Entendiendo la eficiencia energética

Understanding energy efficiency requires distinguishing three concepts that are frequently confused: demand, consumption, and primary energy. Energy demand is the amount of energy the building needs to maintain indoor comfort conditions (20-21°C in winter, 25-26°C in summer, per EN 16798-1); it depends on the envelope (insulation, windows, airtightness), internal gains (occupants, equipment, lighting), and climate conditions. Energy consumption is the energy actually supplied to the building to meet that demand; it depends on the efficiency of the generation, distribution, emission, and control systems. A building with a heating demand of 50 kWh/m²·year and a condensing boiler with a seasonal efficiency of 95% consumes 52.6 kWh/m²·year of gas; the same building with an old atmospheric boiler at 70% efficiency consumes 71.4 kWh/m²·year: 36% more gas to meet the same demand. Primary energy additionally accounts for extraction, transformation, and transport losses: the conversion factor in Spain is 1.19 for natural gas and 1.954 for peninsular electricity (IDAE, 2016).

Energy efficiency operates at all three levels. At the demand level (reducing the need): improving the insulation of a wall from U = 1.5 W/m²·K (uninsulated brick wall, typical of pre-1979 buildings in Spain) to U = 0.27 W/m²·K (with 10 cm of graphite EPS ETICS) reduces heat loss through the wall by 82%. At the consumption level (meeting the demand with less energy): replacing an oil boiler with 75% efficiency with an air-source heat pump with an SCOP of 4.0 reduces final energy consumption by 72% (and primary energy consumption by 67%, switching from oil with a factor of 1.18 to electricity with a factor of 1.954 but divided by a COP of 4.0). At the primary energy level (decarbonizing the source): powering that heat pump with renewable electricity (primary energy factor of 0 for self-consumed photovoltaics) eliminates operational emissions entirely. The combination of all three levels is what enables the transition from a class G building (250-400 kWh/m²·year of primary energy) to a class A building (15-30 kWh/m²·year), a reduction of 90-95%.

The thermal envelope (walls, roof, ground floor, windows) is responsible for 50-70% of a building's energy losses in temperate climates (Perez-Lombard et al., 2008). The key metric is thermal transmittance (U-value), which measures heat flow per unit area per unit temperature difference, expressed in W/m²·K. Typical values for the Spanish building stock are: uninsulated walls 1.2-2.0 W/m²·K (buildings predating the NBE-CT 79 code), walls with basic insulation 0.5-0.8 W/m²·K (buildings from 1980-2006), CTE 2006 walls 0.3-0.6 W/m²·K, and CTE 2019 walls 0.27-0.56 W/m²·K. The Passivhaus standard requires U ≤ 0.15 W/m²·K for walls in cold climates. Each 0.1 W/m²·K reduction in the transmittance of a 100 m² wall reduces annual heat losses by 500 to 1,500 kWh/year (depending on climate heating degree days), equivalent to savings of 40-120 EUR/year in natural gas heating. Thermal bridges — junctions between building elements where insulation is interrupted or reduced (slab edges, shutter boxes, window frames) — can account for 15-35% of total heat losses in an internally insulated building.

Windows are the envelope element with the highest transmittance and the greatest potential for improvement. An aluminum window without thermal break and single glazing — present in 45% of Spanish homes built before 2006 — has a U-value of 5.7 W/m²·K: one square meter of this window loses 10 times more heat than one square meter of wall insulated to U = 0.5 W/m²·K. Replacing it with a PVC window with thermal break and double low-emissivity argon-filled glazing (window U-value = 1.2-1.4 W/m²·K) reduces losses by 75-80%. Air tightness is the second critical factor: uncontrolled air infiltration through window joints, shutter boxes, service penetrations, and construction joints generates losses of 15-40 kWh/m²·year in conventional buildings (equivalent to 20-40% of heating demand). Airtightness is measured using a pressurization test (Blower Door, EN 13829 / ISO 9972) that quantifies air changes at 50 Pa pressure (n₅₀): a conventional Spanish building typically shows n₅₀ of 5-15 ach, one retrofitted with joint sealing reaches 2-4 ach, and Passivhaus requires ≤ 0.6 ach. The difference between n₅₀ = 10 and n₅₀ = 1.0 is equivalent to heating savings of 20-40 kWh/m²·year.

Once demand has been reduced through the envelope, HVAC, DHW, and lighting installations determine the efficiency with which that residual demand is met. System performance is expressed through seasonal indicators that reflect actual behavior over a full year, not just design conditions. For heating: SCOP (Seasonal Coefficient of Performance) for heat pumps (typical values: 3.0-5.0 for air-source, 4.0-5.5 for ground-source), seasonal efficiency η_s for boilers (condensing: 92-98% on GCV; conventional: 75-85%). For cooling: SEER (Seasonal Energy Efficiency Ratio) for compression units (A+++ class: 8.5; class B: 4.4). For DHW: seasonal COP of a dedicated heat pump (2.5-3.5) or solar fraction of the solar thermal system (40-70% in Spain). The combination of a high-performance envelope with an efficient heat pump can reduce primary energy consumption by up to 85-90% compared to a building with a poor envelope and an oil boiler: from 250 kWh/m²·year to 25-40 kWh/m²·year.

Lighting accounts for 10-15% of residential electricity consumption and 20-30% of consumption in non-residential buildings (offices, retail, hotels). Replacing incandescent lamps (12 lm/W, banned in the EU since 2012) or halogens (15-20 lm/W, banned since 2018) with LEDs (80-150 lm/W) reduces lighting consumption by 70-85%. Lighting control systems (occupancy detection, daylight-linked dimming with illuminance sensors, proportional dimming) deliver an additional 20-40% savings in non-residential buildings. Appliances and electronic equipment (standby, refrigerators, washing machines, computers) account for another 15-25% of residential consumption: a class A refrigerator (2021 label) consumes 90-130 kWh/year, compared to 350-500 kWh/year for a class F model from 15 years ago. Disaggregated electricity monitoring through smart meters and home management apps enables identification of inefficient appliances and quantification of actual savings, with documented reductions of 5-15% in electricity consumption simply from the behavioral feedback effect on users (Darby, 2006).

The energy performance certificate (EPC) is the standardized tool for understanding a building's efficiency. In Spain, the EPC has been mandatory for sale and rental since 2013 (Royal Decree 235/2013, replaced by RD 390/2021) and classifies buildings on a scale from A (most efficient) to G (least efficient) based on two indicators: non-renewable primary energy consumption (kWh/m²·year) and CO₂ emissions (kgCO₂/m²·year). Thresholds vary by climate zone: in Madrid (zone D3), class A requires ≤ 29.5 kWh/m²·year of non-renewable primary energy, class B ≤ 47.7, class C ≤ 77.1, class D ≤ 119.1, class E ≤ 198.5, class F ≤ 262.7, and class G > 262.7. The distribution of Spain's certified building stock in 2023 is revealing: class A 0.3%, B 0.5%, C 2.2%, D 8.7%, E 57.4%, F 11.2%, G 19.7% (MITERD, 2023). This means that 88.3% of certified homes are rated E or worse, a figure that quantifies the scale of the retrofit challenge.

The first steps toward improving the energy efficiency of an existing home or building follow a logical sequence. First, obtain an energy audit (cost: 300-800 EUR for a single-family home, 2,000-8,000 EUR for a multi-family building), which identifies losses, quantifies demand, and proposes measures prioritized by cost-effectiveness. Second, address the envelope: insulate the roof (payback in 3-6 years), seal air infiltrations (payback in 1-3 years), and replace windows if they have single glazing (payback in 8-14 years). Third, upgrade the systems: replace the boiler with a heat pump (payback in 5-10 years), install programmable thermostats (8-15% savings with an investment of 50-200 EUR), and switch to LED lighting (payback in 1-2 years). Fourth, incorporate renewable generation: rooftop photovoltaics (payback in 6-10 years with 30-50% self-consumption and surplus compensation). The total investment across these four phases ranges from 15,000 to 50,000 EUR for a single-family home, with cumulative savings of 60,000-150,000 EUR over 30 years. Understanding energy efficiency is the essential first step toward making informed decisions that benefit the homeowner, the community, and the climate.