Building LCC and EN 15459-1 global cost: the cost-optimal calculation

Global cost is a single present value found by summing the discounted investment, operating, energy, maintenance and replacement costs of a building or building energy system over the calculation period and subtracting the residual value; EN 15459-1 defines the procedure and underpins the cost-optimal framework of European building energy performance regulation. The cost-optimal level is the energy performance level at which global cost is lowest. This article covers the formula, the two perspectives, the calculation period rules, the link to EN 16627, an insulation thickness example and two practical difficulties the calculation meets in Turkey.

What the cost-optimal framework requires

The European Energy Performance of Buildings Directive (EPBD) requires member states to set minimum energy performance requirements at the cost-optimal level; the comparative methodology is given in a delegated regulation and the calculation engine is EN 15459-1. The method has three steps: reference buildings are defined, packages of energy-efficiency measures are built for each, and the primary energy consumption and global cost of every package are calculated. The result is a curve: primary energy on the horizontal axis, global cost on the vertical; the lowest point of the curve is the cost-optimal level. Requirements to the right of that level (less efficient) breach the regulation; those to the left are voluntary targets.

The global cost formula

EN 15459-1 defines global cost as Cg(τ) = Cᵢ + Σⱼ [ Σᵢ₌₁..τ ( Cₐ,ᵢ(j) × R_d(i) ) − V_f,τ(j) ]. Here Cᵢ is the initial investment, Cₐ,ᵢ(j) the annual cost of component j in year i (energy, maintenance, operation and any replacement falling in that year), R_d(i) the discount factor for year i and V_f,τ(j) the residual value of component j at the end of calculation period τ. The structure is the same as the NPC logic of ISO 15686-5; the difference is that cost is aggregated per component and energy cost sits at the centre. Energy cost is calculated as annual energy need × unit price and carried forward with a price development scenario (real escalation). The discount factor R_d(i) = 1 / (1 + r)ⁱ is computed with a real rate; the delegated regulation takes 3% real as the reference in the macroeconomic calculation and requires a sensitivity with no fewer than two rates.

The financial and the macroeconomic perspective

The calculation is done from two perspectives, and they answer different questions. The financial perspective is the cost seen by the building owner: prices are taken including taxes and VAT, subsidies may be deducted and the discount rate reflects the owner's cost of capital. The macroeconomic perspective is the cost seen by society: taxes and subsidies are excluded, but the cost of greenhouse gas emissions is added through a carbon price and a social discount rate is used. The cost-optimal level can differ between the two; the carbon price and the lower social rate shift the macroeconomic optimum towards the more efficient side. For a decision such as insulation thickness this is the numerical counterpart of the gap between "optimum for the owner" and "optimum for society", and the perspective a regulation is written from sets how strict its requirements are.

Calculation period: 30 years residential, 20 years commercial

The cost-optimal framework fixes the calculation period by building type: 30 years for residential and public buildings, 20 years for commercial buildings. Because long-lived components such as the envelope and insulation outlast that period, residual value is an inseparable part of the calculation; an insulation layer with a 50-year life has two fifths of its life left after 30 years, converted to value by linear depreciation. Systems with a 15–20-year life such as boilers, heat pumps and lighting are replaced once within the period. Without those two mechanisms a systematic bias arises against insulation and in favour of technical systems; compared with the 50–60-year building RSP of EN 16627, a 30-year period already rewards envelope investments less.

Example: global cost for four insulation thicknesses

Four insulation thicknesses for 1 m² of external wall; calculation period 30 years, real discount rate 3%, 2% annual real escalation in energy price, no replacement because the insulation outlives the period. The figures are illustrative values chosen to show the method.

• 5 cm — investment €18/m², first-year energy cost €9.0/m²: global cost ≈ €251/m²
• 10 cm — investment €26/m², energy €6.2/m²: global cost ≈ €187/m²
• 15 cm — investment €34/m², energy €5.2/m²: global cost ≈ €169/m²
• 20 cm — investment €42/m², energy €4.7/m²: global cost ≈ €164/m²
• With zero escalation: 194 / 148 / 136 / 134 €/m² — the optimum flattens between 15 and 20 cm

The example shows two things. First, the cost-optimum is a plateau rather than a point: between 15 and 20 cm global cost moves by a few euros, which is why regulation is usually written as a range within the plateau. Second, the energy price scenario shifts the optimum: with 2% real escalation 20 cm leads clearly, while with zero escalation the gap falls to two euros. Global cost is therefore always reported with at least two energy price scenarios and two discount rates; a single figure cannot justify a decision without the scenario it was produced under.

The link to EN 16627 and the Turkish context

EN 15459-1 focuses on energy-related systems and energy cost; EN 16627 covers the whole building with its module structure (A0, A1-A5, B1-B7, C1-C4, D). For an insulation thickness or heating system decision the EN 15459-1 global cost is the basis; for the holistic economic assessment of a building design it is the EN 16627 LCC; the two share the same discounting and residual value logic and can be produced from the same cash flows. Energy cost falls into module B6 in EN 16627, so global cost can be read as a B6-weighted subset of a module-based LCC. In Turkey the national calculation method BEP-TR and the insulation standard TS 825 provide the energy need; the hard part of the calculation is the cost input. Building reliable long-term assumptions for the real path of energy prices and for the discount rate is difficult; that uncertainty is managed by working in real terms and reporting several escalation scenarios for energy price.