The Certificate and the City
Exhibit 02, from our own modelling run of 2026-07-06. The full exhibit is live on our home page, with every number drawn from the committed data.
A certificate is one number from one rig test. The machine sits in a laboratory, runs under set conditions, and a figure comes out. The figure is honest. It is also narrow.
A home is a year of hours and a life of changes. Cold snaps and mild weeks. A kitchen extension, a new front door, radiators sized for a boiler that left years ago. The certificate was never built to see any of that.
So we asked a plain question. What does one certified machine do across one city of homes, over a whole year?
What we did
We took one certified 8.5 kW air source heat pump and modelled it in 24,931 homes across Greater London, each home reconstructed from public EPC records. Every home ran a full year, hour by hour, against a London test reference year of weather.
Say the word before the numbers: modelled. No meter was read. No home was visited. This is a simulation of homes as the public record describes them, run through the Home Energy Model at hourly resolution. The measure is seasonal efficiency in the home: annual space heating delivered to the home, divided by the heat pump's space heating electricity input.
What came out
The certificate figure is 5.0. That is SCOP at 35 °C flow, on a test rig.
The median home delivers 3.6.
Nine in ten homes land below the certificate figure. One in ten sits below 3.24. One in ten clears 4.92. Same machine, same certificate, and a citywide spread of outcomes decided by the homes, not the box.
One detail is worth sitting with. The same product carries a second certified figure, at 55 °C flow: 3.6. The median home lands exactly on it. The headline number is the rig at its kindest. The fine print is the reality.
If you make heat pumps
This is not a product story. In this study the product does what its certificate says, at both flow temperatures. What varies is everything around it: fabric, emitters, demand, hours of use.
It is a routing story. You answer for a base of homes, and one number cannot sort it. Which homes are ready now, which need evidence first, which are not yet worth a visit. That call needs a number for each home, before anyone is sent to the door. One figure for the whole range tells you nothing about the home in front of you.
If you run housing
A budget built on the certificate buys 5.0 on paper and 3.6 in the median home. That gap is not slack you can claw back later. It is a promise already made to a board, a funder and a resident before any evidence existed.
And the spread matters more than the median. In the same city, one in ten homes clears 4.92 and one in ten sits below 3.24. The same measure spends well in one street and poorly in the next. Evidence for each home, before the money moves, is what makes a budget defensible when the funder asks what it bought.
Method, honestly
Modelled means modelled. Every number above comes from simulation: a full year at hourly steps, London test reference year weather, a design flow temperature of 55 °C. We measured nothing. There is no monitoring data, no field trial and no installed system behind these figures.
The homes are reconstructions from public EPC records, and those records are imperfect: surveys age, and any single reconstructed home may be wrong in ways its record cannot show. The population is the claim here, not any one address.
The product is anonymised on purpose. It performs to its own certificate, and naming it would turn a finding about housing into a story about a brand. The spread comes from the homes, not the badge.
And the certificate is not the villain. A rig test is repeatable and comparable, and that is its job. The mistake is reading it as a prediction for a particular home. It is one point. A city is a distribution.
Where this leaves you
The full spread is drawn as Exhibit 02 on our home page. If you answer for a base of heat pumps, start at the OEM door. If you answer for homes and the budgets that improve them, start at Housing & Place. Either way, the question is the same: not what the certificate says, but where your homes land.
Source notes
- Exhibit 02, generated 2026-07-06: one certified 8.5 kW air source heat pump modelled in 24,931 Greater London homes reconstructed from public EPC records, full year at hourly resolution, London CIBSE test reference year, 55 °C design flow temperature
- Certificate figures from the public record for the product: SCOP 5.0 at 35 °C flow, 3.6 at 55 °C flow
- Home Energy Model simulation; metric is annual space heating delivered divided by heat pump space heating electricity input