Solar Cells with Battery Storage in Housing Associations in Sonderborg, DK

Main sector

  • Smart buildings
  • Smart electricity


Solar cell plants have been installed onto roofs of housing association buildings in Sonderborg (DK). The excess of produced solar electricity has been sold to the public grid. However, the price for selling solar electricity to the public grid is rather low, and so it is more sensible to use the solar electricity in the apartments themselves. Therefore, in Sonderborg a solar-cells-solution with battery storage in bigger apartment buildings (8-10) will be demonstrated instead of connecting the solar cells to the public grid. If the housing association sells the excess produced solar electricity to the public grid, they will receive a price of maximum 0.05 Euro per kWh. On the other hand, if they store the electricity and use it at a later stage, they will save 0.28 Euro per kWh.
A battery storage solution is an interesting solution for bigger residential houses like apartment buildings, because it means, that the solar electricity produced during the day can be stored until the late afternoon or evening, when the electricity consumption is much higher than during the day, and thereby also contribute to reducing the peak load demand on the public grid.
Normally the feasable area of solar cells and therefore the production of solar electricity can be about 50% higher, if the solar panels are connected to a battery storage. 

Like this, more electricity is produced with CO2 neutral power. The demonstration project in Sonderborg involves three social housing associations with more than 20 departments. Four demonstration projects have been installed during 2020. Ten projects are contracted to be in operation in Spring 2021. Six more projects are expected to be contracted in beginning of 2021. Tenants in housing departments voted to approve the investment in solar-battery solutions. 

Buildings of Housing Association B42 mit PV solar panels and battery storage.

Business model

Roughly it can be stated, that the investment is 60 - 70% higher with a battery solution compared to solar panels connected directly to the grid.
In comparison to a grid connection which as an ecample is most feasible with 100 m2 of solar panels, a battery storage solution would be most reasonable with 150 m2 of solar panels and to connect the panels with a local battery. The simple pay back period will approximately be the same with and without battery, but with the solution you save a lot more external energy and a lot more CO2.
The payback periode, of cource, depends very much on the local electricity price. In Denmark the price is rather high (0.28 Euro per kWh) due to local taxes.
Normally investments in solar cells and battery storage plants can be financed by 20-30 years with external loans from banks or financial institutes.

Citizen engagement

The battery storage projects will be implemented in social housing associations, where the tenants can decide independently, if they want to implement the battery solution or not.
The procedure is, that the administration of the housing associations, together with the consulting engineers, prepare a conceptual plan with estimated investment and estimated energy savings.
The sketch project will be presented to the respective tenants on an anual meeting and followed by a voting procedure among all present tenants.
If there is a majority of attendees that can agree on this investment, it is accepted, and then tender documents will be prepared.
Normally such investments will be accepted, if the simple payback period is less than 10-12 years.



  • Better use of solar energy
  • More feasible investment in solar systems
  • Demonstration of new technology
  • Considerable energy savings
  • Considerable reduction in CO2 emission

Facts & Figures 

  • 2.500 m2 of solar panels are installed together with the batteries.
  • Battery capacity in total is 2 MWh.
  • Energy savings with solar + batteries are 2.950 MWh per year.
  • CO2 reduction is 1.250 tons per year.
  • Payback period for the investment is 10-12 years.


5.5 Mill €
Payback period: 10 - 12 years.

Potential for replication

Obviously, the battery storage solutions in combination with solar cells can be adopted in other cities and countries.
The technology can easily be replicated and the economic feasibility could be even better in other countries (e.g. if the local public grid administrators there do not allow solar cell systems to interfere with the public electricity grid). In many countries it is difficult to manage the input of solar electricity into the public grid, and therefore the public grid will not pay for this solar electricity. In these situations it would be a much better solution to install solar cells in combination with battery storage plants.

Pictures@Sonderborg Municipality