Can you trust BIM Energy’s energy calculations? The answer is yes – and there are three reasons to do so: a technical one, a scientific one and an experience-based one. This insight gives the answer from all three angles.

Technically, BIM Energy is built on a full dynamic model that simulates the building’s energy balance hour by hour across all 8,760 hours of the year, accounting for thermal inertia, solar loads, thermal bridges, air leakage, building services and use – a physics-based digital twin of the building.

Scientifically, the calculations have been tested in two independent research studies, funded by SBUF and E2B2 and carried out together with NCC, EQUA and Lund University. BIM Energy’s calculations were compared against actual post-construction measurements in completed buildings – 26 passive houses and an apartment block built in 28 copies, respectively – and in both cases the measured energy use fell within the calculated range. The calculation core is also validated according to ASHRAE 140.

By experience, BIM Energy rests on nearly 40 years of development and is used today by more than 200 companies, nationally and internationally, including leading energy consultants and property companies.

The conclusion is that what the software simulates during the design phase holds true even when the building is completed – or when a renovation has been carried out and the energy use is followed up.

The question every client asks

When you use an energy calculation program as the basis for investment and design decisions, there is one fundamental question that has to be answered: can I trust the figures? Will what the software calculates hold true once the building is finished and the energy is measured in operation?

It is a reasonable question, and it deserves a concrete answer. This text is meant to give that answer – not through claims, but by explaining how BIM Energy is built on a technical level, and by showing how its calculations have been tested against actual post-construction measurements in completed buildings in two independent research studies.

An established player with deep industry roots

BIM Energy (the continued development of VIP Energy) rests on nearly four decades of experience in energy calculation for buildings. The tool and its predecessors have been developed and refined over more than 30 years, and are used today by more than 200 paying companies – nationally as well as internationally.

Among the users are some of the most highly regarded energy consultancies in the industry and several of the largest property owners in the region. That spread and market foothold says something in itself about the confidence placed in the tool: it is not a niche solution but an established standard tool that the industry’s own experts rely on in their daily work.

But market foothold and experience are indirect arguments. The real reasons to trust the calculations lie in how they are made – and in whether they hold up against reality. That is what the following sections show.

How calculations are carried out in BIM Energy

BIM Energy uses a full dynamic energy calculation model in which the building’s energy balance is simulated hour by hour across all 8,760 hours of the year. Unlike simplified static methods, it calculates how the building responds to changes in weather, solar radiation, use and operation over time. Climate data is taken from climate files with hourly values for outdoor temperature, solar radiation, wind speed, wind direction and humidity.

The model is based on a three-dimensional representation of the building in which geometry, orientation, window areas, floor levels and building envelope are defined. The building’s location and orientation are used to calculate solar loads and shading throughout every hour of the year, where solar radiation is divided into direct, diffuse and ground-reflected radiation and affects heat transfer through windows and other exposed surfaces.

A central part of the calculation is accounting for the building’s thermal inertia. All materials are described by thermal conductivity, density and heat capacity, which lets BIM Energy calculate how heat is stored in walls, floor slabs, roofs and other building elements and later released back into the building. This means a heavy concrete building behaves differently in energy terms than a lightweight timber building – something that affects comfort, peak power demand and energy use alike.

Heat losses through the building envelope are calculated from the real or standardised build-up of the constructions. In addition to ordinary transmission losses, linear thermal bridges are also accounted for: the model identifies the lengths of the thermal bridges and uses ψ-values to calculate the extra heat loss that arises at junctions between, for example, walls, floor slabs, roofs and windows, and includes them directly in the energy balance. Air leakage and infiltration are simulated dynamically based on the building’s air-tightness, wind influence and thermal driving forces – including the so-called stack effect, where temperature differences between inside and outside create pressure differences that drive air through leaks. This gives a more realistic picture of energy losses than standardised methods.

Beyond the envelope, internal heat loads from people, activities, lighting, equipment and domestic hot water are calculated, while ventilation, heat recovery, heating, cooling and heat pumps are simulated hour by hour according to the prevailing operating conditions. The energy balance thereby captures the interplay between the building, its services and the activity taking place inside it.

Calibration against measured data for existing properties

For existing properties, the model can also be calibrated against measured energy data. BIM Energy then uses optimisation algorithms to adjust parameters such as U-values, thermal bridges and infiltration so that the simulated energy use matches the actual figures – a more accurate model for analysing the effect of energy-efficiency measures and investment decisions.

The challenge of the energy performance gap

One of the most persistent challenges in energy calculation is the so-called energy performance gap: the difference between a building’s calculated and actually measured energy use. Ever since the Swedish National Board of Housing introduced requirements for specific energy use in 2006 – verified both through calculation and through measurement in the finished building – the industry has struggled with the fact that reality usually deviates from the calculations.

To address this, BIM Energy was one of two calculation tools in a research programme that tackled the problem with probabilistic energy calculations. The dynamic model was then extended to account for the fact that input data varies in reality – material quality, installation, commissioning and occupant behaviour differ from house to house. The result is a probable range for the building’s energy use, which could then be compared against actual post-construction measurements. The work was carried out together with NCC, EQUA and Lund University, and was funded by the Swedish Construction Industry’s Development Fund (SBUF) and E2B2, a collaboration programme between the Swedish Energy Agency and IQ Samhällsbyggnad.

Stage 1: Simulation versus measurement in 26 passive houses

In the first study (2017), an energy model was built of a single-family passive house that NCC had constructed in several copies. After the houses were completed, their actual energy use was measured and compared against the calculated range. The agreement was very good: the measured values fell well within the calculated span, with most houses around 47–50 kWh/m² Atemp per year. What the model predicted during design therefore held true once the houses were in place and the energy was measured.

Stage 2: The same test on an apartment block

In the second study (2022), the method faced a considerably more complex object: a standardised NCC apartment block of 8 storeys, modelled with 52 zones. This too is a “type building” constructed in many identical copies across Sweden, which makes the comparison unusually robust – the same building and the same technical solutions, but in different locations and with different occupants. The BIM Energy model was compared against actual measured energy use in 28 of the completed buildings.

Once again the link held: the measured energy use fell entirely within the calculated range. What was simulated before the buildings were constructed reflected what was actually measured afterwards – even for a large, multi-zone building.

Conclusion: A basis you can stand behind

What makes the results valuable is that they close the circle between calculation and reality. A simulation is, in the end, only as good as its ability to predict what actually happens in operation. With a physics-based, hourly dynamic model at its core – and two independent, research-funded studies showing that the calculated ranges captured the real energy use in the completed buildings – there is good reason to trust the results.

Together with nearly 40 years of experience, more than 200 paying companies as users and the confidence of leading energy consultants and property owners nationally and internationally, this gives a clear answer to the opening question: yes, the calculations can be trusted – and you don’t have to take our word for it, you can read the independent research yourself.

Further reading

Burke, S., Kronvall, J., Wiktorsson, M., Sahlin, P., & Ljungberg, A. (2017). Beräkningsmetod för sannolik energianvändning i bostadshus (SBUF report 13074). Swedish Construction Industry’s Development Fund (SBUF).

Burke, S., Carling, P., Davidsson, H., Ekström, T., Harderup, L.-E., Hassanie, S., Kronvall, J., Säwén, T., Sundling, R., & Wiktorsson, M. (2022). Beräkningsmetod för sannolik energianvändning i flerbostadshus (SBUF report 13653). Swedish Construction Industry’s Development Fund (SBUF).

What is BIM Energy?

BIM Energy is a platform for energy analysis and action planning for property portfolios. The platform combines energy simulation with financial analysis so that you not only see where the energy is lost, but also what it costs to fix and what it gives back. The result is a decision basis that works equally well for the energy expert who produces it as for the property manager who has to make the decision. You can find more information at bimenergy.com.