Primary Energy

What is primary energy and how is it different from delivered energy?

Primary energy is the raw energy extracted from nature before any processing or delivery—coal in the ground, natural gas in the well, or solar radiation at the Earth's surface. Delivered energy (also called final energy) is what arrives at your building's meter or thermostat. The difference between them represents all upstream losses: fuel extraction, refining, generation of electricity, transmission across power lines, and distribution to your building.

When your building consumes one kilowatt-hour of grid electricity, that single kilowatt-hour required significantly more primary fuel to generate and deliver. A coal or gas power plant converts fuel to electricity with roughly 40–50% efficiency, transmission losses add another 5–10%, and the building itself adds losses in boilers or electrical conversion. In contrast, consuming natural gas directly in a building boiler involves less processing—direct extraction, minimal processing, and local delivery. This is why primary energy, not just delivered energy, gives a complete picture of resource consumption.

How is primary energy calculated?

Primary energy is derived from a simple formula:

Primary Energy (kWh) = Delivered Energy (kWh) × Primary Energy Factor (PEF)

The Primary Energy Factor is a dimensionless coefficient specific to each energy carrier. It accounts for extraction, processing, storage, transportation, generation losses, transmission losses, and distribution losses. In Slovak building energy certificates, the calculation follows this sequence:

1. Measure or model the delivered energy for each end-use (heating, hot water, ventilation, cooling, lighting) based on the building's thermal properties, system efficiency, and occupancy patterns.
2. Apply the appropriate PEF for each energy source (natural gas, grid electricity, biomass, district heating, renewable electricity).
3. Sum the primary energy across all end-uses to obtain the total annual primary energy demand in kWh per square meter of heated floor area (kWh/(m²·year)).

Different energy sources have different PEFs. Natural gas, delivered locally with minimal processing, typically has a PEF of 1.0–1.3. Grid electricity, which requires power plant generation and long-distance transmission, has a PEF of 2.0–3.5 depending on the grid's fuel mix and efficiency. Renewable electricity consumed on-site (solar or wind generated and used immediately) receives a PEF near zero because no extraction or transmission losses occur.

Why do we use primary energy instead of just measuring delivered energy?

Using delivered energy alone would create perverse incentives. A building heated with a high-efficiency heat pump consuming three kilowatt-hours of electricity to deliver 12 kilowatt-hours of heat would appear less efficient than one consuming 10 cubic meters of natural gas directly—even though the heat pump extracts more useful energy per unit of primary fuel burned. Without primary energy metrics, building codes and labeling schemes would penalize electrification and reward fossil-fuel direct use, contradicting climate goals.

The European Union's Energy Performance of Buildings Directive (EPBD) mandated primary energy as the headline metric precisely to solve this problem. By converting all energy sources to primary equivalents, the directive ensures that nearly-zero-energy buildings, passive houses, and other high-efficiency designs are evaluated fairly. A building powered by renewable electricity is no longer artificially penalized; its low primary energy demand reflects its true resource consumption.

Furthermore, primary energy factors decline as grids decarbonize. As a grid shifts from coal and gas toward wind and solar, its average PEF for grid electricity drops. Installed heat pumps and electric heating systems automatically show improved energy performance—without any physical retrofit—because less primary fuel is needed to generate their electricity. This creates a policy mechanism that rewards electrification as renewable infrastructure matures.

How does primary energy relate to energy classes in Slovak buildings?

In Slovakia, the energy certificate (energetický certifikát budovy) displays a building's primary energy demand as its headline performance metric, determining its energy class rating from A0 (most efficient) to G (least efficient). The classification is essential for legal compliance, building valuation, and market transparency.

New residential buildings constructed after 1 January 2016 must achieve a minimum of energy class A1, typically corresponding to a primary energy demand of approximately 54 kWh/(m²·year) or lower. Buildings completed between 2006 and 2016 must meet at least class B. Older buildings (pre-2006) need not meet a minimum class unless undergoing major renovation; however, any building listed for sale or rental is required to have a current energy certificate displayed to buyers or tenants.

The energy class thresholds step progressively. Each class boundary represents a specific primary energy threshold; crossing into a better class typically requires substantial improvements to insulation, windows, heating systems, or renewable energy integration. For passive-house designs, which achieve energy class A0 or A1 with minimal operational heating demand, the primary energy benefit is maximized by combining ultra-low heat demand with highly efficient renewable or district heating.

What are primary energy factors for different energy sources?

Primary energy factors vary widely because different supply chains have different inefficiencies. A building specifying its energy source during design (natural gas boiler, district heating, heat pump on renewable grid, on-site solar) must understand how each choice affects its final primary energy rating.

Natural gas, piped directly to the building with minimal processing, has the lowest PEF among conventional fuels (typically 1.1–1.3). District heating is more variable: a modern district heating system fed by waste heat recovery or biomass may have a PEF of 0.6–1.0, while a coal-fired district heating plant has a PEF of 1.5–2.0. Grid electricity in Central Europe (including Slovakia) historically carried a PEF of 2.5–3.0 due to coal and nuclear generation; as renewables scale, this factor is expected to decline toward 1.5–2.0 by 2030 and lower thereafter.

Renewable electricity consumed on-site (solar, micro-wind, or small-scale hydro) receives a PEF of 0.0–0.2 because the energy is harvested and used without transmission losses. This creates a strong financial and environmental case for pairing heat pumps with on-site photovoltaic systems in new construction and renovation. A passive-house design powered by a heat pump drawing from on-site solar achieves dual benefits: ultra-low delivered energy demand plus minimal primary energy per unit delivered.

How does primary energy affect building design and renovation decisions?

Primary energy calculations drive crucial design decisions. Because grid electricity carries a high PEF, using it directly for heating (resistive electric heating) is rarely the most efficient choice; however, heat pumps extract environmental heat and amplify its utility, making electricity far more efficient for heating. Conversely, direct gas heating carries lower primary energy per unit delivered, but it locks in fossil-fuel dependence and precludes future grid decarbonization benefits.

In renovation, achieving the next lower energy class typically requires multiple interventions: upgrading insulation (reducing delivered heating energy), replacing windows (lowering heating loss), improving airtightness (reducing infiltration loss), and often integrating renewable energy or switching to heat pumps (leveraging low-PEF renewable electricity). Each measure reduces delivered energy; the choice of energy source then amplifies this reduction via its primary energy factor.

For architects and building owners in Slovakia, primary energy is not merely a certification metric—it is the foundation of building economics and climate performance. A renovation that reduces primary energy from 150 kWh/(m²·year) to 60 kWh/(m²·year) typically yields faster payback through lower operational costs, improved tenant comfort via higher indoor temperatures and lower drafts, and substantial carbon reduction because less total fuel—from extraction to combustion—is needed annually.