Photovoltaics in New Construction: Our Experiences with Technology, Costs, and Quotes

It was always clear to us: We wanted a photovoltaic system on the roof. The planning then turned out to be more complicated than we initially thought. We had to find out for ourselves which details matter besides roof area, orientation, and consumption. 

What are the differences between the various manufacturers? Is a battery storage system worthwhile, and what is a reasonable price? Here's how we proceeded with the planning of our PV system and which tools helped us:

Planning the PV System Size: Electricity Consumption

Before you plan modules, roof areas, and storage, you should understand how much electricity you really need in everyday life. This is the only way to sensibly size a PV system.

Our current electricity bill serves as the basis for the calculation. Based on this, we extrapolated/estimated how this will change due to additional building services, network technology, and a BEV.

Heat Pump:

You can roughly calculate how much electricity a heat pump needs for heating and hot water. You need three values for this:

• the final energy demand of your house from the EnEV calculation (in kWh per square meter and year),
• the heated living area in square meters,
• and the Annual Performance Factor of your heat pump, which indicates how efficiently it works. It depends on the model; you can find the value in this table.

If you multiply the final energy demand and the heated living area, you get the total heat energy demand per year. If you divide this value by the APF, you know approximately how much electricity the heat pump consumes.

Example: If your house has a final energy demand of 45 kWh/m²a, a heated area of 150 m², and your heat pump has an APF of 4, the result is:

(45×150)/4=1,688 kWh of electricity per year.

Base Load:

Refrigerator, router, building services, and lighting: many things in the house run permanently or regularly. This base load is important for realistically estimating your self-consumption.

Controllable Consumers:

Other devices can be used flexibly. You can time your washing machine, dryer, or oven to run when the sun is shining. This way, you make the best use of your own solar power.

Electric Car:

An electric car can significantly increase consumption. Example: 20 kWh per 100 km with an annual mileage of 15,000 km results in 3,000 kWh. Even if you only charge a portion of this at home, it makes a noticeable difference.

We assume that we will charge our e-car at home about 80% of the time, and the rest at fast chargers on long-distance trips.

Determining Electricity Consumption: Practical Tools

Tools like the PV Calculator help to realistically estimate your own demand. Here, you can not only enter your consumers but also simulate time windows and consumption curves. This allows you to see not only how much electricity you need in total but also when the consumption occurs. This is crucial for correctly planning the system size.

Our electricity consumption analysis showed that we will consume about 8,200 kWh per year. This means we need at least a capacity of 12 kWp. You can find out why we decided on more further down in the text.

Photovoltaics on the Flat Roof: Orientation, Area, and Green Roof

The roof determines how much space is available for photovoltaics. Size, orientation, and potential shading are crucial.

Our main roof has an area of 216 m². Due to restricted areas for ventilation, cable outlets, and external air conditioning units, 156 m² remain usable for PV modules. The garage roof adds another 36 m². Since the main roof is slightly higher, it partially shades the garage roof.

With a flat roof, we are more flexible in planning the PV system and can determine the orientation ourselves. A south orientation brings the highest yield, but for self-consumption, the even distribution of power production throughout the day is more important. Therefore, an orientation in several cardinal directions makes sense.

However, we do not achieve the optimal angle of 30° to 40° on the flat roof. The mounting system allows for an angle of up to 15°.

A special feature for us: The roof under the PV modules will be greened. This is mandated by the development plan. To implement this, we need a special mounting system. Here, we rely on the K2-Systems Greenroof Vento mounting solution, which is specially designed for green roofs. The feet are installed below the vegetation layer, allowing water to continue to seep through the sediment.

Shade is one of the biggest yield killers. Trees, neighboring houses, or chimneys can significantly reduce the performance of individual modules. An analysis saves trouble later.

Look at the surroundings of your property. Are there buildings, trees, or other objects that could cast a shadow on your house? If so, you can calculate the maximum shadow length with this tool.

If your house is already built, even if only the shell, you can use a drone flight at different times of the day to discover possible shadows on the roof.

We already considered the cabling of the PV system during the factory planning: it was clear that we needed a direct path for the solar cables to the technical room in the basement. Therefore, we integrated a DN100 FleSoDur into the flat roof, through which the cables then enter the house using a conduit.

PV Yield Calculator: Calculating Solar Yield

To find the optimal module orientation, we created a 3D model of our house and tested various arrangements using the Solaredge Designer (account required). Even if you are not planning a SolarEdge system, the tool is helpful. Alternatives are PV*Sol (free trial version) or other simulation programs.

The optimal result was 44 modules divided into two strings, oriented towards southeast and northwest. This achieves a capacity of almost 20 kWp.

As a reserve, the garage roof could later be fitted with about eight modules.

PV Functions: Do We Need Emergency Power and Battery Storage?

In addition to the price, other functions are relevant for the decision: offline operation, local control, smart control, modularity, and emergency power.

These functions are not feasible with every system, so you should look closely when selecting the manufacturer. What these functions can do and why they are important to us:

Local Control

We want to be able to control our PV system independently of the internet. The data should be available locally and not processed via a cloud.

Some manufacturers only offer cloud solutions. This means that without an active internet connection, certain functions can be restricted. For example, they might throttle the available battery capacity or reduce the warranty in case of prolonged offline operation. It is unclear what happens if the company becomes insolvent and its cloud service is possibly shut down.

Smart Home Integration

It is important to us to be able to intelligently control household consumers later. This means that the energy management system can detect when surplus solar power is available and automatically control controllable consumers such as a wallbox.

For local control, we will rely on a combination of Homeassistant, (possibly) evcc, and/or the Warp Energy Manager. Thus, only open-source software that can run on our home server will be used.

This is important for the decision: Not every inverter communicates with external energy management systems. An open API and Modbus interface is necessary for this.

Emergency Power & Backup Power

A PV system does not automatically make you independent of the grid. For your devices to continue working during a power outage, you need an emergency power or backup power function. Specifically: an emergency power-capable inverter and a battery storage system.

With emergency power, the inverter disconnects individual circuits from the public grid and supplies them with stored electricity from the battery storage, while with backup power, all circuits are supplied autonomously. For this, the inverter needs open interfaces, provided you don't want to commit permanently to one manufacturer.

We want to keep this option open: We install an emergency power-capable inverter but will retrofit the battery storage later.

Modularity

A PV system consists of several components, including the inverter, wallbox, and battery storage. In some systems, only a handful of components work with each other.

If you choose SigEnergy, for example, you must also purchase the wallbox and battery storage from this manufacturer. Here there is a risk of having no way to repair the system if the manufacturer becomes insolvent. However, there are systems that are more open and are also compatible with components from other manufacturers.

Obtaining Photovoltaic Quotes

After the initial research and the key data on the number of modules, roof plans, desired functions, and budget, we requested quotes from several providers: two local solar installers, our electrician, and one national provider.

We deliberately left out aggressive sales models (e.g., Enpal) as we did not expect a fair price or good execution there. It was also important to us that the companies did not rely on subcontractors for installation.

The quotes from the providers were easy to compare, but we found it difficult to commit to a specific manufacturer. Especially regarding modularity and local control, we had precise ideas.

Comparison of Photovoltaic Systems

Each system offers its advantages and disadvantages. It's worth looking closely to choose the right system for your situation. Here are the four that we considered:

SigenStor from Sigenergy

• Technically top, emergency power switching time 0 ms
• DC-22 kW Wallbox, perspective support for Vehicle-to-Grid
• Our quoted price: €24,000 for 20kWp capacity/16kWh storage
• All-in-one system, only compatible with proprietary storage/wallbox
• Requires cloud and internet connection. Questionable warranty conditions if cloud connection is missing.
• Company not on the market for long, unclear spare parts supply

Fenecon Home 20

• Completely local operation possible
• Open-source software
• German company
• All-in-one system, only partially compatible with other wallboxes.
• Licensing fees for additional software functions
• Our quoted price: €26,950 for 20kWp capacity/16.8kWh storage

Solaredge

• Modular system, partially compatible with third-party providers
• Local operation possible, but many functions only via Cloud
• Optimizers per module cause additional costs and are extra sources of error (unnecessary for us without shading)
• Our quoted price: €24,860 for 20kWp capacity/10kWh storage

Huawei (SUN2000)

• Completely local operation possible with Modbus dongle
• Our quoted price: €17,200 for 20kWp capacity/No storage
• Storage is ridiculously expensive compared to the competition, not compatible with third-party storage units

We changed our minds several times before we could choose a system. During the research, we repeatedly noticed minor details that led us to exclude a system. There are, of course, many other manufacturers, we can only compare the four we looked at more closely.

Our PV System Overview

We decided on a Huawei system without storage. The Warp3 Charger is planned as the wallbox. We do not rule out storage but want to retrofit it later when prices continue to fall or the technology becomes more attractive. Since the LUNA2000 storage from Huawei is absolutely unattractive in terms of price, we do not rule out using a second inverter later to be more flexible in the choice of battery packs. AC-coupled storage would also be an option to retrofit storage cost-effectively.

On the roof are 44x IBC 450W ES-TA1 Bifacial Modules, producing 20 kWp. This means we generate significantly more electricity than we consume according to our projection. The reason: We want to be able to generate a large part of our energy ourselves even in winter. Even with 20 kWp, we still have to draw a large part of the electricity from the grid from November to January.

Furthermore, the cost of the modules only represents a small part of the total bill. The labor hours of the fitters, scaffolding, and electrical installation are far more costly. So, if someone is already on our roof, they might as well connect a few more modules.

This gives us a system that largely covers our needs, is flexibly expandable, and works independently of manufacturer clouds.