Energy Payback Calculator — Insulation Investment Return

Simple payback is the number of years it takes for annual energy savings to equal the installation cost. For example, if cavity wall insulation costs £600 and saves £180/year, the simple payback is 3.3 years. It's the easiest metric to understand but doesn't account for energy price inflation or the time value of money.

Net Present Value (NPV) accounts for the fact that money in the future is worth less than money today (time value of money). It discounts future savings by a discount rate. The HM Treasury Green Book sets a 3.5% Social Time Preference Rate, but that is a real (inflation-adjusted) rate. Because this calculator escalates savings by nominal energy-price inflation, it discounts at a matching nominal rate — the Green Book 3.5% real plus the ~2% Bank of England CPI target, about 5.5%. A positive NPV means the investment is worthwhile. Energy price inflation works in favour of insulation as future savings rise in cash terms.

Loft insulation (especially a top-up from 100mm to 270mm) usually has the shortest payback — often 2-6 years on Energy Saving Trust figures. Cavity wall insulation typically pays back in around 3-11 years depending on house size and the price you pay. Floor insulation takes roughly 7-17 years. External wall insulation rarely pays back on energy savings alone (15-25+ years) but adds other benefits like weather protection, reduced damp risk and property value. Paybacks shorten as energy prices rise and where grants cut the upfront cost.

Energy price inflation significantly improves the payback of insulation. At 5% annual energy inflation, savings double in about 14 years (Rule of 72: 72 ÷ 5 ≈ 14.4 years). UK domestic gas prices have risen substantially over the past two decades — well above general inflation across the period including the 2022 energy crisis — so future bill savings are likely to grow in cash terms. With sustained energy inflation, even costly measures like external wall insulation can become cost-effective over their 25-40 year lifespan.

Yes — government grants (such as the Great British Insulation Scheme or ECO4) significantly reduce the upfront cost and therefore the payback period. Some measures are fully funded for eligible households. Check GOV.UK for current schemes. Enter the net cost after grants into this calculator for an accurate payback calculation.