Data & Methods8 min read

Discount rate in LCC: real or nominal, and which rate?

The discount rate is the single assumption that moves an LCC result most. The real-versus-nominal distinction, the Fisher relation, typical ranges for public and private projects, the advantage of real terms under high inflation and a sensitivity example.

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The discount rate is the annual rate used to bring a future cost to its present value; in life cycle costing (LCC) it is the single assumption that moves the result most. The short answer: if cash flows are fixed at today's prices a real rate is used, if inflation is built in a nominal rate is used, and the two are never mixed. This article explains what discounting means, the mathematical relation between real and nominal rates, typical rates in the public and private sectors, why working in real terms is safer in high-inflation economies, and how the choice of rate changes the same cash flow.

What discounting means: time preference and opportunity cost

100 units today and 100 units in ten years are not worth the same; today's money can be invested, and money ten years out is uncertain. Discounting expresses that difference in one rate: a cost Cₜ in future year t is brought to today as Cₜ / (1 + r)ᵗ. The higher the rate, the smaller distant costs become. At 3%, 100 units thirty years out are worth 41 today; at 8%, only 10. That is why the discount rate is not a technical detail but a value judgement about the weight of today against the future: a high rate favours the present, a low rate favours the future.

Real or nominal? The Fisher relation

A declining curve drawn on graph paper with a pencil and a ruler resting on it

A nominal rate includes inflation; a real rate has inflation stripped out. The relation between them is the Fisher equation: (1 + n) = (1 + r) × (1 + i), where n is the nominal rate, r the real rate and i expected inflation. With a real rate of 3% and inflation of 2% the nominal rate is about 5.06%; the shortcut "3 + 2 = 5" is an acceptable approximation at low inflation. At 40% inflation the difference becomes visible: the correct nominal rate is 44.2%, the shortcut gives 43%. ISO 15686-5 permits both approaches but sets one rule: real cash flows are discounted with a real rate, nominal cash flows with a nominal rate. Discounting real cash flows with a nominal rate penalises the future twice; the reverse rewards it twice.

Which rate? Typical ranges for the public and private sectors

Public projects use a social discount rate set by national guidance; in Europe typical values sit in the 3–5% real band, and the cost-optimal framework of European building energy performance regulation takes a 3% real rate as the reference for the macroeconomic calculation. In the private sector the rate is derived from the company's weighted average cost of capital (WACC) and is usually higher; a 5–8% real range is common, rising further in risky sectors. The LCC credits of green building certifications mostly do not prescribe the rate but require it to be justified. Whichever rate is chosen, three things go into the report: the source of the rate, whether it is real or nominal, and the sensitivity result at no fewer than one alternative rate.

The advantage of real terms under high inflation

A nominal model asks for an inflation forecast for every year of the analysis period; in a 50-year building LCC that means 50 forecasts. In an economy where inflation has swung between 10% and 70% over the years, none of those forecasts is reliable, and the difference between two alternatives ends up depending on the inflation scenario more than on the products themselves. A real model removes that uncertainty in one move: all prices are fixed at the same date, general inflation leaves the model and only the real rate is entered. Relative price changes — energy rising faster than general inflation, for instance — are modelled separately as a real escalation rate, a far narrower assumption than an inflation forecast. In economies such as Turkey, working in real terms is not a preference but the condition for a meaningful result.

How the rate changes the result: one cash flow, five rates

A simple item: initial investment 100, annual operating cost 4 for 30 years, a replacement of 60 in year 15. NPC with end-of-year discounting:

  • 0% (undiscounted) — NPC 280
  • 1% real — NPC ≈ 255
  • 3% real — NPC ≈ 217
  • 5% real — NPC ≈ 190
  • 8% real — NPC ≈ 164

While the investment stays at 100, the 180 units of future cost shrink to 64 at 8%. The consequence: a high rate makes operating and replacement costs invisible and favours alternatives with low investment and high running cost; a low rate favours efficient but expensive alternatives. When two alternatives are compared, the real question is not "what is the NPC?" but "at which rate does the ranking change?". That threshold rate is given in the report; if it lies well outside realistic rates the result is robust, and if it lies inside the range the decision cannot be taken independently of the rate.

Common mistakes

  • Discounting real cash flows with a nominal rate (or the reverse) — the result is systematically biased
  • Burying energy price growth inside the discount rate — escalation is a separate parameter
  • Using the company's borrowing rate directly as the discount rate — the cost of debt is only one part of WACC
  • Choosing the rate without justification and reporting a single rate — ISO 15686-5 requires a source and a sensitivity
  • Applying different rates to different alternatives — a single rate is the basic condition of comparison
  • Mixing end-of-year and mid-year discounting within one study

Frequently asked questions

What is the difference between a real and a nominal discount rate?
A nominal rate includes inflation, a real rate does not. They are linked by the Fisher equation: (1 + nominal) = (1 + real) × (1 + inflation). The real rate is applied to cash flows fixed at today's prices, the nominal rate to cash flows inflated over time. Built consistently, both give the same NPC.
What is a typical discount rate for LCC?
For public projects European guidance typically uses a 3–5% real band; the cost-optimal calculation of building energy regulation takes 3% real as the reference in the macroeconomic perspective. In the private sector, WACC-derived real rates of 5–8% are common. The rate is always reported with its source and a sensitivity at no fewer than one alternative rate is shown.
How is LCC calculated in a high-inflation country?
In real terms. All prices are fixed at one base date, general inflation is removed from the model and a real discount rate is used. For items that move differently from general inflation, such as energy, a real escalation rate is entered separately. That removes the need for 30–50 years of inflation forecasts, and the result reflects the difference between products rather than the inflation scenario.
Can the discount rate be set to zero?
Technically yes; the result is then the undiscounted total cost and weights the future equally with the present. It is the extreme case that most favours long-lived, low-running-cost alternatives. It is useful as a bounding scenario in a sensitivity analysis, but reporting it as the headline result does not square with the ISO 15686-5 requirement to justify the rate.
How does energy price growth relate to the discount rate?
It is not related and should be kept separate. The discount rate expresses the time value of money; the escalation rate expresses how one item's price moves relative to the general price level. In a real model an annual real escalation of, say, 1–2% is entered for energy; the discount rate is unaffected. Squeezing both into one rate hides both assumptions.

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