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Tips for buying a power storage unit

Do you own a photovoltaic system and would like to use the electricity it generates more efficiently? If the answer is yes, then you should consider buying a power storage unit. With a power storage unit, you actively support the energy transition and relieve the stress on the electricity grid. Surplus electricity is stored in the power storage unit for later use even when the connected photovoltaic system does not generate any electricity. You can also increase your self-consumption and do not have to buy back at a high cost the electricity that you have previously fed into the grid.

Things to consider before you buy

Before you buy a power storage unit, you should think about some essential criteria: Choice of a suitable location for the battery and the desired storage capacity with your target degree of self-sufficiency.  The high overall efficiency of the system should be a basic requirement!


Space requirement

Consider the different designs of the  power storage units. Some models are very compact  requiring little space, whereas others need more space. Use the dimensions of the storage units to establish if you have enough suitable space in your house. Measure the space on the desired installation site. The ambient temperature should be between 15 and 25 °C.


Battery module

If you intend to buy a power storage unit, you should consider a modular system. It is flexible and can be expanded with additional battery modules at a later date. You choose the size of the storage unit aligned with your household consumption. If your consumption is likely to increase in the future, for example, because you want to add a heat pump or an air condition unit, you can upgrade the storage unit with additional battery modules.



The usable battery capacity is, next to the overall efficiency and the charging and discharging performance, one of the most important selection criteria. If you choose a larger battery, you increase your independence from your energy supplier. If you decide on a smaller battery, you optimise the self-consumption of the photovoltaic system. If the selected capacity is too large, you will incur additional acquisition costs for the storage system, and part of the stored electricity remains unused. If the capacity is too small, you will have to buy additional electricity from the grid. Battery capacity is specified in net and gross values. Some manufacturers try and make the battery capacity appear bigger by quoting the gross value for their batteries. You should always compare the net capacities. The size of your household and your PV system plays an important role when you decide on the preferred capacity of your system. For a single or small family household, a lower storage system capacity will be sufficient, while big family households should buy a higher capacity system. 



When you buy a power storage unit, you should pay attention to high overall efficiency. It describes the efficiency of the energy conversion of the system and includes, among others,  performance losses caused by electrical components that control the accumulator. The higher the efficiency, the less energy is lost when storing and converting the electricity. Storage systems components influence the efficiency in different ways. High battery efficiency is no guarantee to achieve a high overall system efficiency. The University HTW Berlin has introduced the system performance index SPI to better and easier evaluate and compare the performance of power storage systems. The SPI rates the systems based on the energy flows at the grid connection point and should be more than 90%. 


Depth of discharge

The depth of discharge indicates what percentage of the gross capacity of your battery you can actually use. The depth of discharge of a storage battery can range between 50 and 90 percent. It should not exceed 90 percent, as this shortens the life of the battery.



Which one is better? An AC- or a DC-coupled system?

There is no simple answer to the question of whether an AC- or a DC-coupled system is better. It will always depend on your individual needs. It is therefore essential that you clarify in advance which one of the two system options is best suited to your requirements.

You already own a photovoltaic system. Do you like to add a power storage unit at a later date, because your feed-in tariff contract is expiring for example? Then the AC-coupled storage system is the right one to get. This system is used to retrofit existing systems. It will help you to increase your self-sufficiency, even if your system is already older.

You have just started the planning process for your photovoltaic system? Then the DC-coupled storage system is the right choice for you. It is designed especially with new installations in mind. It thus ensures the optimal use of your electricity right from the start.


Conventional vs. forecast based charging strategies. Which one is better? 

Before buying a power storage unit, you should learn more about conventional and forecast based charging strategies. Do you receive subsidies for your photovoltaic system? Depending on the framework conditions of these subsidy schemes, the effective power that can be fed into the grid is limited to 30-70% of the nominal power of your system. If the excess electricity cannot be consumed directly, the system cuts off. No electricity is produced despite the sunshine, and the energy is lost. Only power storage units with intelligent energy management and forecast based charging strategy can reduce regulation losses. Charging will be deferred to later hours. Things look different for conventional charging strategies. The surplus electricity generated will be fed into the grid, however, it will be regulated. You should, therefore, buy a power storage unit that has a forecast based charging strategy.


Indoor vs. Outdoor-Systems

As for most consumer products, power storage units can also be classified for in- or outdoor use. Most commercially available power storage systems are designed for indoor installation. The ambient temperature should be kept between 15 and 25 °C. The devices should be installed in a protected location, for example, in a garage or a cellar.


Intelligent energy management and smart-home-ready

Do you already use smart home technology in your household? If so and you are planning to further optimise your energy consumption, you should buy a power storage unit that can be used with smart home ready heat pumps. Heating elements, heat pumps and charging stations for electric cars must also be able to connect intelligently to the power storage.


Intentions for the purchase of a power storage system

There are several reasons to buy a power storage unit:

  • Optimising the self-consumption
  • Backup power in case of a power failure
  • Saving on electricity costs
  • Relief for the power grids
  • More self-sufficiency in energy production and use
  • Increasing property value
  • Investing in the future to support the base of electro mobility
  • Compensating for the steadily decreasing feed-in tariff


Costs for solar storage

Costs are a not insignificant aspect when you buy a power storage unit. Get several offers, compare the unit prices as well as the installation costs. Ensure you check the unit performance parameters. Various grants can be obtained that support the installation of solar power storage units. 


Conclusion: Buying power storage units is a sensible investment

If you use a photovoltaic system, you can add a power storage unit to store excess electricity and use it when it is needed. You save on electricity costs as you do not have to buy back electricity that you have previously fed into the grid. Before you buy, ensure that you determine the space required and also consider what storage capacity is reasonable for you.

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