Primary Energy, Explained

Primary energy is the total energy needed to supply a home, including losses in generating and delivering fuel or electricity, not just the energy delivered to the home. A kilowatt-hour of electricity used at home, for example, required more than a kilowatt-hour of primary energy to generate and transmit, because power stations and networks lose energy along the way. A kilowatt-hour of gas used at home required less additional primary energy, because gas is delivered more directly.

Certificates use primary energy because it allows fair comparison between fuels. Without it, electricity and gas would appear equivalent per delivered kilowatt-hour, hiding the very different upstream losses behind them. Primary energy therefore answers a system question, how much energy does society need to supply this home, rather than a household question, how much energy does this home buy.

Delivered energy, sometimes called final energy, is what a meter records: gas into a boiler, electricity into a home. Primary energy is delivered energy multiplied by a primary energy factor for each fuel, reflecting generation, extraction, processing and network losses. Electricity typically carries a higher factor than gas, because thermal power generation loses a large share of fuel energy as heat, while renewable electricity carries a lower factor in many methods, reflecting its different upstream profile.

Those factors change over time as generation mixes change. As grids add more renewables, the primary energy factor for electricity tends to fall in many national methods, which means the same home, using the same delivered electricity, can show lower primary energy in a later certificate without any physical change. That method effect should be recognised when comparing certificates from different years, and it is a reason to check demand in delivered terms alongside primary terms.

Check which figure a certificate foregrounds. If it shows primary energy, do not compare it directly with a bill, which shows delivered energy, or with a UK EPC energy figure, which is presented on a different basis. Convert or compare like with like, and ask an assessor to explain the basis of any figure you are unsure about, including the factors used.

Check the fuel mix behind a primary energy claim. A measure displacing grid electricity will show a larger primary energy saving than the same delivered saving from gas, because electricity's factor is higher. That difference is real at system level, but it should not be mistaken for a larger delivered saving or a larger bill saving, which depend on tariffs and delivered quantities.

In EU member states, primary energy is central to certificate classes under the EPBD, with national primary energy factors set by each country reflecting its generation mix. A home in a country with a low-carbon grid may therefore show lower primary energy for electric heating than an identical home in a country with a high-carbon grid, even though delivered use is the same, because national factors differ.

In the UK, EPCs do not foreground primary energy in the same way; they present an energy efficiency rating based on modelled costs and an environmental rating based on carbon, with estimated energy use shown separately. A UK reader encountering a primary energy figure, for example on a European certificate or a detailed audit, should treat it as a different metric with its own factors, not as a direct equivalent of a UK rating figure.

One mistake is treating primary energy as a bill. A primary energy figure will usually be higher than delivered energy, especially for electrically heated homes, and comparing it with a bill total will make the home look far more wasteful than it is. Bills should be compared with delivered energy, and primary energy should be compared with primary energy, on the same national factors.

Another mistake is ignoring factor changes over time. A home whose primary energy improves between certificates may have improved physically, or may simply have benefited from a lower electricity factor in a newer method. Checking delivered demand alongside primary energy separates building improvement from method improvement, and prevents a method change being celebrated as a retrofit success.

Keep certificates showing primary energy, delivered energy where available, fuel mix, and method date, so factor changes can be spotted. If a future certificate shows a primary energy change, compare delivered figures first, and ask whether the change reflects building work, fuel switch, or factor update.

Ask an assessor or auditor to explain primary energy in the context of your home: which fuels contribute most, what factors were used, and how the figure would change with a fuel switch versus a fabric improvement. That explanation turns an abstract system metric into a useful decision aid.