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Every Energy Performance Certificate for an existing home carries a rating from 1 to 100, banded A to G. This page explains what that number actually measures, what moves it, and lets you test it yourself: pick a home like yours on the interactive explorer below and change its heating, insulation, glazing and solar panels, with every figure calculated by our own RdSAP engine.
The rating is a running-cost index: the estimated annual cost of heating, hot water, lighting and pumps under standardised occupancy, adjusted for floor area so a large home and a small one can be compared fairly. It is not a forecast of your bills. The assessment method for existing homes is called RdSAP (Reduced Data Standard Assessment Procedure): the assessor records what can be seen and evidenced, and the calculation fills every gap with a standard assumption based on the home's age.
Because the index is cost-based, what heats the home and what that fuel costs dominate the result. The price table behind lodged certificates puts mains gas at 3.48 pence per kilowatt hour against 13.19 pence for standard-tariff electricity, nearly four times as much, with heating oil at 5.44 and bulk LPG at 7.60 between them. Those few numbers explain more low ratings than anything else on this page.
Thirty synthetic homes, five types by six ages, each built on the same age-based assumptions an assessor's software applies when no better evidence exists. Every combination of home and measures, 77,760 states in all, has been run through our RdSAP engine in advance: nothing here is estimated on the fly. The tool shows the rating, the modelled running cost behind it, the environmental impact rating from the same calculation, and whether the state clears band C, the minimum the government proposes for rented homes from 2030. If a home starts below C, the tool will also work out the cheapest priced route up to it.
An assessor who can see no insulation evidence must record each element as built, and the calculation then applies the standard for the home's construction era. That is why age moves the rating before a single improvement is made. The grid below shows all thirty homes on the same specification: a modern gas combi and ordinary double glazing, everything else as built.
| Gas combi, as-built fabric | Pre-1930 | 1930-1966 | 1967-1975 | 1976-1995 | 1996-2011 | 2012 onwards |
|---|---|---|---|---|---|---|
| Detached house | 54 E | 55 D | 61 D | 69 C | 75 C | 77 C |
| Semi-detached house | 58 D | 58 D | 63 D | 70 C | 75 C | 77 C |
| Mid-terrace house | 64 D | 61 D | 67 D | 71 C | 74 C | 78 C |
| Top-floor flat | 60 D | 59 D | 68 D | 74 C | 76 C | 79 C |
| Bungalow | 49 E | 50 E | 59 D | 67 D | 74 C | 76 C |
Two things stand out. First, most of the gap between the eras is the loft and the walls each era was built with: homes built before the mid-1960s began life with no loft insulation at all, which is why the two oldest columns start so far back, and why loft insulation is such a large, cheap win on older homes. The bungalow row makes the same point loudest: a single storey exposes the whole footprint as roof and floor, so an uninsulated bungalow starts lower than any house of its age. Second, attached homes are cheap to heat: in every age group up to 1995 the mid-terrace and flat sit ahead of the detached house, because party walls lose far less heat than outside walls. Only in the two newest groups do well-insulated outside walls close that gap. There is even a subtlety the tool can show you: top both terraces up to a 300 millimetre loft and the pre-1930 one edges ahead of its late-1960s neighbour, because a solid brick party wall loses almost nothing while a mid-century unfilled cavity party wall leaks.
The same 90 square metre semi-detached home, walked through five specifications. Every figure is read from the same precomputed matrix the tool uses, so the table and the tool agree row for row.
| The same semi-detached home, built 1967 to 1975 | Rating | Modelled running cost |
|---|---|---|
| As many stand: older gas boiler, single glazing, as-built fabric | 51 E | £1,118 a year |
| Modern gas combi and double glazing (the explorer's opening state) | 63 D | £841 a year |
| Combi plus cavity fill and 300 mm loft insulation | 72 C | £651 a year |
| Air source heat pump on the as-built fabric | 42 E | £1,341 a year |
| Heat pump with the fabric done and 5 kWp of solar | 85 B | £345 a year |
The first two rows are the renovation many homes have already had: a modern combi and double glazing are worth twelve points here. The last two rows carry the sharpest lesson on the page: the identical heat pump rates 42 on the as-built fabric and 85 with the fabric done and solar fitted. A heat pump multiplies whatever the fabric gives it; it cannot rescue a leaky home on its own.
Across every home and combination of measures this tool models (77,760 states in all), 43,748 reach band C or better, about 56%. Reaching C is rarely impossible; the question is which measures a particular home needs, and what they cost.
Because the rating prices every unit of energy, the fuel ladder is visible in the tool's running-cost line: switch the same home from gas to oil, LPG and electric panel heaters and watch the cost climb in the order of the price table. Direct electric heating pays 13.19 pence for every unit of heat; a gas boiler pays 3.48 for a unit that mostly becomes heat.
Electricity claws back in three ways. Storage heaters charge overnight at the 5.50 pence off-peak rate, and the tool separates the older large-volume type from modern high heat retention units, which hold their charge better and respond to the room. A heat pump delivers around three units of heat per unit of electricity, more on a well-insulated home, fewer on a leaky one, which is why its chip in the tool changes as you move the insulation levers. And solar panels offset bought electricity directly, which lifts an electrically heated home much further than a gas one. Carbon runs the other way entirely: grid electricity now causes less CO2 per unit than gas, so the certificate's second chart, the environmental impact rating, rates electric options far better than the headline number does. The tool prints that rating under the running cost: put a heat pump in a leaky home and watch the cost rating fall while the carbon rating climbs.
Rented homes in England and Wales currently need band E. The government has confirmed its intention to require band C for new tenancies from 2030, subject to legislation, with a cost cap and an exemptions regime. The tool marks the C line on every state, and for a typical rented semi the distance is smaller than it looks: the explorer's opening home, a 1967 to 1975 semi on a modern combi, rates 63 D. Filling its cavity walls, typically £500 to £1,500, takes it to 68 D, one point short of the line. Topping the loft up to 300 millimetres and completing the switch to LED lighting carries it to 72 C and takes the modelled running cost from £841 to £651 a year. The tool's route button does this arithmetic for any home below the line: it finds the cheapest priced combination of measures that clears C from wherever you currently stand.
Solid-walled and off-gas homes have further to travel, and the cost cap and exemptions exist for exactly those cases. The measured route matters too: our assessors record the evidence that stops the calculation assuming the worst, which is often the cheapest point on the whole list.
The thirty homes here are synthetic: real geometry, real age-band assumptions, real calculations, but no real address. A real assessment measures your actual home and, crucially, records what can be evidenced. A filled cavity with no drill marks, paperwork or borescope check must be recorded as unfilled. A boiler with a readable model plate is looked up in the product database rather than taken from an age table. Documentation matters: the same home with the same measures can land on different ratings depending on what the assessor could prove on the day.
That is also why the tool's figures are illustrative. Use it to learn which levers matter for a home like yours; use a survey to find out where your home actually stands.
Our accredited domestic energy assessors survey existing homes across the UK, and record the evidence that gets every point your home has earned.
Get a quoteTwo changes are coming, and they are often confused. The Home Energy Model replaces the current calculation engine behind certificates. Separately, the single A to G rating is being replaced by four metrics: fabric performance, heating system, smart readiness and energy cost. Reformed certificates are targeted for the second half of 2027, and the existing A to G rating continues alongside the new metrics until at least October 2029. The levers on this page keep their physics either way: insulation, efficient heating and solar will matter under any methodology.
Not directly. The software calculates the rating from what the assessor records, so the result only moves when the inputs move. What a good assessor does is record everything the evidence supports: proof of cavity fill, a boiler's model plate, loft insulation photographed at depth. Where there is no evidence, the calculation falls back on age-based assumptions, and those defaults are usually worse than what is actually in the building.
Usually one of three things: a different heating system or fuel, insulation the neighbour could evidence and you could not, or a different dwelling form. Attached homes share party walls that lose little or no heat, so a mid-terrace or flat typically out-rates a detached home of the same age and spec. The explorer on this page lets you test all three.
It depends on the home, which is the honest answer the tool is built to show. On a cavity-walled house, filling the cavities is usually the biggest jump per pound. On homes with a hot water cylinder, a proper insulation jacket costs very little. Completing the switch to LED lighting is cheap on any home. Solid-wall insulation and heat pumps move the rating too, at much higher cost.
Not yet. The current legal minimum for rented homes in England and Wales is band E. The government has confirmed its intention to raise the minimum to C for new tenancies from 2030, subject to legislation, with a cost cap and exemptions. The tool's C marker shows which combinations of measures would clear that bar.
Yes, in two separate ways. The Home Energy Model replaces the current calculation engine behind certificates, and the single A to G rating is being replaced by four metrics covering fabric, heating system, smart readiness and energy cost. Reformed certificates are targeted for the second half of 2027, and the existing A to G rating continues alongside them until at least October 2029.
Related reading: SAP EPC ratings explained (new builds and conversions, with its own explorer), commercial EPC ratings explained, RdSAP vs SAP EPC ratings, EPC rating C, EPC rating D, and EPC changes explained.