The 10 most important criteria for buying energy storage devices

April 2019

The 10 most important criteria for buying energy storage devices

Sun and wind are infinitely available – but they’re not always there when we need them. On sunny days, vast quantities of solar electricity might be loading up the grids, whereas during the night, the wind drops and there’s a critical shortfall. That’s why energy storage systems are an important component in the energy transition. They allow you to use self-generated solar electricity and relieve the power grids, making it possible to increase PV expansion. This is how all of us can make an important contribution to climate protection while also protecting themselves from rising electricity prices. But what are the essentials that you should take into account when buying an energy storage system? Here are some valuable tips on the relevant features and what you should consider when making a purchase.

Every energy storage device on the market has different dimensions. Some are wide and flat, others are narrow and deep. Some are extremely compact, while others take up lots of space. Check the availability of space in your house. Flat batteries are suitable for mounting in narrow hallways or behind doors. Narrow batteries fit well in small equipment rooms. You also need to leave clearance for ventilation.

Modular systems can accommodate varying numbers of battery modules, which allows you to select the battery size that fits your consumption profile. It’s especially important that you be able to retrofit more batteries later on – in case, for example, you purchase an electric car or heat pump.

Some manufacturers specify the gross value as their batteries’ capacity in order to make them seem larger. But every battery has a limited charging capacity, so check the data very carefully and compare it to the systems’ net capacity.

The PV modules on the roof generate direct current (DC), whereas our domestic mains supply works with alternating current (AC). That’s why a PV inverter always converts the direct current that’s generated to alternating current. Energy storage can either be integrated directly on the DC side between the PV installation and an inverter (DC-coupled) or flexibly mounted in the AC network (AC-coupled).


System preference is more a question of philosophy. With a DC system, you can eliminate one inverter, because the alternating current doesn’t have to be converted back to direct current for the batteries. However, most manufacturers factor the cost savings into the battery price, and the reduction in loss of efficiency is negligible. DC batteries also have to be precisely dimensioned to the PV installation, and when consumption is high you can’t simultaneously draw power from the PV installation and the battery: You have to buy additional electricity from the grid. With AC systems, on the other hand, the capacity is completely flexible, and you can both connect several power generators and draw electricity from the grid. This is especially valuable if you ever want to take advantage of day/night tariffs or trade your energy on the electricity market.

Integrated systems come from a single manufacturer and already contain all the important components: batteries, inverters, and even control intelligence. As a result, the components are optimally coordinated with one another and the manufacturer offers a system guarantee.


Generally speaking, however, this type of premium provider only works with selected partners and exclusive distribution rights. Many small electrical manufacturers have to make do with individual components purchased wholesale from different manufacturers and laboriously piece them together. Then if there’s a problem, each component manufacturer can blame the malfunction on the others without having to offer a solution. And if in the meantime your electrician goes bankrupt or retires, you have no one to contact. That’s why it’s recommended that you buy a complete system. Then you’ll always have access to a customer hotline and a service technician when you need one.

Energy storage can be purchased for different purposes. Most people are thinking about optimizing their consumption of self-generated electricity. They want to be independent of conventional energy sources, contribute to the energy transition, and save money in the process. But there are also people who are afraid of blackouts and prioritize the emergency power function: If configured correctly, their domestic mains supply can continue to operate during a power outage, while their neighbors are left in the dark. However, the average blackout time per person in Germany in 2016 was a mere 11.9 minutes, which is statistically and rationally insignificant – similar to a being in a plane crash.


Whatever you decide, the important thing to remember is that you can’t have both. Emergency power systems not only consume more power, they always have to block out battery capacity that then isn’t available for optimizing the consumption of self-generated power. This makes them much less efficient than other systems.

EEG- or KfW-subsidized PV installations are obligated to limit their active power feed-in to 70 percent (EEG) or 50 percent (KfW) of the PV’s rated power. If the excess electricity isn’t consumed immediately, there can even be curtailment losses: This means that even if the sun is shining, no more electricity is produced.

Energy storage can reduce these losses – but only if they have an intelligent energy management system that uses a predictive charging strategy. Whereas conventional batteries simply store excess energy and have to be curtailed at about noon because they’re fully charged, predictive systems can delay charging until the later hours and avoid curtailment losses. The University of Applied Sciences (HTW) in Berlin conducted a large-scale that determined that conventional batteries have losses in yield of about eight percent, whereas predictive systems lose less than two percent. Intelligent systems can save money and also make a significant contribution to grid relief.


Storage study

Some manufacturers offer outdoor systems that can, for example, be installed in the garage or mounted on a wall of the house. This is especially practical for the American and Australian markets because the houses often lack basements. These systems are often less efficient because they require both a water pump for cooling and a heater, both of which are constantly consuming power. In addition, these systems are often closed, meaning that their capacities can’t be upgraded later to meet increased need. And if the pump fails after a few years, the entire system has to be replaced. So unless you absolutely have to have an outdoor system, you should think carefully before purchasing one.

If you want to optimize your consumption of self-generated power in the future, you’ll need an intelligent energy management system that interconnects all the electrical consumers in your household and acts to minimize power consumption and maximize coverage with self-generated solar electricity. It’s especially important that the heat pump, heating element, and electric vehicle wall box be intelligently connected to the battery.

Many manufacturers advertise long guarantee periods to attract customers: But the devil’s in the details. Hidden costs for travel, repair, and spare parts are often shifted onto the customer, even when a guarantee is in place. That’s why the Verbraucherzentrale NRW (Consumer Association of North Rhine-Westphalia) recently admonished several manufacturers of electricity storage systems. Our tip: Before you buy an energy storage system, carefully read through all the guarantee conditions.


Verbraucherzentrale NRW amonished five manufacturers of electricity storage systems

Released by Marlon Hassel