actually Means
A 4 kWp system has approximately four kilowatts peak of module capacity under standard test conditions. It is a useful description of array size, but it is not a promise of a fixed annual output.
Predicted annual generation is normally expressed in kilowatt-hours, or kWh. That prediction should be calculated for the individual roof and system design.
What Changes Annual Generation
- Roof orientation and azimuth
- Roof pitch
- Shading from buildings, trees and roof features
- Geographic location and local irradiation
- Module position and usable roof area
- Electrical design and system losses
The Same Capacity Can Use Different Roof Areas
One 4 kWp array may fit on a suitable unshaded south-facing elevation. Another may need to be split across several elevations because rooflights, hips, valleys or dormers interrupt the most productive roof area. Both quotations can state 4 kWp while showing different predicted annual generation.
This is why system comparisons should include the proposed layout and predicted kWh as well as the headline kWp and price.
How Smaller Modules Can Help
SolarTyle uses a smaller-format module than a conventional full-size solar panel. On a complex roof, that can give the designer more options around rooflights, dormers, hips and changes in shape.
The benefit is project-specific. It does not mean every SolarTyle layout will generate more than every conventional system. It means more of the usable roof can be considered during design.
On suitable SolarTyle projects, better use of productive roof elevations has allowed the required installed system size to be reduced by up to approximately 20% while still meeting the required predicted annual generation. This is a project-specific outcome, not a universal saving.
See the existing Future Homes Standard roof-space example and our guide to solar on complex roofs.
A Better Solar Quotation Comparison
| Measure | What it tells you |
|---|---|
| Installed capacity (kWp) | The peak-rated size of the module array |
| Predicted annual generation (kWh) | The modelled electricity output for the proposed layout |
| Elevation and pitch | Where the modules will sit and at what angle |
| Shading assumptions | How obstructions have been allowed for |
| Installed price and scope | What the quotation includes and excludes |
Compare kWh as Well as kWp
A lower price per installed kWp is not the only measure of value. Ask for the design assumptions and predicted annual generation, then compare them with the total scoped price.
Our SolarTyle cost guide explains the current indicative price comparison, while the roof orientation guide covers the role of each elevation.
Sources and Tools
- European Commission Joint Research Centre: PVGIS, site-specific photovoltaic performance modelling.
- SolarTyle data sheet, current module power specification.
- SolarTyle roof-space and annual-yield project example.
