In short: solar output depends on the amount and angle of light reaching the panels, which is shaped by roof orientation, pitch, shading, temperature and how the system is designed. Shading can reduce output, but the effect depends on when, where and how much of the array is shaded. This page explains the factors that matter, not a guaranteed production figure for any specific property.
Orientation
In the UK, a south-facing roof generally receives the most sunlight over the course of a year. East-facing roofs capture more morning sun; west-facing roofs capture more afternoon sun. North-facing roofs receive the least direct sunlight and are typically the least suitable for solar PV, though partial solutions may exist depending on the property.
East-west split arrays (panels on both east and west slopes) can provide a broader generation curve throughout the day rather than a single midday peak. This may suit households with morning and evening demand patterns, but total annual generation is typically lower than an equivalent south-facing array. The best orientation for your home depends on roof availability, shading and usage - subject to survey.
Pitch (Roof Angle)
Roof pitch affects how directly sunlight strikes the panel surface. In the UK, pitches between 30° and 40° are often considered favourable for year-round performance. Flat roofs use angled mounting frames to achieve a similar effect. Very shallow pitches may accumulate dirt and water more readily; very steep pitches may be harder to access for maintenance.
Panels can be installed on a wide range of pitches with varying performance trade-offs. The optimal angle depends on latitude, orientation and whether year-round or summer-weighted generation is prioritised. System designers account for this when estimating output for a specific roof.
Shading
Shading is one of the most significant factors reducing solar output. Even partial shading from chimneys, dormers, trees, neighbouring buildings or aerials can disproportionately affect performance - particularly on string-wired systems where one shaded panel can reduce the output of an entire string.
Shading varies by season: a tree bare in winter may shade little, but in full leaf during summer it can block significant sunlight during peak generation months. Shading analysis using specialised tools (or on-site assessment at different times of day) is a standard part of professional site surveys. Microinverters or optimisers can mitigate shading losses but add cost.
Roof Space
The available unobstructed roof area limits how many panels can be installed. A typical residential panel occupies roughly 1.7-2.2 square metres. A 4 kW system might require 10-12 panels and corresponding clear roof space. Obstacles such as velux windows, soil pipes and access pathways reduce usable area.
Roof structural integrity must also be assessed. Not all roofs can support the additional load of panels and mounting hardware without reinforcement. This is a property-specific consideration that cannot be determined from a satellite image alone.
Seasonality
UK solar generation varies significantly between seasons. Summer days are longer and the sun is higher in the sky, producing more generation per panel. Winter days are short, the sun is low, and output can be a fraction of summer peaks. This is normal and expected - not a sign of system failure.
Annual generation estimates typically sum these seasonal variations into a yearly total. Monthly monitoring will show the pattern clearly: high output from April to September, lower from October to March. Battery storage and usage patterns can help capture summer surplus for winter benefit, but cannot fully compensate for reduced winter generation.
Panel Efficiency
Panel efficiency measures what percentage of sunlight striking the panel is converted to electricity. Modern residential panels typically range from roughly 19% to 22% efficiency. Higher efficiency panels produce more power per square metre, which matters on space-constrained roofs.
Efficiency is measured under standard laboratory conditions and real-world output will differ. A highly efficient panel on a shaded north-facing roof will still underperform a standard-efficiency panel on an unshaded south-facing roof. Site conditions generally outweigh panel efficiency in determining overall system performance.
Why Winter Output Is Lower Than Summer
Three factors combine in winter: shorter daylight hours, lower sun angle (meaning sunlight passes through more atmosphere), and increased cloud cover in many UK regions. Together, these can reduce daily winter generation to 10-20% of a bright summer day for the same system. This is a physical reality of UK geography, not a defect in the installation.
Understanding this seasonal pattern helps set realistic expectations. Solar is a long-term investment that benefits from annual generation totals, not consistent daily output year-round.
Why a Site Survey Is Needed
Generic online estimates cannot account for your specific roof's orientation, pitch, shading profile, structural condition, consumer unit capacity and local obstructions. A site survey - conducted in person or with detailed remote assessment tools - provides the data needed for accurate system design and realistic output estimates.
No responsible installer or reviewer should quote precise generation figures without site-specific information. Output depends on roof, shading, usage and tariff. A technical review is needed before installation decisions are made.
No output guarantees: This guide does not quote kWh figures for your property. Any generation estimate should come from a qualified assessment of your specific roof and conditions. Performance varies between properties and years.
What a Technical Review Should Check
- Actual roof orientation and pitch, measured on site rather than estimated from satellite imagery
- Shading from trees, chimneys, dormers and neighbouring structures across different times of day and seasons
- Roof structural condition and its suitability for the additional weight of panels and mounting hardware
- Whether microinverters or optimisers would meaningfully help with any shading identified
Frequently Asked Questions
Does shading ruin solar panels?
Shading reduces output rather than ruining the panels themselves. The size of the effect depends on how much of the array is shaded, when it happens during the day and season, and whether the system uses microinverters or optimisers that limit how much one shaded panel affects the rest of the string.
Does a roof have to face south for solar to work?
No. South-facing roofs generally receive the most sunlight over a year in the UK, but east, west and even east-west split arrays can still generate useful electricity, usually with a different daily generation pattern and a lower annual total than an equivalent south-facing roof.
Why is solar output lower in winter?
Winter combines shorter daylight hours, a lower sun angle and, in many UK regions, more cloud cover. Together these factors can reduce daily winter generation to a fraction of a bright summer day. This is a normal seasonal pattern, not a sign of a fault.
Does panel angle matter?
Yes. Roof pitch affects how directly sunlight strikes the panel surface. In the UK, pitches between roughly 30 and 40 degrees are often considered favourable for year-round performance, though panels can perform on a wide range of angles depending on the property and priorities.
Do trees make solar unsuitable?
Not necessarily. Trees can cause shading that reduces output, and the impact varies by season as foliage changes. Whether this makes a roof unsuitable depends on the size, position and timing of the shading, which is assessed during a site survey rather than assumed in advance.
Can shading be reduced with system design?
Partial mitigation is possible. Microinverters or optimisers allow each panel to operate more independently, so one shaded panel has less effect on the rest of the array. They add cost and do not eliminate the underlying loss of sunlight, but they can reduce its impact compared to a standard string system.
Understand Your Property's Solar Suitability
If you understand the basics and want your own property assessed, you can book a technical solar review. Suitability depends on the property, roof, shading, usage and tariff - subject to survey.
Book a Technical Solar Review