Lower-carbon testing
Testers drive to every plot. TestConnect gives work to testers who are already passing, puts plots on the same site into one visit, and stops wasted trips with readiness checks. This page explains exactly how we estimate the travel that saves, and what we don't count.
Method version 1 · Last checked: October 2026
What we measure
We measure testing travel emissions: the miles testers drive to carry out tests booked through TestConnect, converted to kilograms of carbon dioxide equivalent (kg CO2e). Every figure is an estimate, made with the published method below and the UK government's conversion factors.
We call this lower-carbon testing and testing travel emissions. It covers travel only, not the carbon in building materials, and we never claim that testing, a site or TestConnect has no climate impact. We don't buy or sell offsets.
The method (version 1)
1. The tester's vehicle
In their profile each tester can set a vehicle type: a small, medium or large car or van, running on diesel, petrol, hybrid, plug-in hybrid or electric. If a tester hasn't set one, we assume a medium diesel van and flag the estimate.
2. Conversion factors
We use the Department for Energy Security and Net Zero (DESNZ) greenhouse gas reporting conversion factors, 2025 edition, in kg CO2e per mile for each vehicle type. Platform administrators can update them and add each new year's set. Every estimate stores the factor set and year it used, so an old report never changes when the factors do.
| Vehicle | DESNZ 2025 category | kg CO2e per mile |
|---|---|---|
| Small diesel van | Vans, Class I (up to 1.305 tonnes), diesel | 0.25329 |
| Medium diesel van (also used when the vehicle is unknown) | Vans, Class II (1.305 to 1.74 tonnes), diesel | 0.30996 |
| Large diesel van | Vans, Class III (1.74 to 3.5 tonnes), diesel | 0.44866 |
| Medium electric van | Vans, Class II, battery electric (electricity used, including losses) | 0.09298 |
| Small diesel car | Cars by size, small, diesel | 0.23078 |
| Medium diesel car | Cars by size, medium, diesel | 0.27639 |
| Medium petrol car | Cars by size, medium, petrol | 0.28121 |
| Medium hybrid car | Cars by size, medium, hybrid | 0.18869 |
| Medium plug-in hybrid car | Cars by size, medium, plug-in hybrid (including electricity used) | 0.14193 |
| Medium electric car | Cars by size, medium, battery electric (electricity used, including losses) | 0.06246 |
| Large diesel car | Cars by size, large, diesel | 0.33808 |
Where DESNZ publishes no factor for a vehicle (hybrid vans, for example), we use the nearest published factor and flag the estimate.
3. Each visit
- Actual miles are the extra miles this visit added to the tester's day: the day's route with the visit, minus the route without it, from route optimisation. If the tester logged mileage against the job, we use the mileage log.
- Stand-alone miles are a return trip from the tester's base to the site: what the visit would have taken as a booking on its own.
- Saved miles are stand-alone minus actual, and never below zero.
- Plots tested on the same visit share one journey, so batching plots shows up as savings.
- An avoided wasted visit, where the site said "not ready" before the tester set off, counts as a full stand-alone trip saved. It only counts when the readiness check was answered "not ready" before the journey started.
kg CO2e and kg CO2e saved are the miles multiplied by the factor for the tester's vehicle. If road distances aren't available, we use the straight-line distance multiplied by 1.25 and flag the estimate.
A worked example
A tester in a medium diesel van is based 25 miles from a site. A stand-alone visit would be a 50-mile round trip. The tester is already testing at a site 4 miles away that day, so adding this visit puts 8 extra miles on their route.
- Saved: 50 − 8 = 42 miles
- kg CO2e for the visit: 8 × 0.30996 = 2.48 kg
- kg CO2e saved: 42 × 0.30996 = 13.02 kg
Reports
- Site and developer report: tests, visits, plots per visit, miles driven, miles saved, kg CO2e and kg CO2e saved, by month, as CSV and PDF with these method notes included. It is purchased-services data developers can use in their Scope 3 reporting.
- Tester report: miles and fuel cost saved (using the tester's own fuel cost per mile), and CO2e.
- The website counter: platform totals, shown on our home page only once real totals pass a threshold set by an administrator. Until then, nothing is shown. It is never a made-up number.
What we don't count
- Upstream emissions from producing and delivering fuel ("well-to-tank"). The factors we use are for the vehicle in use; for electric vehicles they include the electricity used and grid losses.
- The carbon in building materials, and the energy a home uses once it's lived in.
- Making the vehicles and test equipment.
- Travel that isn't to a test booked through TestConnect, such as a tester's private customers' jobs, unless they record them in TestConnect.
- Retests avoided because a home was built tight first time. That is real, but we can't reliably say what would have happened otherwise, so we don't claim it.
The bigger saving is in the build
Travel is the part of testing we can measure directly. The larger effect is on the homes themselves: research by UCL with ATTMA on test results from over 100,000 new homes found a disproportionate number of results just inside the targets, suggesting temporary sealing to pass. A home that is built airtight, rather than sealed for the test, uses less energy for as long as it stands. That is why TestConnect keeps every result, including failures, and never rewards a pass. Read Trust and feedback.
Sources
- DESNZ: greenhouse gas reporting conversion factors 2025 (flat file, published 10 June 2025)
- DESNZ: 2025 conversion factors methodology paper (PDF)
- UCL Bartlett: "Hitting the target and missing the point" (2017)
- TightVent: BCTA analysis of the ATTMA lodgement database (2021)
DESNZ publishes new factors each June. We review the factor set every year and keep the year used with every estimate. Last checked: October 2026.