Energy Efficiency Glossary
The terms worth knowing before you buy, install, or advise. Plain English, with links to the deeper guides where they exist.
A
- Air Source Heat Pump (ASHP)
- Electric heating system that moves heat from outside air into your home.
- Covered by the Boiler Upgrade Scheme (£7,500 grant). Far more efficient than electric resistance heating, but works best with a well-insulated, lower-temperature heating system.
- BUS guide →Schemes →
- Annual Fuel Petroleum Equivalent (AFUE)
- Measure of how much of the fuel a boiler consumes actually becomes usable heat.
- Modern condensing boilers reach 90%+ AFUE. Older boilers can sit below 70%, wasting fuel and money.
C
- Cavity Wall Insulation
- Insulation blown into the gap between the inner and outer leaves of a cavity wall.
- One of the cheapest, highest-return measures for the ~8 million UK homes with unfilled cavities. Not suitable for solid-wall homes.
- Insulation guide →
- Condensing Boiler
- Boiler that recovers heat from exhaust gases to improve efficiency.
- Mandatory for new gas boilers in the UK since 2005. Replaces older non-condensing units during a boiler swap.
E
- Energy Company Obligation 4 (ECO4)
- Supplier-funded scheme improving efficiency for low-income and fuel-poor households.
- Delivered via obligated energy suppliers, often accessed through LA Flex. Closes to new applications on 31 December 2026; there is no ECO5.
- Schemes →
- Energy Performance Certificate (EPC)
- Rating from A (best) to G (worst) showing how energy-efficient a property is.
- Previously needed for many grants; the Boiler Upgrade Scheme dropped the EPC requirement on 28 April 2026. Still useful for understanding fabric performance and for EPC-related grants.
G
- Great British Insulation Scheme (GBIS)
- Closed scheme offering one free or subsidised insulation measure.
- Ran to 31 March 2026 for lower-income and eligible households in Council Tax bands A-D (England).
- Schemes →
- Ground Source Heat Pump (GSHP)
- Heat pump that pulls heat from the ground via buried loops.
- Higher upfront cost than ASHP but very stable performance. Also BUS-eligible (£7,500).
L
- LA Flex
- Local Authority Flexible Eligibility — a council referral route into ECO4.
- Lets councils refer low-income households that miss the standard rules into funded energy efficiency measures.
- Schemes →
M
- Microgeneration Certification Scheme (MCS)
- Quality mark required for most UK low-carbon heating and generation installs.
- Installers must be MCS-certified for the Boiler Upgrade Scheme and most grant routes. Use it to vet installers before you commit.
- Find installers →
P
- PAS 2035
- UK standard for whole-house retrofit: assess, design, coordinate, and monitor.
- Requires a Retrofit Coordinator for funded or PAS-compliant projects. Prevents "value-engineering" that causes damp and cold homes. Core knowledge for advisors and installers.
- Retrofit careers →
R
- Retrofit
- Upgrading an existing building to use less energy and run on low-carbon heat.
- Covers insulation, ventilation, glazing, and heating. Whole-house planning (PAS 2035) avoids unintended consequences like condensation.
- Retrofit guides →
- Retrofit Coordinator
- Qualified professional who plans and oversees a PAS 2035 retrofit project.
- A growing career path; coordinates assessor, designer, and installers. Often requires a Level 5 qualification.
- Career guide →
S
- Solid Wall Insulation
- External (EWI) or internal (IWI) insulation for homes without cavity walls.
- Costlier than cavity fill but essential for the ~8 million UK solid-wall homes. EWI also refreshes the facade; IWI steals internal space and needs careful ventilation.
T
- Thermostatic Radiator Valve (TRV)
- Valve that sets the temperature per radiator rather than the whole house.
- Cheap comfort and savings win — heat only the rooms you use. Pair with a room thermostat and timer.
W
- Warm Homes Plan
- Flagship government programme launching January 2027.
- Targets up to 5 million homes and 1 million out of fuel poverty by 2030, bundling BUS, the Social Housing Fund and the Local Grant (up to £30,000 per home).
- Schemes →
Not sure where to start?
Browse all our plain-English guides, or check which government schemes you might qualify for.