Powerful PV system for a home in Sofia

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Powerful PV system for a home in Sofia

With batteries and charging station

29.06.2021 25.02.2024

At the beginning of 2021, the owner of a frame house under construction near Sofia addressed us to install a solar power plant on the roof. An important criterion was uninterrupted power supply.

In the process of negotiations, we stopped on a hybrid system that can work both in parallel with the external grid and autonomously. The customer approved the design with the location of photovoltaic (PV) modules on all roof slopes.

3D-model of the arrangement of photovoltaic modules on the roof of the house

3D model of the PV Modules’ arrangement

In most cases, our clients avoid compromises and choose equipment of the highest level. Therefore, we used the time-tested solar modules from the Japanese company Sharp Solar. During installation, the customer had the pleasure to watch the process from a bird’s height and kindly shared photos.

View of the village from a height

We installed 94 modules: 86 with 445 Wp and another 8 with 440 Wp. The total peak power of the PV installation was 41.8 kWp. For several years we have been collecting detailed production statistics of solar power plants we built and we know that in gloomy weather (for example, during ordinary protracted rain), the generation is approximately 10% of the peak power. It can be 5 or 30%, depending on the thickness of the clouds and other factors, but in most cases about 10%. This means that the owner of the house can count on an average of 4 kilowatts of power «from the sun» in rainy weather.

Installation of solar modules on the roof of the building

At the same time, our experience shows that in gloomy weather all the slopes of the roof «work» similarly, therefore, if there is a sufficient budget, do not neglect the northeastern and northwestern slopes. In sunny weather, the use of various roof slopes allows getting more uniform generation throughout the day from sunrise to sunset.

Construction of a hybrid solar power plant in Sofia

Apart from its direct purpose — converting solar energy into electrical — photovoltaic modules on the roof of the house give their owners two pleasant bonuses: in hot weather, they significantly reduce the heating of the space under the roof, and during the rain they notedly minimize noise.

Photovoltaic modules produce direct current, and in everyday life, as a rule, alternating is required: 1-phased or 3-phased. In on-grid solar power plants (without batteries), the modules are connected to the grid-tie PV inverter, which converts direct current into alternating. In off-grid and hybrid systems (with batteries), the modules are usually connected to solar charge controllers or hybrid PV inverters. In our case, we connected the solar modules to five charge controllers and one grid-tie inverter.

Installation of Sharp solar modules on the roof of a private house

For effective operation of the power plant, we used the charge controllers SmartSolar from the Netherland company Victron Energy. They lower the voltage obtained from the solar modules to the level that is necessary for the batteries, proportionally increasing the charging current. Battery inverters are responsible for converting direct current into alternating in such systems.

Three charge controllers — MPPT 250/85. The first number in the name of the modification means the maximum allowable voltage at the input (from the solar modules) — no more than 250 volts. The second number shows the maximum possible current at the output (to the battery) — up to 85 amperes, which at the voltage of the battery, 55 volts, provides a charging power of about 4.7 kW.

To each of those controller we connected 12 modules (blue in the scheme), grouping them into 3 strings of 4 modules. Solar modules in each string are connected sequentially, which increases the total voltage of a string. Between themselves, they are connected in parallel, which increases current strength. For the effective operation of the modules, all strings connected in parallel should be the same and be in identical conditions (azimuth, angle of inclination, illumination, temperature).

Connection scheme of the solar modules to charge controllers and invertor

Six modules we connected to a charge controller MPPT 150/45 (also blue in the scheme, 3 strings with 2 modules each) and another 16 — to the newest controller MPPT RS 450/100 (green in the scheme). This was the first example RS 450/100, installed in Bulgaria. The higher permissible voltage at the input of this controller made it possible to combine sequentially eight modules in each string. In addition, we were able to place two strings on different roof slopes, since the modification RS 450/100 has two independent MPPT (Maximum Power Point Tracking).

The other 36 modules (red in the scheme) we connected to a 3-phased on-grid PV inverter Fronius Symo (Austria) with power of 15 kW. Such a combination of charge controllers and an grid-tie inverter increases the overall efficiency of the hybrid system and deserves a separate article.

Installation of the PV Sharp Solar modules according to the scheme

As usual, we used the special «solar» cables and connectors of the highest quality by the company Stäubli (Switzerland) and mounting elements by Aerocompact (Austria).

Installation of Sharp modules using the Aerocompact mounting elements

We fixed each module at six points, which is somewhat redundant (usually four is enough), but gradually becomes a trend in the installation of large modules in difficult climatic conditions.

Fixing the Sharp modules at six points

In the technical room, we placed a ventilated cabinet with solar charge controllers and a monitoring system, bidirectional battery inverters Victron Energy MultiPlus-II (one for each phase) and a 3-phase on-grid PV inverter Fronius.

The full power of each battery inverter is 5 kV·A (about 4 kW active power depending on the type of load). This means that in off-grid mode they can give together up to 12 kW, while converting direct current from the solar modules and batteries into alternating. During daytime the on-grid PV inverter Fronius is helping the battery inverters Victron, adding up to 15 kW, depending on the illumination of the solar modules.

Installation of Victron Energy battery inverters in a technical room

Here, an attentive reader may object, since the grid-tie PV inverter is called «grid-tie», because it works only in parallel with the external grid and cannot work autonomously. Yes, in a classic on-grid PV system, the inverter really cannot work without a grid, but in our case it is part of the Victron hybrid power plant, and battery inverters form the grid for it. This type of system is called «Micro Grid».

It turns out that in off-grid mode, the AC power of the entire system (output power at the alternating current side) will be limited to 12 kilowatts in the dark time of the day, with the possibility of its increase to 27 kilowatts in a clear sunny noon. In the presence of an external grid (hybrid mode) there is no such restrictiom, since every battery inverter, additionally to its own power, can transmit up to 50 amperes from the grid, and this means that the total AC power of the system, even in the dark time of the day, will be about 40 kilowatts, which is more than enough for the given object.

The risk of overloading the system occurs only with a large consumption in the house in combination with an emergency shutdown of the external grid. In such cases, in order to prevent the overload of inverters or a quick discharge of batteries, all consumers are usually divided into two groups: critical and nonritical. For example, in an accident with the grid, water heaters and sauna can be automatically turned off.

Firmware update and tuning of the BYD LVL batteries

BYD Battery System Setup

We also installed three lithium iron phosphate BYD batteries LVL with a capacity of 15.36 kW·h each. This type of battery is the safest on the market at the moment. The total stock of energy in the batteries is about 46 kW·h. Based on our recommendation, the customer also purchased two backup solar modules and stores them in the technical room «just in case».

We installed a Victron Energy touch display on the door of the equipment cabinet, which allows you to locally monitor the power plant and change settings. The Victron monitoring system also allows doing this remotely via the Internet using the free VRM Portal.

Victron color display in NENCOM cabinet

A few months after the system was put into operation, the owner of this beautiful and energy-efficient home sent us new photos with already green lawns. According to him, the power plant works very well, significantly reducing electricity bills and providing uninterrupted power to the entire house.

Sharp Solar photovoltaic modules on the roof of a house in Sofia

Thanks to the «top-down» angle we were able to verify that the actual result of the location of the photovoltaic panels exactly matches the 3D model:

Comparison of the 3D model with the actual arrangement of solar panels

The cost of this powerful hybrid PV system in 2021 was 115 590 lv (59 100 €) with delivery, installation, commissioning and VAT. This is comparable to the price of a compact crossover like Audi Q3 or Mercedes GLB in medium configuration with a diesel engine.

System expansion

At the end of 2022, after a year and a half of operation of the power plant, the owner bought an electric car and decided to upgrade the system. By that time, in the range of Victron MultiPlus-II inverters appeared more powerful versions of 8, 10 and 15 kV·A. After a detailed analysis of peak loads, we came to the conclusion that 10 kV·A would be sufficient. Our client himself sold the three «old» MultiPlus-II of 5 kV·A (Victron Energy equipment is highly valued in the secondary market) and we installed new ones in their place. Thus, the minimum AC power of the system in off-grid mode has doubled: from 12 to 24 kW.

We also installed one more battery, after which the total energy reserve was 61.4 kW·h. The new version of the BYD LVL battery is visually different, but has the same characteristics.

Replacement of Victron inverters and installation of an additional BYD battery

And the main update of the system was the addition of a proprietary Victron Energy charging station. Now excess solar energy can not only heat pool water, but also charge electric cars for free.

Charging the hybrid Jeep Wrangler from the sun

The cost of all these improvements in 2022 was 47 900 lv (24 490 €), which increased the total investment in the system to 163 490 lv (83 590 €). With this money you could buy a regular business class sedan, like an Audi A6 or BMW 5 Series with medium configuration with a diesel engine. In addition to this, the client returned part of the investment when selling the «old» inverters.


During 2022, the family consumed 30 543 kW·h electrical energy, without saving or denying themselves anything. Herewith, from the grid (red colour in the diagram) only 11 633 kW·h was consumed, which made about 38.1% from the total consumption. The other 18 910 kW·h — this is the energy from the photovoltaic system: one part of it was consumed directly (yellow colour), and the other part — by the batteries (blue colour). The batteries in this system are only charged by excess solar energy during the day, although technically they can also be charged from the grid (for example, at a night tariff).

Solar power plant performance statistics

Statistics for 2022

The following year, total consumption was slightly higher and amounted to 34 738 kW·h. At the same time, from the diagram we see that the share of energy consumed directly from the PV system has remained almost unchanged, and the share of consumption from the grid decreased by approximately 2% due to an increase in consumption from the battery. This is largely due to the fact that we installed an additional battery at the end of 2022, thereby increasing the total energy storage capacity.

Analysis of energy consumption in 2023

Statistics for 2023

It is not difficult to notice that consumption from the grid mainly occurs in winter: partly due to less solar energy, partly due to the periodic accumulation of snow on the roof. However, the settings of the system have the greatest impact in this case. Unfortunately, emergency grid outages often occur in this region, especially in winter. Therefore, many owners of such systems, during these periods, temporarily reduce DoD («depth of discharge») to 20-30%, instead of the usual 70-80%. As a result, the battery almost does not work while there is voltage in the external grid.

In the event of a grid failure, Victron inverters switch to off-grid mode in less than 20 milliseconds; it is almost impossible to notice. Therefore, at the client’s request, we set up sending automatic push notifications to his phone when the external grid is turned off and on. Now the owner knows that the power plant has switched to off-grid mode and can control the situation taking into account the current battery charge, weather conditions and his plans.

Let’s look at the hourly chart on a typical partly cloudy winter day. Here we see that consumption from the grid mainly occurs at night, at a low tariff:

Hourly operating data of NENCOM solar power plant

January 28, 2024, partly cloudy. Pie charts show data at the time the screenshot was taken (February 24, 2024)

Because of the clouds, the battery managed to charge only to 90% during the day, and in the settings of the system the lower limit of the SoC («state of charge») was set to 50%. Accordingly, in the evening the battery was discharged to this level, leaving a reserve in case of an accident. Even with such restrictions, consumption from the grid mainly occurred during the period of the night tariff.

Thus, the owner of a solar power plant received detailed consumption statistics, complete independence from grid failures, and also significantly reduced electricity bills.


At the time the system was put into operation, its components had the following warranty periods: Sharp solar modules — 25 years to power; BYD batteries — 10 years to capacity; Fronius inverter — 7 years; Victron Energy equipment — 5 years with the possibility of a paid extension up to 10 years for 10% of its cost. Our client took advantage of Victron’s extended warranty option.


By mutual agreement, we signed a contract with the customer for distant support of the system. The service includes periodic remote checking of the operation of the power plant, updating software, changing settings and automatic notifications of events if necessary.


In this folder we have collected documents with the characteristics of the main components of the power plant:



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