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Home energy demand is estimated using simplified heat loss models: Heat loss (W) = U-value (W/m²K) × Area (m²) × ΔT (°C), where U-value reflects insulation quality and ΔT is the indoor-outdoor temperature difference.
Estimated annual heating demand = Heat loss rate × Heating degree days × 24 ÷ (1,000 × system efficiency)
Heating degree days vary by climate: approximately 2,000–3,000 in temperate northern Europe; 1,000–2,000 in Mediterranean climates; 3,000–5,000 in colder continental climates.
Reference U-values (W/m²K): uninsulated solid wall ≈ 2.1; cavity wall with insulation ≈ 0.45; well-insulated loft ≈ 0.13; single glazing ≈ 5.7; double glazing ≈ 2.0; triple glazing ≈ 0.8.