In-Roof vs On-Roof Solar Panel Energy Output Calculator

How this calculator works

This calculator works in ranges rather than single figures, because real quotes, yields and tariffs genuinely vary that much. It starts from typical UK price, output and savings ranges drawn from published 2026 industry guides and official scheme documentation, then scales them by the quantities you enter and adjusts them for each choice you make below. The result is a low-to-high band that a realistic outcome is likely to fall within, not a promise of the figure you will achieve.

Standard on-roof panels benefit from an air gap behind them that keeps them cooler, and cooler panels generate more: integrated in-roof systems typically give up around 3 to 8 percent of annual output because they run warmer, roughly 780 to 900 kWh per kWp against 830 to 950 for on-roof. On a 4 kWp system the difference is typically 100 to 300 kWh a year, worth to at 2026 prices.

System size

Solar systems are sized in kilowatts peak (kWp), the output of the panels under standard test conditions. A 4 kWp system of around 10 modern panels is the most common choice for a UK three bedroom home, while larger households, or homes with an electric vehicle or heat pump, often go to 6 kWp or more. Bigger systems cost less per kWp because fixed costs such as scaffolding, the inverter and installer overheads are spread across more panels, and the calculator reflects that economy of scale.

Region (solar resource)

The same panels generate more in Cornwall than in Caithness. The industry baseline is around 850 kWh per year for every kWp installed on an unshaded south-facing roof in an average UK location, with southern England running around 5 percent above that and Scotland 10 to 15 percent below. The calculator scales its output and savings figures accordingly.

What else affects the figures

In-roof systems trade that output for looks and weathering: the panels replace the tiles beneath them, sitting flush with the roof line, which planners and owners of period properties often prefer, and on new builds or re-roofs they save the cost of the displaced roofing. On-roof mounting is cheaper on an existing sound roof, easier to service and marginally more productive. The temperature penalty varies with the specific in-roof system and how well it ventilates, and both approaches use identical panels otherwise, so orientation, shading and region matter far more to total output than the mounting choice does.

Certification, planning and grid connection

If a calculation here prompts a project, the practical next step is an MCS-certified installer’s survey: MCS installers must provide a location-specific generation estimate using the standard industry methodology, which turns the broad ranges on this page into a figure for your actual roof. Most domestic solar work falls under permitted development, with the usual exceptions for listed buildings, flats and conservation areas, and installations must be notified to the district network operator under the G98 or G99 procedures, which a competent installer handles as part of the job.