Solar powering our apartment complex

Climate change, probably the biggest threat of our time. Even before the IPCC’s Special report, we had decided on solar powering our apartment complex to both promote sustainable living and save some money. Our project started already when we built the house, but it took a giant leap forwards this year when we tripled the capacity and did a change in the metering of the electricity.

Where we came from?

We live in an apartment complex (link in Finnish) built in 2014. During the construction, we decided that we need to be solar powering our apartment complex and invested in a solar power system of 14 photovoltaic (pv) panels, with a total power of 3,2 kWp and a single 5kW inverter. The panels were connected behind the main electricity meter of the apartment complex, allowing us to utilize the produced electricity in heating both the apartment and the consumed water.

However, each of the four apartments have their own electricity meters, efficiently eliminating the economic use of the produced electricity in the apartments. Even though the electrons produced in the solar panels were being used in the apartments, the two electricity meters incurred tariffs for both the sell side and the buy side. We sold one kWh for €0,04 and bought it back in at €0,12.

The biggest consuming factors in our apartment complex were heating and lightning. However, both of these electricity consumers were the most active when the sun wasn’t shining – thank the location of Finland with long cold, and especially dark winters.

Electricity consumption of the  apartment complex’s common consumers, year 2015

To get some facts and economics behind the future investments, we did some calculations and planning.

Month Measured production (kWh) Measured excess from own production (kWh) Measured consumption (kWh) Difference in price (loss) (€)
January 4,49 0 2598,54
February 28,48 1,19 2166,91  €0,09
March 179,56 49,66 2005,13  €3,63
April 293,54 111,06 1426,28  €8,11
May 421,4 144,84 979,46  €10,57
June 410,58 213,84 701,84  €15,61
July 400,06 207,1 662,14  €15,12
August 482,82 281,39 733,8  €20,54
Septmeber 210,6 92,16 893,62  €6,73
October 100,73 17,81 1546,54  €1,30
November 16,29 0,47 1882,25  €0,03
December 4,67 0 2271,83

Table 1: Calculating the excess electricity sold to the network

Increasing utilization efficiency
Measured consumption and production a day in July 2017

Evaluated solutions to increase the utilization rate

In essence, we talked about increasing the utilization rate in two different methods: 1) Changing the metering so that all of the apartments consumption would be metered through one meter and thus all produced electricity would be available to all apartments and 2) storing the excess electricity into batteries to be used when the consumption is higher and production lower.

Metering change

The idea behind changing the metering logic is that instead of each apartment having their own contracts, the apartment complex would have only one contract and connection to the network (the blue box in the image below). This means that the electricity company and the network owner will invoice the apartment complex (its own legal entity in Finland) per used electricity in all apartments. The complex can then invoice it’s inhabitants, as long as it measures the consumed electricity per apartment.

Image source:

The financials behind the metering change is that the apartments consume more electricity when their combined electricity consumption is taken into account. Refrigerators, computers, wifi-routers etc consume electricity during the day. In addition, we have air conditioning units for the apartments that are mainly used during really sunny days, when the sun is up. This is a great value proposition, as we are able to use the sun that heats our apartments during the summer months to cool them down as well.

Return on investment (ROI) Calculations for the metering change

Cumulative consumption of all apartments on a day in July 2017
Solar powering apartment complex
The utilization is increased to 100% with current production
Year Cumulative savings Remaining value of the metering change
1  €                 86,31  €            3 413,69
2  €                 172,96  €            3 327,04
3  €                 259,61  €            3 240,39
4  €                 346,26  €            3 153,74
29  €              3 205,75  €               294,25
30  €              3 292,40  €               207,60
31  €              3 379,06  €               120,94
32  €              3 465,71  €                  34,29
33  €              3 552,36
34  €              3 639,01

Table 2: The ROI calculation for the metering change. At this stage, we had no knowledge of the changes in electricity network tariffs. It would’ve shortened the ROI, as we’d save an additional € 20 / month  i.e € 240 / yr! Thus, the correct payback time would be 11 years. The fixed calculations can be found here

The law in Finland is somewhat obscure about this. The law states, that each inhabitant has to have the freedom of choice from which provider they buy their electricity. This means, that the complex cannot dictate the contract to each inhabitant. In a large apartment complex, this may be difficult as there are many people living in it and finding consensus may be difficult. However, in our four apartment complex this was easy – we all share the same values and have been involved in the construction of the house from the beginning.

We held an annual shareholders meeting, where we decided to do the changes, and changed the rules of our apartment complex to accommodate the complex to invoice its inhabitants. It was relatively easy, as we already have a similar system in place for water.

The law still states, that if an inhabitant wants to change the electricity provider, they have the possibility but they need to make the changes to the main electricity board on their own expense and the complex doesn’t need to take part in those expenses.

Energy storage via batteries

The idea about batteries is that when there is production excess, the excess is stored in  a batter and released when there is no production but need for the energy.

Battery concept
The idea behind the battery installation. Utilizitaion of the energy when it is not produced. The x-axis represents time in hours.

To calculate the rationale on investing to a battery system, we used the recently published Tesla Powerwall. It might not be the cheapest of the alternatives, but it was the easiest to be researched at the time. Tesla recommended us three units of 14kW batteries at a price of €23.000. Without knowing the savings from the different contract with the network company, we calculated the ROI to be 250 years. A bit too much perhaps…

ROI Calculations for the powerwall

Year Cumulative savings Remaining value of Tesla Powerwall
1 €               86,31 €          22 313,69
2 €             172,96 €          22 227,04
3 €             259,61 €          22 140,39
4 €             346,26 €          22 053,74
5 €             432,91 €          21 967,09
249 €       22 269,04 €               130,96
250 €       22 355,69 €                  44,31
251 €       22 442,34

Table 3: The ROI Calculation of a Tesla Powerwall system. This initial calculation is flawed the same way as the metering change calculation. The actual payback time would be “only” 61 years. The fixed calculations can be found here

Solar powering our apartment complex – the installation

So we decided to do the metering change AND increase the capacity of our solar plant. We asked around some contractors, with the criteria that they represent the manufacturers we already have, mainly NAPS and SMA. We received offers from two of them. During the proposal stage, we looked at the site and possibilities with three contractors and ended up receiving very similar proposals from all of them:

  • Add solar panels to the maximum capacity of the existing inverter
  • Add another inverter and add panels close to its maximum, based on the amount of roof space available.
  • Change the electricity contract with the network company from five separate contracts to a single contract.
  • Install new electricity meters for us to measure the consumed electricity between the apartments to invoice the inhabitants based on consumed electricity.

We ended up choosing PlayGreen, a company that had never done the metering changes but were looking for the reference, as it is a topic that is very hot at the moment in Finland. The actual proposal thus consisted of the following:

  • 27 pcs of NAPS Saana 305 Wp PV-panels
  • 1 SMA Sunny Tripower 5000TL-20
  • Installation, cabling, supports
  • 5 Carlo Gavazzi electricity meters
  • 1 Carlo Gavazzi VMUC-EM Datalogger

The installation itself was fairly straight forward. PlayGreen started with the metering changes, contacted the network operator Caruna to liason all changes with the contracts and connections. After a day of installation, the metering changes were done. There was a slight pause of few weeks before they installed the panels, but they were installed eventually in two days.

Solar panel supports
The supports installed. One of them was installed to a wrong position and we had to request to move it.
Close up of the solar panel supports
1/3 of the newly installed panels
The same panels from another angle


The new electricity consumption meters were significantly smaller than the previous

Connection type challenges with the network operator

Even though solar powering our apartment complex was relatively simple and our financial math so far had proven to be accurate, we didn’t know about the different tariffs the network operator charges, when the connection type goes over a certain limit.

Each of our apartments had 3 x 25A main fuses, and the apartment complex had 3x80A main fuses. When we removed the individual apartment meters, the connection type was determined by the size of the main fuses. In our apartment complex’s case, we were given a “power connection”, i.e. a connection with at least 3 x 80A main fuses.

This connection type is planned for significant electricity consumption, as the € / kWh is somewhat lower, but the monthly fixed fee is ten times bigger and additional fees are charged for reactive power and a maximum power peak, but 40kW the least. This connection type would increase our yearly electricity bill by over 1500 EUR! Which is almost the same as the calculated saving was supposed to be.

Monthly fee (€/kk)     42,50 €
Power fee (€/kW, kk; min 40kW)       1,25 €
Reactive power, input (€/kVAr, kk)       4,05 €
Reactive power, output (€/kVAr, kk)       4,05 €
Transfer (c/kWh)       2,32¢

Table 4: The current pricing for the “Power connection” (Tehosiirto 1 PJ)

Monthly fee (€/kk)       4,76 €
Transfer (c/kWh)       2,53¢

Table 5: The current pricing for the “general connection” (Yleissiirto)

So after few frustrated phone calls to the electricity company and our solar contractor, we eventually got them to change our main fuses down to 3 x 63A, which is the maximum without being forced to the “power connection” category.

During the one month we had the bigger connection, it costed us 150EUR more, than what the general connection would have.

This is one of the biggest learning of the project. The electricity network operator is naturally keen on following their processes and charging according to their policies. Since the metering changes aren’t very common, nobody knew, that this would happen. So our recommendation is to take this into consideration already in the planning and installation phase.

Final data

solar powering our apartment complex
Daily consumption in September 2018. Notice the sunny days, when the consumption is near zero. y-axis units is kWh
Solar production sep 13
Sep 13th was a sunny day. During the day, the consumption was significantly below the production.
solar production sep 13
September the 13th from the solar power analysis point of view. Two bigger clouds were easily seen in the data.
cumulative production 2018
The total production of 2018 so far. The installation of the new panels is easily seen. Installation date was 29th of August

I will need to write a followup once we have a year full of data. After some time, we’ll start looking into storing the energy. Not necessarily batteries, but perhaps the water boiler. Also, if our neighbors will build a new house, we should try to negotiate to found an energy community (link to Finnish law), to share electricity between the houses, instead of the entire network.

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