Two houses. Same Energy Performance Certificate (EPC) rating. Yet one feels warm and energy-efficient, while the other leaves its occupants cold and struggling with high bills.
This isn’t a rare anomaly—it’s a frequent frustration for homeowners, landlords, and tenants. Identical EPC ratings don’t always mean identical energy performance. But why does this happen?
Let’s uncover the technical and behavioural reasons why EPCs don’t always reflect real-world performance, even when two homes appear equal on paper.
📋 What an EPC Actually Measures
The UK’s EPC system uses:
SAP (Standard Assessment Procedure) for homes
SBEM for commercial buildings
In homes, SAP calculates a theoretical energy efficiency score, based on:
Building materials (walls, roof, windows)
Heating systems
Hot water systems
Insulation levels
Ventilation systems
Lighting
The final rating is based on typical usage, not real-life behaviour. This is a key source of discrepancies.
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🧩 Why Two Homes with the Same EPC Score May Perform Differently
1. User Behaviour Varies Dramatically
One household may:
Keep heating on 24/7 at 23°C
Leave windows open frequently
Use inefficient appliances
While the other may:
Use smart thermostats and timers
Wear warm clothing indoors
Rely more on passive solar gain
EPCs don’t factor in personal energy habits—so two homes rated ‘C’ can behave like an A or an E in practice.
2. Maintenance and Age of Systems
Even if two homes have the same type of boiler or insulation:
One may have an ageing, poorly serviced system
The other could have a newer, well-maintained one
Small things—like a clogged radiator, draughty windows, or a miscalibrated thermostat—can cause significant energy loss, even in a high-rated home.
3. Installation Quality
An EPC assumes that insulation, double glazing, and heating systems are installed to spec. But in reality:
Loft insulation might be compressed or missing in places
Cavity wall insulation might be patchy
Heating controls might not be used correctly
These flaws can’t always be seen or measured in a short EPC visit.
4. Orientation and Microclimate
Two identical homes on a street can experience different conditions:
One faces south, enjoying solar gain
The other faces north, staying cold and damp
Trees, hills, or nearby buildings can block light or wind
EPC models don’t adjust for microclimates, which can significantly affect how warm a house feels and how much heating it requires.
5. Air Leakage and Thermal Bridging
Two homes may look alike, but one might:
Be more airtight (e.g. better seals, fewer cracks)
Have fewer cold spots or draughty areas
Avoid thermal bridging at wall junctions
These factors aren’t always captured in an EPC unless a blower door test is conducted—rare in standard assessments.
6. Occupancy Density
A home with 5 people generates more internal heat (from body warmth, cooking, showers, etc.) than a home with 1 or 2. This means:
Less need for central heating
Better internal humidity regulation
More ‘free’ heating from daily activity
Again, EPCs assume standard usage, not real-world occupancy patterns.
⚠️ The Consequences of Misleading Similarities
Identical EPCs can cause:
Overconfidence in poor-performing homes
Missed opportunities for energy savings
Frustrated tenants paying higher bills than expected
For landlords and property buyers, this reinforces the need to look beyond the certificate.
✅ What You Can Do
1. Request a Detailed EPC from a Trusted Assessor
We go beyond the basics to help identify real performance issues.
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2. Book a Post-Assessment Review
Discuss EPC findings in detail and learn what improvements will actually save money.
🔗 Book an EPC | See Our Prices
3. Conduct an Airtightness Test or Thermal Survey
If your energy bills feel high despite a decent EPC, thermal imaging or a blower door test can pinpoint hidden heat loss.
🔎 Final Thoughts: EPCs Are a Guide, Not Gospel
An EPC is a helpful tool—but it’s a theoretical model, not a performance guarantee. Identical scores don’t always mean identical efficiency.
For a more accurate picture:
Consider post-occupancy evaluations
Pay attention to occupant feedback
Consult experienced energy assessors who understand the limits of the model
