How MCS Calculates Solar Performance
When you receive a quotation for solar panels, one of the most important figures is the estimated annual electricity generation.
Many homeowners assume this figure is simply an educated guess. In reality, MCS-certified installers are required to use recognised calculation methods that consider your property's location, roof characteristics and the technical specifications of the proposed system.
Understanding how these calculations are produced helps explain why two properties with the same number of solar panels can generate very different amounts of electricity.
What Is an MCS Performance Estimate?
The Microgeneration Certification Scheme (MCS) sets standards for the design and installation of renewable energy systems across the UK.
As part of the design process, certified installers calculate the expected annual generation of a solar PV system using recognised methodologies and software.
The estimate provides homeowners with a realistic prediction of how much electricity their system is expected to generate under typical UK conditions.
It is an estimate rather than a guarantee, as weather conditions and household circumstances naturally vary from year to year.
Solar Irradiance
One of the biggest factors affecting solar performance is solar irradiance.
Solar irradiance refers to the amount of sunlight that reaches a particular location over the course of a year.
Different parts of the UK receive different levels of solar energy.
For example, southern England generally receives higher annual irradiance than northern Scotland. However, Scotland still receives sufficient sunlight for solar panels to perform extremely well, particularly during the long summer daylight hours.
MCS calculations use recognised irradiance datasets appropriate to your property's location.
Roof Orientation
The direction your roof faces affects both the quantity and timing of electricity generation.
Generally:
- South-facing roofs produce the highest annual generation.
- East-facing roofs generate more electricity during the morning.
- West-facing roofs generate more electricity during the afternoon and evening.
- East-west systems spread generation over a longer part of the day.
Roof orientation is one of several factors included in every MCS performance calculation.
Roof Pitch
Roof pitch also influences the amount of sunlight reaching the solar panels.
Fortunately, solar panels perform efficiently across a broad range of roof angles commonly found on UK homes.
Rather than assuming a standard roof angle, MCS calculations use the actual roof pitch measured during the survey or obtained from design software.
Shading Analysis
Shading can significantly reduce solar generation.
Potential sources of shading include:
- Trees
- Chimneys
- Dormers
- Nearby buildings
- Roof-mounted equipment
Professional surveys assess the impact of shading and incorporate it into the expected performance calculation where necessary.
This helps provide a more realistic prediction of annual electricity generation.
Panel Efficiency
Not all solar panels produce the same amount of electricity.
Panel performance depends on factors including:
- Cell technology
- Module efficiency
- Temperature characteristics
- Manufacturer specifications
Higher-efficiency panels allow more generating capacity to be installed within the same roof area.
System Losses
Not all electricity generated by the solar panels reaches your consumer unit.
MCS calculations account for normal system losses, including:
- Inverter conversion losses
- Cable losses
- Temperature effects
- Dirt and soiling
- Manufacturing tolerances
These losses are expected in every solar installation and form part of a realistic performance estimate.
Why Actual Generation May Be Different
No two years have identical weather conditions.
Actual generation may be higher or lower than the estimate depending on:
- Annual sunshine hours
- Cloud cover
- Snow
- Temporary shading
- Panel cleanliness
- System availability
- Household electricity usage (which affects self-consumption but not generation)
Many homeowners find their systems outperform the estimate during particularly sunny years, while poorer weather may reduce generation in others.
Why Two Installers May Produce Different Estimates
It is not unusual for two quotations to show different generation figures.
This may be due to differences in:
- Roof measurements
- Shading assumptions
- Panel selection
- System layout
- Software settings
- Design methodology
For this reason, it is important to compare more than just the annual generation figure when assessing quotations.
Understanding how the system has been designed is equally important.
Why MCS Estimates Are Important
MCS performance estimates help homeowners make informed decisions by providing a consistent method of comparing different system designs.
They are also used to:
- Support financial calculations.
- Demonstrate expected system performance.
- Meet MCS certification requirements.
- Provide transparency during the design process.
Although estimates cannot predict future weather, they provide a robust basis for evaluating the likely performance of a solar installation.
How Zero Carbon Designs Solar Systems
At Zero Carbon, every solar system is designed using professional MCS-compliant software.
Our designs consider:
- Property location
- Solar irradiance
- Roof orientation
- Roof pitch
- Shading
- Panel performance
- Inverter selection
- Future battery storage
- Household electricity demand
Rather than relying on generic assumptions, we tailor every design to the property's specific characteristics to produce realistic generation estimates and maximise long-term performance.
Frequently Asked Questions
Is an MCS performance estimate guaranteed?
No. It is an estimate based on recognised calculation methods and typical weather conditions. Actual generation will vary depending on environmental conditions and system operation.
Why is my neighbour's estimated generation different?
Even neighbouring properties can have different roof orientations, pitches, shading and system designs, all of which influence expected generation.
Can my system generate more than the MCS estimate?
Yes. During years with above-average sunshine, many systems produce more electricity than their estimated annual generation.
Does shading always reduce solar performance?
Yes. The extent depends on the severity, duration and location of the shading, which is why a professional site survey is important.
Why do installers use MCS calculations?
MCS provides a recognised and consistent methodology for estimating solar performance, helping homeowners compare system designs with confidence.
Want an Accurate Solar Performance Assessment?
A reliable solar design is based on far more than the number of panels on your roof.
Zero Carbon uses MCS-compliant design software to assess your property's orientation, pitch, shading and expected energy generation, providing tailored recommendations for homeowners across Scotland.
Contact our team today for a professional solar assessment and personalised system design.
