**5. Conclusions**

Analyzing the hourly power demand of buildings helps in planning future generation capacity and backup and energy storage investments. The hourly heating and electric power demand of the Finnish detached house building stock was simulated using four di fferent age categories of buildings, four di fferent main heating systems and three levels of energy performance (reference, low cost retrofit D, and high impact retrofit B). Energy retrofits to improve energy e fficiency had a significant e ffect on the peak and average power demand in all examined buildings. The main contribution of this paper was to show the power demand distribution before and after retrofits. Typically retrofit studies only show the e ffects of retrofits on the annual level, but this study presented the seasonal changes in power demand, to better understand what additional changes to the energy system are needed inside and outside the building sector. Another important contribution was the presented estimate of the net change in power demand in the building stock level if large-scale building energy retrofits are done.

The lower emissions of electricity compared to on-site boilers or district heating favor electrification of heating, through the use air-source heat pumps. This resulted in increased electricity demand in buildings with district heating or on-site wood boilers. At retrofit level B, the peak power demand of these building rose by 60 to 70%, but the absolute impact was low. On the other hand, buildings with direct electric heating significantly lowered their demand through the retrofits (peak demand down by 27 to 40% in retrofit B), as did buildings with ground-source heat pumps (peak demand down by 36 to 68%), with significant absolute impact.

These e ffects were combined in scenarios where all single-family houses of the whole building stock were retrofitted, which resulted in a net decrease in annual electricity use, −11% for low cost retrofits (scenario D), and −38% for high impact retrofits (scenario B). On the building stock level, peak power demand increased by 19% for low cost retrofits, but remained unchanged for the combined high impact retrofits. However, it is not likely that all buildings could be retrofitted in the same way in practice, due to both social and technical issues related to di fferent conditions in each building.

The optimal solar electricity generation capacity on the individual building level was high. When the individual optima were utilized in the whole building stock, the peak excess power of solar electricity was 3.5 GW for the low cost retrofit scenario and 5 GW for the high impact retrofit. Such high values for unnecessary power generation could be di fficult for the grid to handle. Such a scenario is also sensitive to price assumptions and might not be feasible if increasing excess production were to reduce solar energy value. This calls for further research on the optimization of individual building retrofits together with the power system as a whole. Future studies need to combine the changes in buildings and conventional power sector, as well as include new potential ways to use the available renewable energy. Seasonal thermal energy storage could be one way to solve the problem of overproduction, along with electric cars and power-to-X technologies.

Retrofitting old detached houses in Finland can reduce emissions significantly by improving thermal insulation values and by utilizing electrified heating with air-source or ground-source heat pumps. Fears of increasing the marginal electricity demand seem to be unfounded. While the amount of heat pumps is increased, reducing the energy demand in buildings with direct electric heating can prevent both the total electricity demand and peak power demand from rising at the building stock level. This bodes well for major retrofit projects based on electrification of the heating market. However, more accurate modelling of the building stock is needed. A future study should consider how the Finnish building stock could realistically be retrofitted, taking into account both the addition and removal of buildings as well as regional trends in population and economic activity.

**Author Contributions:** Conceptualization, J.H., J.J. and R.K.; methodology, J.H., J.J. and R.K.; software, J.H.; validation, J.H., J.J. and R.K.; formal analysis, J.H.; investigation, J.H.; resources, J.H., J.J. and R.K.; data curation, J.H. and J.J.; writing—original draft preparation, J.H.; writing—review and editing, J.H., J.J. and R.K.; visualization, J.H.; supervision, J.J. and R.K.; project administration, R.K.; funding acquisition, R.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Academy of Finland, gran<sup>t</sup> number 309064. The study was made as part of the project Optimal transformation pathway towards the 2050 low-carbon target: integrated buildings, grids and national energy system for the case of Finland. The study received further funding from the FINEST Twins project, that is co-funded by the European Union (Horizon 2020 programme, gran<sup>t</sup> number 856602) and the Estonian government.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
