Whole-Home Energy Retrofit in Cornwall: The Right Order of Works

Why sequence matters

A whole-home energy retrofit is not just a list of upgrades to install in any order. Each measure affects the sizing and performance of the next one. Install a heat pump in a poorly insulated house and it will be oversized, expensive to run, and potentially noisy. Install solar before you know how much electricity your heat pump will use and you will size the array incorrectly. Install a battery before your solar is sized and you may buy the wrong capacity.

The correct sequence also maximises grant eligibility. Some grants (ECO4, GBIS) require you to address insulation before other measures. The BUS grant for heat pumps requires outstanding insulation EPC recommendations to be actioned first. Getting the order right avoids having to undo or modify work.

Step 1: Address the fabric first

Insulation, draught-proofing, and double or triple glazing should come first. The principle: reduce the amount of energy the house needs before designing a system to deliver it. A well-insulated house needs a smaller, cheaper heat pump. It also needs less solar energy to cover heating costs. The hierarchy:

• Loft insulation — almost always cost-effective, often free through ECO4 or GBIS.
• Cavity wall insulation (where applicable) — quick, relatively cheap, significant saving.
• Draught-proofing — inexpensive, immediate impact.
• Solid wall insulation (internal or external) — more disruptive and expensive, but often essential for older Cornwall granite properties.
• Secondary glazing or window replacement — large upfront cost; typically only worth it when windows need replacing anyway.

Step 2: Heat pump installation

Once the fabric is addressed, size and install the heat pump. The insulation improvements reduce the design heat loss, which determines the kW output needed. Undersizing is a myth — a correctly specified heat pump is always sized to cover the coldest design day. An insulated house needs a smaller system.

In Cornwall, the £7,500 BUS grant is available for heat pump installations replacing fossil fuel heating. Apply at this stage, not earlier.

Step 3: Solar panels

Now you know the total electricity consumption of the house — including the heat pump's annual usage. This is the correct point to size the solar array. Sizing solar before the heat pump is installed means guessing the largest single electrical load in the house. Size it after and you get it right.

Target self-consumption rate: 40–60% without a battery, 60–75% with. For Cornwall's irradiance, a typical heat-pump household needs 5–8kWp to cover a significant proportion of annual demand.

Step 4: Battery storage

After 6–12 months of solar generation data, you have real self-consumption figures. Battery storage is easiest to size at this point. The rule of thumb: size the battery to cover evening and overnight demand that the solar doesn't cover — typically 5–15 kWh for a Cornwall family home. Adding a battery immediately post-solar installation (rather than waiting for data) is fine; just use our sizing tool rather than guessing.

Step 5: EV charger

Install the EV charger last if pairing with solar. The charger should be a solar-diverting smart charger (Zappi or equivalent) that can absorb surplus generation into the car. Installing it before solar means you miss this integration. If no solar is planned, install the charger at any point — it does not depend on the other measures.

Timing and grant eligibility

For grant maximisation: check ECO4/GBIS eligibility for insulation before starting anything. ECO4 is income and benefit-dependent; GBIS is EPC-band dependent. Process these applications first, as they can fund Step 1 at little or no cost. Then proceed with BUS grant application for the heat pump (Step 2), noting that outstanding EPC insulation recommendations must be addressed or exempted first.

The 0% VAT applies to all five steps automatically on eligible residential works.

Book a free survey or see all available grants to start your Cornwall retrofit.