A standardised guess at how much energy a building needs
An Energy Performance Certificate rates a property from A, the best, to G, the worst, and is valid for ten years. You need one to sell, to let, or on completing a new building, and it has to be ordered before the property goes on the market.
The important word is standardised. An assessor is not measuring your energy use. They are recording the building itself, the walls, the windows, the insulation, the heating system, and running it through a model that assumes a standard household living a standard life. That is deliberate, because a rating that moved depending on whether the occupant liked the heating on would tell a buyer nothing about the house.
Which is why the certificate is a poor predictor of your own bills and a decent comparison between two properties. It answers "which of these is cheaper to run", not "what will I pay".
The one place it has teeth
For most people the EPC is paperwork. For landlords in England and Wales it is a gate: a property rated below band E cannot be let under the minimum energy efficiency standard unless an exemption has been registered, and that has covered all relevant tenancies since April 2020. A band F or G property is not a poor investment, it is one you are not permitted to rent out.
Everybody else measures it too, and nobody agrees which way is up
If you have moved between countries, or you are reading advice written in another one, this is the thing that will mislead you. Five schemes, five scales, and they do not point the same way.
The reason for the split is that the schemes are answering two different questions. A band, a score or a star count is a measure of efficiency, so more is better. A HERS Index number and an EnerGuide figure are measures of consumption, so less is better. Once you know which kind you are looking at, the direction stops being arbitrary.
Two specifics catch people out. In the United States, a HERS 60 is a good house and a Home Energy Score of 6 is a middling one, and both are sixty-ish numbers on American homes. And in Canada, EnerGuide used to run 0 to 100 with higher being better until that was retired at the end of 2018, so anything written before then means the opposite of what a current rating means. The two scales cannot be converted, because the newer one counts energy uses the older one ignored.
Australia is the one with a legal floor comparable to ours, but at the other end of a property's life: the 2022 National Construction Code raised new homes from six stars to seven, adopted from October 2023 in most states. Britain regulates what you may rent out, Australia regulates what you may build.
Improving it costs money, so start with where the heat is going
Every one of these schemes rewards the same things, because they are all modelling the same physics: keeping heat in, and needing less of it in the first place. What none of the certificates tells you is which of those is worth doing on your particular building.
The heat loss calculator is the honest starting point, because it splits the loss between the fabric and the air changes, and those have completely different fixes. From there, the energy bill estimator puts the difference into money, the boiler sizing tool covers what happens when the heat demand drops and the existing boiler is suddenly far too big for it, and solar payback handles the generation side that the newer whole-of-home ratings have started counting.
Common questions
How long does an EPC last?
Ten years, and it belongs to the property instead of to you. So a house sold twice in five years keeps the same certificate, and buying somewhere does not oblige you to get a new one. You can look up whether a valid certificate already exists before paying for another, which catches a lot of people out: the assessment gets ordered as a matter of routine when a perfectly valid one is already on the register.
When do I legally need one?
Selling, letting, or completing a new building, and it has to be ordered before the property is marketed rather than produced at the point of sale. There are fines for not having one when you need one. The letting case carries a second requirement on top: in England and Wales a property below band E cannot be let at all under the minimum standard rules unless a valid exemption is registered, which has applied to all covered tenancies since April 2020.
Does the rating mean my bills will be that high?
No. The assessment is made on standardised assumptions about occupancy, heating hours and temperatures, because the point is to let two properties be compared with each other. That is the most common misreading of the whole document. Your actual bills depend on how many of you there are, what temperature you keep the place at and when you are in. Two identical flats with the same rating can be hundreds apart in real spending, and that is the scheme working as designed rather than failing.
What is the American equivalent?
There are two, they are not the same thing, and they run in opposite directions. The Department of Energy Home Energy Score is one to ten with ten being the most efficient, so higher is better. The RESNET HERS Index sets a reference home at 100 and a net zero home at 0, so LOWER is better, and each point represents about one per cent of energy use. Neither is a general legal requirement the way an EPC is, though some cities and states require disclosure at sale.
What does Australia use?
A NatHERS star rating, from zero to ten stars, where more stars means less energy needed to heat and cool. Since the 2022 National Construction Code took effect, adopted from October 2023 in most states and territories, new homes have to reach the equivalent of seven stars, up from six. Note what it covers: the star rating is about the thermal performance of the building fabric, and the Code added a separate whole-of-home rating out of 100 for the appliances, hot water and any solar.
And Canada?
EnerGuide, and it changed in a way that inverts the answer, so old advice is actively misleading. Ratings used to run on a 0 to 100 scale where higher was better. That was retired at the end of 2018, and evaluations since then are expressed in gigajoules per year, which is a measure of consumption, so a lower number is now the good one. The two cannot be converted between, because the newer rating counts energy uses the old one did not.