Fabric first, almost always. Insulating before the heat pump shrinks the unit you need, cuts its running cost forever, and can be grant-funded together. Here’s the maths — and the exceptions.
A heat pump sized for a leaky home costs more to buy and run — every kWh of heat loss is paid at electricity prices, forever. Insulate first and the same house may need an 8kW unit instead of 12kW (£1,500–£2,500 cheaper), run lower flow temperatures (higher SCOP), and often keep existing radiators. Loft + cavity at ~£3,600 can cut the pump’s lifetime bill by thousands.
Dead boiler now: the £7,500 BUS makes a pump viable immediately — insulate in parallel. Already-decent fabric (EPC C, filled walls, 270mm loft): go straight to heating. Solid-wall homes waiting on funded EWI: don’t delay heating years for a wall job.
The quiet winner: ECO4 and WH:LG both fund insulation AND air-source heat pumps in one whole-home package (WH:LG dual caps = up to £30,000). Eligible households shouldn’t choose — sequence both under one scheme via the checker.
Loft under 200mm or unfilled cavity → insulate first. Boiler failing or off-gas → pump now via BUS, insulate alongside. Scheme-eligible → both together, funded. Sequence your exact home.
AC-coupled (battery has its own inverter) is the retrofit default: no rewiring of your array, works with any brand or age of system, £3,500–£6,000 installed for 5–10kWh usable. DC-coupled only makes sense if your inverter is due replacement anyway — a hybrid inverter + battery swap can be cheaper combined.
Feed-in Tariff payments are unaffected by adding storage — generation payments continue and deemed export isn’t metered down. On SEG, a battery lets you shift export into peak-price windows instead.
Match usable capacity to your evening consumption and summer surplus — the sizing guide has the tables. Most 4kWp homes land at 5–8kWh usable.
Yes in most homes — smaller unit, higher efficiency, permanently lower running costs. Exceptions: failed boiler (pump now via BUS) or already-good fabric.
Yes — ECO4 and WH:LG both fund whole-home packages combining fabric and air-source heat pumps.
Materially — a well-insulated 3-bed may need 8kW instead of 12kW, saving upfront cost and enabling efficient low flow temperatures.
Insulation first: smaller outlay, faster payback, improves whichever heating you run meanwhile.
Facts checked 21 August 2026. We link to official sources and show when facts were last verified. If a scheme changes, we update the page.
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