A solar power storage unit is in essence a rechargeable accumulator or battery pack. It stores the excess solar power that your PV system has generated during hours of sunshine. A battery inverter takes over the transformation of direct current into alternating current and controls the charging and discharging of the solar power storage unit.

When is a solar power storage unit worthwhile?

The production costs of self-generated and self-consumed solar energy are considerably less than the costs of buying energy externally. Today, consumers already pay an average of 36 cents per kilowatt-hour to their energy suppliers. At the moment, the difference is between 20 and 25 cents per kilowatt-hour and is expected to increase over the years. It will shorten the time to make the installation of solar power storage units profitable. Boosting the degree of self-consumption will also increase the cost savings. Without a solar power storage unit, the self-consumption rate of a PV system is between 15 and 30 per cent. By installing a solar power storage unit it can be tripled.

How economical is a solar power storage unit?

The economic efficiency of a solar power storage unit is calculated as the ratio from the sum of additionally self- consumed kilowatt-hours of self-generated electricity and the service life of the storage unit set against the investment costs less possible subsidies. However, the calculation must consider the projected development of the electricity price and household electricity consumption as well as the self-discharge rate of the solar power storage unit. The projections assume that retail consumer electricity prices will continue to rise. They also expect household consumption of electricity to increase as new electrical appliances are added.

For many people, economic efficiency is not the only reason to invest in a solar power storage unit. Protecting the climate is equally, if not more important to them. Only with a solar power storage unit, you can use your self-generated solar power around the clock. You will thus support a climate-friendly energy transition towards a clean, decentralised and sustainable energy production.

How important is efficiency?

The efficiency of a solar power storage unit is significantly important when you want to maximise the use of self-generated electricity. Depending on efficiency and location, a PV system generates per year between 800 and 1,200 kilowatt-hours (kWh) of solar electricity per kilowatt peak (kWp).

The unit kWp describes the rated output of a PV system under standardised test conditions. To simplify the example calculation, we assume a value of 1,000 kWh. With a solar power storage unit efficiency of 98 per cent, you can use 980 kWh of the generated 1000 kWh. If the efficiency is 85 per cent, you will lose 130 kWh simply due to the inferior efficiency. At today's electricity prices, this translates into more than 30 Euros lost per year for every 1,000 kWh of generated energy. (For this example calculation, we assume that 100% of the generated solar energy can be stored in the solar power storage unit.)

Important! The solar panel orientation, the generated electricity output and the available electricity consumption periods of PV systems are correlated. South-facing PV systems generate most of their electricity output at midday. For east-facing PV systems, the peak output will be during the morning hours, while for west-facing systems, it will be during the afternoon. If, in your household, you generally consume electricity in the morning and towards the evening and your solar panels are south facing, you should install a larger solar power storage unit to store the generated energy for self-consumption.

What are the battery technologies?

Why lithium iron phosphate-based batteries?

Lithium iron-based batteries are currently the gold standard for solar power storage units. Reason number one - The batteries have a very long service life. After 5000 charging and discharging cycles, the batteries still have 80% of their initial capacity. RCT Power, therefore, provides a current value guarantee of 10 years for its power storage units, while the actual service life of the battery is considerably longer.

Reason number two - The battery safety is very high. LFP batteries have a lower energy density than NMC or NCA batteries. To achieve the same power output, the LFP batteries are bigger. A benefit of the lower energy density is that thermal runaway and overheating of the battery units are virtually excluded and thus also the risk of a battery fire.

Reason number three - LFP batteries have excellent performance values. RCT Power's solar power storage units have an average efficiency of over 97 per cent. They are the most efficient on the market which has been confirmed in the annual Berlin University of Applied Sciences (HTW) Energy Storage Inspection. RCT Power's 10-kWp storage system was the most energy-efficient in the 2021 and 2022 editions. The 5-kWp system topped the ranking in 2020.

Reason number four - The raw materials used to produce LFP-based batteries for solar power storage units are natural minerals. Their mining has a considerably lower environmental and social impact than batteries that use cobalt, which is mined under ethically and environmentally very controversial conditions.

Reason number five- The LFP battery technology is tried and tested, and has been deployed in various settings for almost 20 years.

What is the required capacity of a solar power storage unit?

You will need to know the PV system output and the daytime electricity consumption to establish the best fitting size of a solar power storage unit. You can then derive an initial benchmark for the appropriate storage size. As a rule of thumb, the storage capacity in kilowatt-hours is between 0.9 and 1.6 times the PV system output in kWp with an assumed energy consumption of 1000 kWh per year and kWp. Translated into a specific example: For a PV system with 5 kWp, the solar power storage unit capacity should be between 4 and 8 kWh. RCT Power offers a wide range of solar power storage units with capacities from 3.4-to 10.4 kWh.

If your PV system is designed to approximately generate the annual power consumption, the solar power storage unit will enable you to achieve an almost 100 per cent degree of self-sufficiency in the summer months. The more of the total electricity consumption you require during daytime hours, the smaller the size of the storage unit can be. It is because solar power storage units are mostly used in the morning and evening hours and at night, as already mentioned in the section 'What is the mode of action of a solar power storage unit?'.

Oversizing the solar power storage unit will not necessarily increase the degree of self-consumption as you will not be able to generate enough energy in the months with low sunshine hours.

Can the capacity of the solar power storage unit be increased?

If the electricity demand increases, for example due to offspring, heat pump or newly purchased appliances, you can increase the capacity of your solar power storage. Because: RCT Power batteries are modular and expandable. This allows you to design the usable capacity from 3.4 to 10.4 kWh. This gives you the necessary flexibility with the best possible economic efficiency.

What is the meaning of the acronyms AC and DC?

PV systems generate direct current (DC). The direct current must be converted into alternating current (AC) to use in the household or to feed it into the grid. Power storage systems are distinguished as to whether the storage system is connected on the DC or AC side.

This distinction is important for existing and new PV systems. AC-coupled solar storage systems are mostly used for retrofitting existing PV systems. For new PV systems installations, DC-coupled solar storage systems will be used. PV and storage systems are installed simultaneously.

Can solar power storage units be connected to existing PV systems?

You can easily retrofit your existing PV system with an AC-coupled solar power storage system. This option is interesting if your PV system no longer qualifies for solar power subsidies. If your current feed-in tariff contract expires, you will only receive a small amount of money for the electricity you feed into the grid. The core component of an AC-coupled solar power storage system is the battery inverter. At first, the inverter converts the rectified solar electricity into alternating current. Subsequently, the AC battery inverter transforms the alternating current back into a direct current. This process is the prerequisite for storing the electricity in the battery. An AC-coupled solar power storage system is connected directly to the existing PV system without tools via plug-and-play. The commissioning is performed with the RCT Power App.

What is a DC-coupled storage system?

DC-coupled solar power storage systems are used with new PV systems installations. A DC-coupled storage system combines the PV inverter with the battery controller. When in charging mode, the direct current generated by the PV system is stored directly in the power storage unit batteries via the battery controller. When in discharging mode, the previously stored solar power is available for consumption in the household via the battery controller and the inverter.

One benefit of DC-coupled storage systems is that they manage to operate with a single inverter. The installation of DC-coupled storage systems is of low effort. Commissioning begins by connecting the power storage system to the PV system's existing DC circuit. The system's integrated battery charge controller feeds the generated solar power into the battery

What are the operating costs of solar power storage units?

The RCT Power solar power storage units with LFP-based batteries are virtually maintenance-free. They are conveniently controlled via the RCT Power App.

Lead-acid-based batteries are more maintenance-intensive, another reason to choose a solar power storage unit with an LFP-based battery.

What does grid-serving storage mean?

Modern solar storage systems store electricity in a way that serves the grid, i.e. they have an intelligent charging management system. Grid-serving storage is often a prerequisite for funding.

What does this mean? RCT Power solar power storage systems have a forecast-based charging management system. The system calculates the yield forecast for the next few hours. The expected hours of sunshine are related to the expected consumption in the same time window. Based on this calculation, the management decides at what time the battery should be charged. On sunny days, for example, the battery only charges during the midday hours. The electricity produced in the morning is fed into the grid. The result: the solar power storage unit still has charging capacity available at midday. Grid-serving storage prevents grid overload at peak times.

What are the benefits of an intelligent charging management system?

What happens in the event of a power failure?

Usually, PV systems and solar power storage units do not work in the event of a power failure. They cannot continue to supply the house with self-generated electricity. RCT Power has a solution for this scenario, the RCT Power Switch.

It is a backup power function that switches on automatically as soon as the power fails. It supplies selected domestic consumers with electricity. The switch-over process only takes a few seconds. The RCT Power Switch is only suitable for DC-coupled storage systems and can be retrofitted.

What is the role of solar power storage systems in the energy transition?

In the future, decentralised energy sources with storage capabilities will play a central role in the built environment. They will serve as a buffer between power generation and consumption peaks, relieving the load on the electricity grid. They will also act as short-term storage units counteracting the day and night solar power generation gap. These are reasons why the government is subsidising and promoting the uptake and installation of solar power storage systems.