Storage has landed

11 / 2012, Storage & smart grids | By:  Michael Fuhs, Shamsiah Ali-Oettinger

Storage overview: The smorgasbord of storage solutions is on an upward trend as the products on offer increase this year. The first market overview on storage provides an orientation to integrated solutions and to this hot topic.

The market overview supplies information on over 80 storage systems from around 30 companies.

Whoever seeks for a storage solution on the market today faces a mammoth task. The selection is aplenty and the data is not all that easy to compare, or even decipher at times. There are no clear regulations and directives to look for either. The market overview on storage systems that we present consolidates the information through filled questionnaires from companies, and tries to shed some light on the complicated details. Around 30 manufacturers participated and the answers can be found on pages 102 to 105.

The request for storage with solar installations is a trend that is set to take off exponentially at some point. According to EuPD Research’s “Dawning of a New Era – Photovoltaics at the Edge of Grid Parity” in this “post-grid parity” market, the fundamental structure of PV systems is set to be transformed. From residential PV systems feeding power into the grid, existing and new systems are likely to feature storage as the report states. This scenario is seen as the third demand cycle for industry stakeholders who want to differentiate themselves and provide solutions with energy management tools, smart PV inverters and energy storage solutions.

Now before racking one’s brains on which solution to purchase, one should ask if the system itself is deliverable in the first place. Purchase at times does not mean it is instantly delivered to the doorstep. Some manufacturers state that their system is being introduced or will be within the next half of the year. This means there is a high possibility of delay. “There are not many firms at the moment who, apart from their demonstration projects, have concrete solutions with fixed delivery options,” explains Matthias Vetter, Head of PV Off-Grid Solutions and Battery System Technology, Fraunhofer ISE.

Jan van der Walle, for one, has already installed solar panels with battery storage systems at four households in summer in Germany. The owners came to him with the request for storage as they were not particularly looking to generate returns. “They are afraid of acute electricity price increases,” he explains. What Van der Walle did when the request came to him was to proceed to take a look at different offers on the market. However “good functional systems were rarities” as the installer laments. He finally decided on Nedap that doubles as a solar inverter. The installer could connect a battery and as he says, it did not cost any more than a high-end solar inverter.

One challenge lies in a detail that few ponder over. Looking at Germany, grid operators in rural areas often have concerns about single-phase power plants. They worry that these can eventually increase the load imbalance. The electricity grid has three phases that ought to be equally supplied and drained whenever possible. With PV plants, up to 4.6 kW are allowed, but only on single phase feed-in. Hence with storage a grey area appears. “We have mounted three storage systems at three installations and connected each to every phase,” says van der Walle. That is one, if not the only, solution. Single-phase systems can be installed too, though it is recommended to have a word with the grid operator first. The critical point is whether the single-phase storage systems are allowed to feed at that one phase contrary to what household devices use.

Many electricity meters still remain so that the aim of storage is fulfilled: to achieve cost savings. There is a legislative catch here, one that is interpreted in different ways. On the one hand are companies like Eon Bayern, a grid operator who does not see a problem. On the other side are companies like Prosol Invest. The smaller single-phase storage systems from this manufacturer go offline and connect all three phases to one as soon as the battery kicks in for the consumer. This is “to be on the safe side,” says company head Torsten Stiefenhofer. This theme has manifold dimensions and will be covered in upcoming editions.

Jan van der Walle chose lead batteries for storage that cost around €1,000/kWh of capacity. In this case, it was easy to find a spot. There was an unused storage area that was airy enough for setting the system up. A spot for setup, the battery and inverter sizes and prices are only a few of the details that we asked manufacturers and dealers about. The questionnaire contained 100 questions requiring answers in order to assess the systems.

Selection criteria

It’s clear that complex information is a challenge to decipher. One possibility for clarity is to get the dealer’s opinion. Many systems are integrated with the right inverter and battery combination. Some like those from SMA and Nedap come without batteries. Others from Solutronic or Prosol Invest come with them.

Of course price plays a role. Some manufacturers have given the unbinding suggested price for the end-customer. With the cheaper variations, a kWh of usable capacity comes to €600, with the pricey ones at €3,000. The prices do not allow practical comparison. The services scope and the dimensions of the components just vary too much. A huge portion of the price variation comes from the differences in the batteries employed, their quality and effective parameters. When it comes to replacement of batteries, some say five years while others state that the battery can pull through 20.

A cheaper battery might actually do the trick here. Buy a cheaper one to feed the speculation that the pricier variation will become cheaper in a few years.

The dimensioning of PV plants and battery capacities is a sticky subject. It is rather fixed what type of storage systems come into question in the first place. With regards to the so-called DC-coupled systems, the battery and inverter sizes are coupled together within a small frame. With AC-coupled systems, the flexibility is bigger. Still more often than not dealers offer only one battery size.

How much is too much?

It is essential to ask what the end-customer wants, whether a large rate of self-consumption from his own power or a high autarky degree. This is linked to the question of whether the consumer wants to achieve high returns, or when it is enough that he does not have to pay additional fees, that a certain level of autarky is appreciated. An accurate return calculation is not really possible as it varies highly with electricity price development prognoses, battery life and price decline of replacement batteries.

A rough estimation shows the sum for a period of 20 years for a system in a household that utilizes 4,700 kWh per year and manages 50% autarky. Under the speculative assumption that the electricity price up until 2030 reaches €0.35 per kWh and the investment has a 2% interest rate, the solar and storage system with 2013 prices would save about €12,000 in electricity costs- where the battery replacement investment has not been deducted. And with the compensation for grid-fed electricity added in, the sum comes up to €18,000 to €20,000, depending on the maintenance costs and module degradation.

An estimated 50% autarky status can be achieved with a 5 kWp solar installation and approximately 3.5 kWh of storage capacity (see Graph DC-and AC-Storage system, p. 98). Other configurations are possible too. The market overview and the questions posed on project designs show that according to the entries made by manufacturers these rates are well within the price frame. Even with the rough estimation of 2% interest rates amidst the Euro crisis, such combinations are still doable. Whoever seeks to optimize economically needs to adjust the size of his installation and the storage system.

The proportion of own consumption from self-generated power and the autarky degree will increase with a bigger storage capacity. The effect is lower with increasing size (see Graph “Influence of battery and solar capacity on the autarky degree”, p. 98).

In other words, for every extra kWh of battery capacity there is a little less extra cost savings. With the 4,700 kWh usage scenario and a 5 kW installation, the possible sensible limit will be a 4 to 6 kWh battery capacity.

The tendency is such that smaller solar installations can be financed easier with smaller batteries than bigger ones due to cost savings. They do not promise exceptionally high autarky.

Battery choice: lead or lithium?

For economic purposes, some brochures and websites of dealers and manufacturers provide the cost calculations. The harder question for customers and installers is the selection of the battery itself.

Opinions differ sharply when it comes to picking lead or lithium. Lead batteries have more experience on the field so they can be assessed relatively well. Lithium batteries, nevertheless, also hold advantages.

Lead batteries used with solar installations can achieve efficiencies of around 86% according to Matthias Vetter. These are estimations from the off-grid segment. With lithium batteries 95% is possible.

With regards to lifetime lithium batteries also seem to surpass lead. “With German irradiation values, a battery system that is attached to a solar system running for 20 years can be estimated to undergo 3,000 to 4,000 cycles, granted that the batteries also have a calendar life of 20 years,” says Vetter. Special battery technologies like those based on lithium titanate can have over 7,000 cycles even.

The number of cycles withstood depends on the depth of discharge and the effective capacity usage. Hence we also asked how the single systems are arranged and how many cycles can be reached under these conditions. The cycle number is represented when the battery capacity sinks to 80% nominal capacity. That is why it is not indicated that the battery must be replaced after this number. When the customer accepts the lower capacity they can still let it function.

The systems in the table differ from one another vastly with regards to charging and discharging capacities. The charging capacity is dependent on how fast the battery is charged in the morning, assuming the solar installation is big enough and the sun shines. The discharge capacity depends on the peak loads that can be supplied.

Vetter explains, “When the power electronics are cheap then there is nothing against high charge and discharge capacities”. But when it comes to money, then it makes more sense to cover the peak loads with grid power. C-rates of one and above are not important in this aspect.

“High charge and discharge capacities allow the system to be flexible enough to react to weather and consumption fluctuations,” says Armin Schmiegel from voltwerk. He adds that high capacities can then be essential during changing weather conditions or with east-west roofs as the batteries can be quickly charged. An important point is that in emergency operation, the AC-power is limited due to the battery discharge capacity.

AC versus DC

A basic question is whether a DC-or AC-coupled system is chosen (see Graph DC-and AC-Storage system, p. 98). The first devices that come from the off-grid segment or are for emergency power supply are AC-coupled, like the Sunny-Backup series from SMA. That means that the batteries are connected to the household grid over a converter and a charge regulator and are independent from the solar installation. The converter adjusts for charging the AC current to DC, and for discharging DC to AC. Converters and regulators are also indicated as controllers and battery inverters. DC-coupled systems have not been in operation for that long. One of the first was developed by voltwerk and its R&D partners in the research project Solion. Last year, more joined the bandwagon with similar systems, namely E3DC, Solutronic and Nedap. Their systems have the regulators for the batteries in the inverters’ DC-intermediate circuits. That way the inverter power electronics are used for the power out of the solar generators and the batteries. The biggest advantage of the so-called DC-high voltage system is that they have higher efficiencies than AC-systems.

The actual specifications given by manufacturers on the system efficiencies (to be found on pv magazine online) show no fundamental differences between AC-and DC-coupled systems. They vary between 80 and 90%. Still there is no standard as to how these figures are derived, so a comparison is not necessarily straightforward. That is why we asked for the efficiency of the “solar generator to battery” as well, where the DC-systems can exhibit their advantages. The devices with the highest values overall are DC-systems but they are not necessarily better than AC-systems. There are also numerous positives with AC-systems. They are more flexible in the planning and deployment stages. They can be subsequently installed without needing to switch the inverter and the battery system can be sized independently of the solar inverter.


For the selection of a storage system, there is the fundamental question of whether one chooses a DC-or AC-coupled product. The production meter is not needed with a small DC-coupled system, as the self-consumption bonus does not apply and there is no need for verification. For installations over 10 kW in Germany this is needed because from 2013 only 90% of solar capacity will be remunerated.

A stipulated autarky degree can be achieved through the different layouts of storage and solar installations. When a customer wants to generate 50% of the electricity himself, accordingly a 7 kWp PV capacity and a 3 kWh storage capacity would be needed. It also works with a 3 kWp capacity and a 7 kWh storage capacity. This is calculated based on a single-family household with an annual electricity consumption of 4,700 kWh as given by experts at HTW Berlin.

Measurements of different load profiles taken over the course of a day in summer and one in winter. The electricity used by household devices fluctuate vastly. Working with average figures will not be able to know the capacity that the battery really needs to provide.

On the left is the AC-coupled system Sunny Backup S from SMA, on the right is the DC system from voltwerk. The AC system has more components while the DC system has only the device on the right.

Self-consumption degree based on battery capacity and solar installation performance for a single-family household with a yearly electricity consumption of 4,700 kWh as given by experts at HTW Berlin.

Autarky degree based on battery capacity and solar installation performance for a single-family household with a yearly electricity consumption of 4,700 kWh as given by experts at HTW Berlin.

Project layout for single-family house and commercial business

We have asked the market overview participants for typical project layouts for two exemplary applications.

Example 1 – private household: Annual usage 4,000 to 5,000 kWh (single family house with two working parents and two children, and there is no solar power installed as yet on the 70 square meter roof space.)

Result: The suggestions from participants vary vastly: Solar capacity of 3 to 11 kWp, inverter capacity of 2 to 10 kW, storage capacity of 5 to 13 kWh. According to the entries, a 30 to 90% self-consumption rate and an autarky degree of 40 to 90% can be achieved. Costs vary between €17,000 and €35,000.

Example 2 – commercial business: 15 kWp installation, annual usage 30,000 to 60,000 kWh (according to German Energy and Water Industries, BDEW, consumption profile L1, dairy farm).

Result: The suggestions vary again vastly: Inverter capacity between 6 and 15 kW, storage capacity of 10 to 40 kWh. According to the entries, a 30 to 100% self-consumption rate and an autarky degree between 10 to 45% can be reached. Costs vary between €31,000 and €58,000.

The answers come from: Akasol, Azur Solar, Frankensolar, IBC Solar, juwi, MHH, Prosol Invest, SMA, Solon, Tritec, voltwerk and Würth Solar.

Market overview storage systems pages 102-105

The market overview is based on the entries of manufacturers or dealers. Due to the reasons that many systems are still under development and that entries are complex, it is best to directly ask the companies should questions arise.

The print version has been shortened. Subscribers can find the full version of the table at: www.pv-magazine.com/services/product-overviews/

The online version will hold the full table with answers from manufacturers and dealers covering more than 100 questions and comments. As standardization of the systems is still relatively new, the comments are extremely useful.

We thank Martin Rothert from SMA and Armin Schmiegel from voltwerk for the support rendered in conceptualizing this questionnaire.

Table glossary: The efficiency of the solar inverters, the maximum discharge capacity of the battery converter and the actual battery capacity are special characteristics that describe a storage system. Thus we have highlighted these columns.

Solar inverter AC nominal power: With DC-systems this is directly coupled with the battery system. With AC-systems this is eventually limited due to the maximum capacity of the switching components. But there is free selection.

Battery converter maximum charge capacity: The maximum charge capacity determines how quickly the battery can be charged, provided the solar installation is big enough and the sun shines.

Battery converter maximum discharge capacity: This determines which household loads can be supplied with the device. The charge and discharge capacities are determined by the power electronics, comprising the control and the battery characteristics. The C-rate shows how quickly the battery, with regards to its capacity, is discharged. 1C means that the battery completely discharges within one hour. The energy available to be drawn is also dependent on the C-rate.

Nominal battery capacity: With systems that are delivered with different capacities, there is a given range. Quite a number of systems are delivered with a fixed battery size, but most can be increased.

Actual battery capacity: According to how the control electronics are programmed, the actual battery capacity differs from the nominal battery capacity.

Phases: The electricity grid has three live phases in addition to the neutral cable. Many storage systems feed in on single-phase – just like many small PV systems.

Battery regulation: The battery system is for example connected to Phase 1 and a stove to Phase 2. If the battery system is regulated on the current from one phase, the stove cannot operate with the battery system. If the battery system is regulated by the total current over all phases, it feeds in over Phase 1 what the stove requires over Phase 2. That counts as self-consumption, but legally this falls into a gray zone in Germany.

Full feed-in: The device allows battery power to be fed into the grid. That is useful at the moment only to allow the battery regulator to regulate the total current over all three phases to null.

Standalone operation: Details found in online version. Many devices also function in standalone systems, some automatically switching to standalone when the battery overtakes power supply. Some devices can work unsymmetrically on three phases, feeding in every phase what is needed, while others feed in symmetrically. When phase coupling is applied to a device, the three phases are interconnected and supplied. That works for users who do not need three-phase current.

Efficiency: Details found in online version. System and individual component efficiencies.

Lifetime in cycles: Cycles up when nominal battery capacity sinks to 80%. This number depends on how heavily the battery is charged and discharged. This is given as DOD or Depth of Discharge. This is often measured as 80% DOD, which is when the capacity is used up to 80% every cycle. This entry is given online. In the print edition, we have indicated the cycles for the actual load in the specific storage system layout.

Minimum and maximum state of charge: Details found in the online version. This system setting determines the DOD and influences the battery life.

Calendric lifetime: Lifetime when no load cycles exist and the nominal capacity falls to 80%.

Details energy manager: The energy manager regulates when the battery is charged and discharged. It can be set so that self-consumption is maximized. It can also be set to relieve the grid via peak shaving. Some manufacturers offer an extra energy manager with more functions. Details found in the online version.

Mechanical data: The number of components, their sizes and weights can be found online.

Certificate and guarantees: Details found in the online version.

Price: Companies tend to be reluctant providing industry magazines with their price information. Some have given the recommended retail price for the end-customer, others for installers.