The most important home energy decision UK homeowners face in 2026. The honest answer: it depends on what your home is missing. Here is the complete unbiased comparison.
| Factor | Loft insulation | Cavity wall | Solar panels 4.5kWp |
|---|---|---|---|
| Cost (private) | £300–£900 | £1,600–£2,800 | £6,500–£9,000 |
| Annual saving | £100–£200 | £100–£300 | £500–£900 |
| Payback period | 2–4 years | 5–15 years | 8–13 years |
| ECO4 funded | ✅ Yes | ✅ Yes | ❌ No |
| WH:LG funded | ✅ Yes | ✅ Yes | ✅ Yes |
Insulate first if underinsulated: If you have less than 270mm of loft insulation or unfilled cavity walls, address those first. Faster payback, lower cost, ECO4 fundable. Solar on a poorly insulated home returns less.
Solar first if well-insulated: If your home already has good loft and wall insulation, solar is the logical next step. Annual savings of £500–£900 outstrip insulation improvements at that stage.
Both if you qualify for WH:LG: The Warm Homes Local Grant can fund both — solar was 38% and insulation 24% of all WH:LG installations to end-July 2026.
Yes, ideally. A well-insulated home needs less energy to heat, so solar covers a larger proportion of your needs. Most retrofit assessors recommend addressing fabric efficiency before renewables.
Yes. The Warm Homes Local Grant can fund both in one delivery programme. Government data shows 38% of WH:LG installs were solar and 24% were insulation.
Loft insulation: 2–4 years. Solar panels: 8–13 years. Insulation wins on speed. Solar wins on total lifetime saving.