How the calculator works

The calculator estimates the size, cost, running-cost savings and carbon reduction of a heat pump, solar PV system, battery, or any combination, for a specific UK property. It is an estimation tool, not a quote, design or MCS survey. It doesn't account for listed-building constraints, planning considerations, grid-connection issues, or site-specific plumbing.

Three accuracy tiers run the same underlying model but with different inputs:

• Simple – postcode and property basics only.
• Building-specific – uses your EPC certificate if we can find one.
• Professional – uses your actual energy bills.

More input data narrows the uncertainty band; the arithmetic is the same.

Every result comes out of four stages, in order:

1. Energy profile – how much electricity, heating and hot water the home uses in a year.
2. Sizing – what capacity of kit fits that demand and the roof/site.
3. Costs – install cost, running cost before vs after, payback.
4. Carbon – annual and lifetime emissions saved.

Rather than asking the user lots of questions, we look up a building archetype: a combination of property type (detached, semi, terraced, flat, bungalow) and construction era (pre-1930, 1930–70, 1970–90, 1990–2006, post-2006). Each archetype has a base electricity intensity and heating intensity, expressed in kWh per m² per year, derived from UK housing-stock research.

That base figure is then adjusted by:

• Floor area – your input, or archetype average for your bedroom count.
• Occupancy – ±15% per person vs a 2.4-person baseline for electricity; hot water scales more directly with people.
• Region – from postcode: heating demand, wind exposure, solar irradiance, regional performance gap.
• Wall construction – solid brick, cavity (un/insulated), modern, timber frame.
• Glazing – single, double, triple, low-E – weighted by the archetype's glazing ratio.
• Performance gap – a factor accounting for the well-documented tendency of homes to miss their theoretical performance.

If you supply energy bills, we skip the estimation entirely and use your annual kWh, splitting gas 75/25 between space heating and hot water (a standard ratio). If we find an EPC, we overlay band-specific factors on top of the archetype model.

Worked example

A 1930s semi-detached, 90 m², 3 bedrooms, 2 occupants, Manchester (NW), cavity uninsulated walls, double glazing.

Electricity:

Heating + hot water:

Numbers rounded for clarity. The live calculator carries full precision to the final step.

Heat pump. Peak heating demand is annual heating kWh ÷ 2,000 (approximate full-load hours over a UK heating season), floored at 3 kW, then oversized by 10% for the coldest days.

Solar PV. We size to the smaller of: electricity consumption ÷ specific yield (kWh/kWp), or available roof area ÷ 6 m²/kWp. Clamped 2–20 kWp. Specific yield depends on region and orientation (south = 1.00, north = 0.60).

Battery. With solar, sized to store roughly 50% of daily generation (capped at 40% of daily consumption). Without solar, 20% of daily consumption for time-of-use arbitrage. Clamped 5–20 kWh.

SCOP (seasonal performance): 3.2 for air-source, 4 for ground-source. Source: EST Electrification of Heat field trial medians.

Unit rates

Install cost formulas

BUS grant: £7,500 subtracted when a heat pump is selected (eligibility applies – check gov.uk).

Running cost

Current cost = kWh × unit rate (or your actual bills). Projected cost applies each technology in turn:

• Heat pump: gas cost → 0; add (heating kWh ÷ SCOP) × electricity rate. Gas standing charge also saved.
• Solar PV: subtract generation × self-consumption rate (40% without battery, 70% with) × electricity rate.
• TOU variant – heat pump: heat-pump electricity priced at the blended off-peak rate; household electricity stays at the standard rate.
• TOU variant – battery: a portion of household electricity flows through the battery (charged off-peak, discharged at peak). That portion is repriced at the off-peak rate divided by round-trip efficiency (0.9), so storage losses are accounted for. Annual kWh shifted ≈ capacity × 0.85 × 1 cycle/day × 0.9 × 365, capped at the home's non-HP electricity use. If solar is also selected, cycles used for storing PV are subtracted first (they already save at the standard rate via the solar offset).

Emissions = kWh × carbon intensity. Current grid electricity is 0.131 kgCO₂e/kWh; natural gas is 0.182 kgCO₂e/kWh. Source: DESNZ/Defra GHG Conversion Factors 2026.

Lifetime savings use a 25-year system life. The "trees planted" and "car miles" equivalences (÷22 kg/tree/year and ÷404 g/mile) are communication aids, not precise accounting.

If you supply actual bills and your heating costs look unusually low vs what the archetype predicts (below 85%), we assume you're heating selectively – rooms off, thermostat low – to save money. Running a standard projection off those numbers would flatter the heat pump artificially.

So we show two results:

• Current pattern – heat pump run the same way you heat now. Lower savings, same comfort.
• Full comfort – heat the whole home properly. Higher running cost than "current pattern" but a fairer like-for-like with a well-heated home.

Heat-pump size is held constant across both – only the run-hours change.

The confidence interval starts at ±20% and widens with missing data, older buildings (pre-1930 adds 30%, pre-1970 adds 15%), and estimated floor areas. It's a statement of model uncertainty – not a guarantee that your bill will fall within the band, because behaviour and prices also move.

• Energy prices – Ofgem Price Cap
• Carbon intensities – DESNZ/Defra GHG Conversion Factors
• Heat pump field performance – Electrification of Heat (BEIS/DESNZ)
• BUS grant – gov.uk/apply-boiler-upgrade-scheme
• Housing-stock archetypes – UK building performance research and EST publications