Understanding Battery Storage: Terms, Categories, and Their Place in the Market

Battery storage systems are a key component of the energy transition – and at the same time an area where terminology quickly becomes confusing. Depending on the context, people speak of home storage systems, commercial storage systems, large-scale storage systems, C&I storage systems, or BESS. However, these terms do not all describe the same level: some refer to size categories, others to fields of application, and still others to technical system boundaries.

This report classifies the most important terminology and explains how they are interrelated. It is aimed at planners, investors, project developers, and decision-makers who come into contact with battery energy storage projects and are looking for a solid foundation for technical classification.

What is a BESS?

BESS stands for Battery Energy Storage System and is the internationally established generic term for stationary battery storage systems. This refers not only to the battery itself, but to the complete system consisting of:

• Battery modules (cells → modules → racks or containers)
• Battery Management System (BMS) for monitoring and protection functions
• Power Conversion System (PCS) or inverter
• Energy Management System (EMS) for control and optimization
• Fire Protection and Safety Systems

The term BESS is technology-neutral and size-independent—it applies equally to a 10-kWh home system and a 100-MWh utility-scale storage system. Because BESS describes the entire system, it is the most precise term to use in technical texts, requests for proposals, and project documents.

What terms are used in the market?

Several terms have become established simultaneously in the German market. They overlap to some extent and are not always used consistently in practice. The following classification provides clarity.

Home storage

Home storage systems are BESS for residential households. Typical capacities range between 5 and 20 kWh, the power between 3 and 10 kW. Their primary purpose is almost exclusively to optimize self-consumption in combination with a PV system. Home storage systems are typically installed as BTM (behind-the-meter) systems, meaning they are located behind the household’s grid connection.

Commercial storage

Commercial storage facilities cover the range from approx. 50 kWh to 1 MWh and are used in businesses, retail, and small commercial operations. Typical applications include peak shaving, optimizing self-consumption, and improving grid connection capacity. Commercial storage systems are also generally BTM systems and are therefore primarily geared toward operational energy needs. These include traditional sectors such as Load shifting, Peak Shaving or the Self-consumption optimization.

C&I; Storage

C&I stands for Commercial & Industrial and refers to storage solutions for commercial and industrial applications. The term is more market- and customer-segment-oriented than technical: it describes the field of application, not a fixed performance class. C&I; Storage overlap with commercial storage facilities, but typically fall within the range of from 100 kWh to 10 MWh and often include market-related applications such as Control energy, the Day-Ahead Trading or Intraday Trading one.

Utility-scale energy storage systems / Utility-scale BESS

Large-scale energy storage systems—also known internationally as utility-scale BESS—are energy storage facilities starting at approximately 1 MWh, which are typically operated by grid utilities or market participants. In Germany, projects in the field of from 10 to several hundred MWh no longer a rarity. Large-scale storage systems are typically designed as FTM (front-of-the-meter) systems—that is, as standalone grid installations with a direct connection point to the medium- or high-voltage grid.

The most important distinction: FTM & BTM

Beyond size categories, the distinction between front-of-the-meter (FTM) and behind-the-meter (BTM) is the most fundamental classification in the professional BESS market. It determines not only the technical configuration, but also regulatory requirements, grid fees, and business models.

FTM – Front-of-the-Meter

• System at or upstream of the grid connection point (independent grid facility)
• Direct participation in energy markets: balancing power, spot market arbitrage, direct marketing
• Typical for large-scale storage systems, utility-scale BESS, and grid-supporting applications
• Treated for regulatory purposes as a generation facility or grid facility
• Mandatory MaStR registration, BImSchG permit if required, grid connection in accordance with KraftNAV / EnWG

BTM – Behind-the-Meter

• System located behind an end-user's meter
• Primarily geared toward self-consumption, load optimization, or self-sufficiency
• Typical for residential storage, commercial storage, and C&I systems;
• Exemption from grid fees is possible (Section 19 of the StromNEV) under certain conditions
• Section 118 of the Energy Industry Act (EnWG): Protection Against Double Charging for Grid Fees for Storage Facilities

An Overview of Typical Use Cases

The terminology used in use cases is closely linked to the FTM/BTM distinction and is often used interchangeably in technical texts, even though the terms describe different levels:

• Self-consumption optimization  BTM – Utilizing PV Surplus, Reducing Grid Purchase Costs
• Peak Shaving  BTM – Peak load reduction, avoidance of high capacity charges
• Arbitrage / Spot Market Optimization:  FTM & BTM – Charging at low prices, discharging at high prices
• Balancing energy (FCR / aFRR / mFRR):  FTM – Provision of system services for TSOs and DSOs
• Current reserve: FTM – Provision of extremely short-term system services for the stabilization of the public power grid
• Emergency power supply / Off-grid operation:  BTM – Security of supply during power outages, USV unit
• Value, respectively. Revenue Stacking:  FTM & BTM – Combining multiple use cases in one system for yield maximization

Regulatory Framework

Battery storage systems are subject to a complex regulatory framework in Germany. Here are some important reference points:

• MaStR registration obligation:  All BESS starting from 1 kW must be in the Market Participant Registry (MaStR) be registered with the Federal Network Agency.
• § 19 of the Electricity Regulation (StromNEV):  Under certain conditions, BTM storage systems are exempt from grid fees—a key economic factor. (Ends 2029)
• § 118 EnWG:  Protection mechanism against double charging of grid fees during the charging and discharging of storage systems. (Ends on August 4, 2029)
• KraftNAV / EnWG:  Govern the grid connection of storage systems, particularly in the FTM sector.
• Federal Immission Control Act  Larger BESS systems may require permits under environmental protection laws (particularly once certain capacity thresholds are reached).
• Fire protection:  Requirements from building code regulations and VdS guidelines depending on the location and system size.

Overview: Terms Compared

Why differentiation is important

In tenders, project documents, and technical articles, the terms described above are often used inconsistently. In practice, this frequently leads to misunderstandings—for example, when a bid for a commercial storage system lacks information on FTM/BTM integration or when terminology remains unclear during consultations.

Therefore, it is advisable to explicitly define at the beginning of project-related documents which size segment, integration form, and use cases are meant. This not only increases clarity, but also protects against regulatory and contractual ambiguities.

Conclusion

Battery storage systems cannot be neatly described with a single term. Home storage, commercial storage, C&I, utility-scale storage, and BESS represent different perspectives on the same core technology. The decisive dividing line in the professional market is less the size category than the FTM/BTM integration: it determines regulation, business model, and economic viability.

CUBE CONCEPTS accompanies projects across all segments – from commercial behind-the-meter solutions to front-the-meter large-scale storage systems in the utility-scale sector. Contact us if you would like to classify your project technically or structure a solution.