How the EPC C checker works
The checker gives an estimate. It is not an EPC, which only an accredited assessor can produce after visiting the property. This page explains every step and where each figure comes from, so you can judge the result for yourself.
1. The rules it checks against
In January 2026 the government confirmed that privately rented homes in England and Wales must reach EPC C, or a registered exemption, by 1 October 2030 (government response). The checker uses these confirmed points:
- Band C starts at a score of 69.
- The most a landlord has to spend is £10,000 per property, or 10% of the property's value where it's worth less than £100,000 (Hamptons summary, Mortgage Solutions).
- Qualifying spending from 1 October 2025 counts towards that cap. If the cap is spent and the home still can't reach C, a cost cap exemption lasts 10 years.
- The maximum fine rises to £30,000 per property, per breach (Pinsent Masons).
- Homes rated C or above on the current scale before 1 October 2029 are treated as compliant until that EPC expires.
2. The starting score
The best starting point is the score on the property's current EPC, which anyone can look up free on the government EPC register. If you only know the letter, the checker uses the middle of that band. If you don't know either, it makes a rough estimate from your answers about the property, and says so clearly in the result.
3. Turning upgrades into points
An EPC score isn't a simple tally of features. It comes from the home's modelled energy cost per square metre of floor area, called the energy cost factor (ECF). The SAP methodology behind every EPC converts the ECF into a score with two published equations (source):
If ECF is 3.5 or more: score = 117 - 121 x log10(ECF) If ECF is under 3.5: score = 100 - 13.95 x ECF
The checker runs those equations in reverse to get the ECF from your current score. Each upgrade then cuts the modelled energy cost by a typical percentage, and the new ECF is turned back into a score. This matters for two reasons:
- The same upgrade adds more points to a poorly rated home than a well rated one, as it does on a real EPC.
- Upgrades are applied one after another, so stacking them doesn't double count the savings.
Each step shows a range, from 70% to 130% of the typical cut, because real results depend on the property.
The typical cut for each upgrade
These percentages are an accredited assessor's judgement of typical results, informed by published savings figures. They aren't from a single official table, so treat them as estimates.
| Upgrade | Typical cost cut | Points from D61 | Points from E45 |
|---|---|---|---|
| Loft insulation to 270mm (from bare) | 20% | +8 | +11 |
| Loft insulation top-up to 270mm (from under 100mm) | 7% | +3 | +4 |
| Loft insulation top-up to 270mm (from 100 to 200mm) | 2.5% | +1 | +1 |
| Cavity wall insulation | 18% | +7 | +10 |
| Internal solid wall insulation | 22% | +9 | +12 |
| Suspended timber floor insulation | 5% | +2 | +3 |
| Replace single glazing with A-rated double glazing | 7% | +3 | +4 |
| New A-rated condensing gas boiler | 18% | +7 | +10 |
| Modern high heat retention storage heaters | 25% | +10 | +14 |
| Replace panel heaters with high heat retention storage heaters | 40% | +16 | +22 |
| Full heating controls (programmer, room stat, TRVs) | 8% | +3 | +4 |
| Complete heating controls | 3% | +1 | +2 |
| Hot water tank jacket and pipe lagging | 3% | +1 | +2 |
| Low energy (LED) bulbs throughout | 3% | +1 | +2 |
| Swap remaining bulbs to LED | 1.2% | +0 | +1 |
| Solar panels (about 3.5kWp) | 22% | +9 | +12 |
4. Choosing the cheapest route
The checker only considers upgrades that apply to the property, based on your answers. At each step it picks the upgrade with the lowest cost per point from where the home is at that moment, until the typical result reaches 69. Anything else that could help is listed separately.
Where you answered "not sure", the checker fills the gap from the property's age. For example, it assumes a 1950s house has unfilled cavity walls. Upgrades that depend on a guess like this are tagged "check first" in the result.
5. Where the costs come from
| Upgrade | Source |
|---|---|
| Loft and cavity wall insulation | Energy Saving Trust figures, via Rightmove and HomeOwners Alliance |
| Solid wall insulation | Energy Saving Trust (about £12,000 internal, £18,000 external, for a typical semi) |
| Suspended floor insulation | Energy Saving Trust (£1,400 to £2,500) |
| Double glazing | Energy Saving Trust figure, via EcoWatch |
| Boiler replacement | Energy Saving Trust figure, via Rightmove |
| Solar panels | Which?, citing Energy Saving Trust |
| Heating controls, storage heaters, tank jackets, LED bulbs, loft top-ups | Estimates. No single published source. Marked "estimate" in the results. |
Costs for solid wall insulation and glazing are published for a typical semi-detached house. For other property types, the checker scales them by a size factor (flat 0.45, mid-terrace 0.65, semi 1, detached 1.5, bungalow 0.9). All costs are before grants.
6. Bills before and after the upgrades
The checker already works out how much the chosen upgrades cut the home's modelled energy cost (step 3). It applies that same percentage cut to the part of the bill that goes on heating and hot water.
- Prices: the Ofgem price cap for 1 October to 31 December 2026, GB average, paying by Direct Debit: electricity 26.32p per kWh and 54.83p a day, gas 7.97p per kWh and 29.68p a day. Electricity has 0% VAT from 1 October 2026 to 31 March 2027 (Ofgem).
- Typical bill: if you don't enter your own bill and the home has a gas boiler, the checker uses Ofgem's typical usage of 2,500 kWh of electricity and 9,500 kWh of gas a year (Energy UK). At the prices above that comes to £1,723 a year, the typical bill quoted for this price cap. For electric, oil, LPG or communal heating there's no comparable typical figure, so you need to enter your own bill.
- The split: standing charges are paid whatever you use, so they're taken off first. Appliances, lighting and cooking are taken as the typical 2,500 kWh of electricity, about £658 a year. What's left is treated as heating and hot water, and that's the part the upgrades cut. The small saving from LED bulbs is folded into the same percentage, so it's only approximate.
- The range: the low and high savings use the same low and high estimates as the points (70% and 130% of each upgrade's typical cut).
Heating and hot water = bill - standing charges - appliances, lighting and cooking Saving = that amount x (1 - modelled cost after / modelled cost before)
Treat the result as a guide. EPC savings assume a standard household heating the home to a standard temperature. Many homes, especially poorly insulated ones, are kept cooler than that, so real savings are often smaller. Some of the benefit then shows up as a warmer home rather than a lower bill.
Upgrades don't change what appliances cost to run. To see those, use the appliance running costs.
7. What the checker can't do
- It can't see the property. A survey will find things the questions can't, good or bad.
- It uses the current EPC scale. The 2030 standard will be judged on new EPC metrics, which haven't been fully published. Results are a guide to the direction and rough size of the work.
- It doesn't cover heat pumps, room-in-roof insulation, flat roof insulation or community heating in detail.
- It isn't legal advice on whether a property is compliant or exempt.