Solar panels and EPC ratings

A solar array is recognised in an Energy Performance Certificate assessment and generally improves the rating, but by how much depends entirely on the property. The assessment models the whole building — fabric, heating, hot water and lighting — and generation is one input among several. An array on a poorly insulated house with inefficient heating moves the rating far less than the same array on a well-insulated one. Nobody can tell you the exact points gain before the assessment is done. What you can do is make sure the system is properly evidenced, so it is counted correctly.

How solar enters the assessment

EPCs for existing homes are produced using the reduced data standard assessment methodology. The assessor collects data about the building and enters it into approved software, which calculates energy cost, carbon and primary energy figures. The rating on the certificate is derived from those.

For photovoltaics, the inputs are essentially:

• Installed capacity in kWp
• Orientation of the array
• Pitch or inclination
• Overshading category
• Whether the system is connected and operational

The software converts those into an annual generation estimate using standard assumptions, and that generation offsets modelled energy use. Because it is a standardised model, it will not match your actual metered output — that is not what it is for.

Why the improvement varies so much

Three reasons.

The rating is a whole-building figure. Generation offsets a portion of modelled energy use. Where that use is large — poor insulation, old heating — the offset is proportionally smaller. Where the fabric is already good, the same generation moves the needle more.

Orientation and overshading are modelled inputs. A south-facing unshaded array is credited more than a north-facing or heavily overshaded one. The overshading category the assessor selects has a real effect.

The bands are not evenly spaced. A property sitting just below a band boundary can cross it with a small improvement. One sitting in the middle of a band may absorb a much larger improvement without changing letter.

This is why the honest answer to “how many points will I gain?” is that it depends on your specific property, and why we will not quote one.

What the assessor needs to see

Evidence matters. An assessor working to the convention rules needs documentary support for what they enter, and in the absence of evidence the software has default assumptions that are generally conservative.

Have ready:

• The MCS certificate for the installation
• System size in kWp
• Orientation and pitch of each array
• Commissioning documentation and the system design
• Anything recording the inverter and panel specification

Where an array is split across orientations, make sure the assessor knows — the split matters to the model.

Where it matters practically

Selling. The EPC is part of the marketing pack. Buyers increasingly read it, and an array with its documentation in order is a straightforward positive. An array without paperwork raises questions during conveyancing instead.

Letting. Minimum energy efficiency standards apply to rented property in England and Wales, and the applicable requirements have been subject to policy change over time. Landlords should check the current position rather than relying on a summary in an article.

Mortgages and green products. Some lenders offer products linked to EPC bands. The details are lender-specific and change; treat it as something to ask your broker about rather than a reason to install.

What solar does not fix

An EPC is dominated by fabric and heating. Loft and wall insulation, glazing, draughtproofing, and the efficiency and fuel of the heating system carry more weight than generation for most properties.

If the objective is a specific rating band, get an assessor to model the options before spending. It is entirely possible that insulation improvements deliver more rating movement for less outlay, and that solar makes better sense afterwards — both for the rating and because a better-insulated house uses more of its own generation proportionally.

We say this to customers regularly. Solar is worth doing on its merits: reducing what you import, generating on site, and adding a genuine asset to the property. Treating it primarily as an EPC instrument tends to disappoint.

Any published estimates of typical EPC movement are illustrative industry figures, not Solar Fx figures for your property. Actual generation, savings and payback depend on the property, roof orientation and pitch, shading, system design, your electricity usage and tariff, export rates and future energy prices. Figures shown are estimates and are not guaranteed.