Britain has just had its strongest month for solar electricity. That makes the obvious household question more relevant than the national headline: what could a system produce on one particular roof?
How much electricity do solar panels generate?
There is no honest single figure for every UK home. The current Energy Saving Trust reference system is around 4.5 kilowatt peak (kWp), usually about 12 panels and 20 to 30 square metres of roof. Its annual electricity output depends on the postcode, roof angle, direction, shading and system losses.
Here is one transparent example. We modelled a 4.5 kWp system in Doncaster using the European Commission's PVGIS tool. The roof faces south, sits at a 35-degree pitch and uses a 14% system-loss assumption.
That is an illustration, not a promise. It assumes no separate near-shading loss and averages a full year into one daily figure. A bright June day and a dark December day will not look remotely alike. The installer’s performance estimate should use the real roof and site details.
What happened in July 2026
Solar farms and rooftop panels supplied 14.4% of Great Britain's electricity in July, according to reporting based on National Energy System Operator data. The previous monthly record was 12.4% in May. July's share was also 50% higher than the 9.6% recorded a year earlier.
The same update reported 142,536 new solar installations during the first half of 2026, the strongest six-month total since 2011. DESNZ updated its official solar deployment tables on 30 July.
Those figures matter, but they do not tell a homeowner what one roof will generate. The national result combines large solar farms with smaller systems, different locations and thousands of roof layouts. It was helped by long periods of clear sky, unusually strong sunshine and the growing amount of installed capacity.
kW, kWp and kWh are different numbers
Many confusing solar quotes begin with three similar abbreviations. They measure different things.
| Term | What it measures | How to use it |
|---|---|---|
| kWp | The array's rated peak power under standard test conditions. | This describes system size. It is not the amount the system will produce every hour. |
| kW | Power being generated or used at a particular moment. | A monitoring app may show this as live output. |
| kWh | Energy generated or consumed over time. | Annual output, household use and electricity bills are normally compared in kWh. |
A 4.5 kWp array is not expected to produce 4.5 kW all day. Morning and evening sun, cloud, the season, panel temperature and the roof itself move the live figure throughout the day.
How MCS estimates annual solar output
The current MCS Solar PV Standard, MIS 3002 issue 6.0, sets out a site-based performance method. It starts with the array rating, identifies the postcode region, records the roof pitch and orientation, uses the relevant regional kWh-per-kWp value and then applies a shading factor.
MCS identifies inclination, orientation and shading as the three main site factors. Drawings, maps and photos may establish pitch and direction, but the standard notes that an accurate shade assessment will normally need a site visit. If a remote evaluation misses a site condition, the customer must be told when that condition changes the expected performance.
This is why a postcode and a panel count are not enough for a dependable quote. Two identical systems on the same street can produce different results when one roof faces west or loses afternoon sun behind a chimney or tree.
What changes the amount of electricity generated?
Installed kWp
More panel capacity generally increases annual generation, provided the roof, inverter and electrical design support it.
Direction and pitch
Energy Saving Trust says east- or west-facing systems tend to produce around 15% to 20% less than an equivalent south-facing system.
Postcode and weather
Solar resource varies around the country and between years. MCS uses postcode regions rather than one UK-wide irradiance figure.
Near and far shading
Trees, chimneys, dormers and nearby buildings can reduce output at particular times, even when the roof looks clear in an aerial image.
Modules, inverter and losses
Module behaviour, inverter sizing, cabling, dirt, mismatch and other system losses affect the energy that reaches the home.
Generation is not self-use
The array may generate well while the home exports much of it. Occupancy, timed loads and battery storage change how much stays on site.
Why hotter weather does not always mean better efficiency
Solar panels need light, not heat. Long, cloudless summer days can produce a large daily total because the array receives strong sunlight for many hours. At the same time, the cells become less efficient as they get hotter.
UK government research reports that solar efficiency can fall by roughly 0.2% to 0.5% for each degree above 25°C. This does not mean panels stop working in a heatwave. It means that a very hot panel can convert a slightly smaller share of the available sunlight than it would on a cooler bright day.
There is no contradiction in July setting a generation record. More installed solar, lengthy clear spells and more daylight can outweigh the temperature penalty across the month. A cool, bright day can still be excellent for panel efficiency, while a hot, bright day can deliver a strong total because so much solar energy is available.
How to read a solar performance estimate
A useful estimate separates four questions:
- How large is the array? Check the installed kWp and proposed panel count.
- How much should it generate? Look for an annual kWh estimate based on the actual postcode, pitch, orientation and shading.
- How much will the property use directly? Compare daytime consumption with the generation profile.
- What happens to the surplus? It may be exported, diverted or stored for later use.
A battery does not increase panel generation. It changes when the household can use the electricity. This distinction matters when a proposal combines generation, self-consumption, export income and bill savings in one headline figure.
Ask to see the assumptions. If the roof direction, pitch or shade position is wrong, a precise-looking annual number is still the wrong number.
What the solar survey needs to capture
A practical solar survey should give the designer enough evidence to model the real property and give the installation team enough information to plan the job. For installer teams, that means more than a roof photograph.
- Each usable roof plane, including direction, pitch, dimensions and obstructions.
- Nearby trees, chimneys, dormers, buildings and other likely shade sources.
- Roof covering, visible condition, access limitations and areas that need further structural advice.
- Meter, consumer unit, supply details and a plausible inverter and cable route.
- Battery location and electrical context when storage is included or likely to follow.
- Photos and notes that let the office see where the performance and installation assumptions came from.
For a closer look at the evidence, read the solar PV pre-design checklist or the solar panel survey guide. The earlier two-million-installations update covers the wider growth in the UK market.
Quick answers
Do solar panels work on cloudy days?
Yes. Panels use daylight and still generate when it is cloudy, but output is usually lower than in strong direct sunlight.
Is the average daily figure what I will see each day?
No. Dividing annual generation by 365 is useful for scale, but it hides the large seasonal difference between summer and winter.
Does a battery make the panels produce more?
No. It stores some of the electricity for later. It can increase self-consumption, not the amount generated by the array.
Can an online calculator replace a site survey?
It can give an early indication. It cannot reliably confirm near shading, visible roof condition, access, the electrical route or the details that may change the design and installation price.
Vertex provides solar roof, access, shading and electrical evidence for UK installer teams. See the solar survey scope, current report format or guide pricing.
Sources checked
- The Guardian: solar powers a record share of Great Britain's electricity in July
- DESNZ: Energy Trends UK renewables and solar PV deployment tables
- NESO: historic demand, wind and solar data for 2026
- MCS: MIS 3002 Solar PV Systems, issue 6.0
- Energy Saving Trust: solar panels, costs, savings and suitability
- GOV.UK: impacts of extreme heat and heatwaves on energy assets
- European Commission Joint Research Centre: PVGIS solar performance tool