Grey Water Storage: Definition, How It Works, and Applications

A Grey Water Storage Tank is a battery energy storage system (BESS) that can be operated without a dedicated connection to a renewable energy source. A renewable energy connection is possible, but it primarily charges and discharges from the public grid. However, the difference to the green electricity storage system is increasingly losing significance due to regulatory developments through MiSpeL: Today, the crucial factor is no longer the origin of the charging current, but the metering concept and the preparation of the system for all applications.

What is a grey electricity storage system?

A grey electricity storage system is a battery storage system that its Charging current from the general power grid can draw from. In co-location with solar or wind power plants, it may of course also charge using renewable energy. The term „grey electricity” describes the origin: the electricity comes, among other things, from the grid electricity mix, which also combines conventional and renewable generation.

The counterpart is the Green electricity storage, which exclusively or predominantly absorbs electricity from PV, wind, or hydroelectric power plants. For a long time, this classic distinction was relevant for regulatory purposes—particularly regarding EEG funding eligibility. However, with the Solar Peak Act of February 2025 and the Federal Network Agency's MiSpeL determination taking effect by 2027 at the latest, it is increasingly losing its distinctiveness: A BESS can fulfill both roles simultaneously in the future with a clean metering concept.

The term „grey power storage” is also not an officially defined legal term. It is used in the market as a counterpart to green power storage—and remains relevant wherever there is no renewable energy generation system or where metrological delimitation is not provided for.

Typical use cases

Gray electricity storage systems are used in two fundamental contexts:

Front of the Meter (FTM) — the grid-scale electricity storage system as a market participant

In FTM operation, the grey electricity storage system acts as an independent market participant without an underlying consumption system. It charges from the grid when spot prices are low and feeds back into the grid when prices are high — a classic arbitrage model. In addition, grid services such as Control energy (FCR, aFRR, mFRR) and Current reserve the central revenue sources. This approach corresponds to the Standalone storage - a BESS without a coupled RES plant, acting exclusively as a market participant.

Behind the Meter (BTM) – the grey electricity storage system in industrial operations

In a BTM operation, the grid-electricity storage system is located behind a company's metering point — without its own PV or wind system. The primary levers here are:

• Peak Shaving — Capping peak loads at the grid connection point and thereby reducing the capacity charge component of the grid fees
• Load shifting — Buying grid power at cheap times, using it yourself at expensive times
• Grid fee optimization Atypical grid usage through targeted control of the load profile

For industrial companies without their own generation plant, the BTM grey power storage system is often the easiest entry point into active energy cost optimization.

MiSpeL as a turning point: Why the distinction between green and grey is losing importance

The classic separation between green and grey electricity storage was regulatory-forced: under the previous exclusivity option (§ 19 para. 3a EEG), a storage facility immediately lost its claim to EEG subsidies as soon as it absorbed even a single kilowatt-hour of grid electricity. This forced operators to make a choice: either EEG subsidies or flexible market optimization — both together were not possible.

With the Solar peak law on February 25, 2025, the legislature broke this logic. The new Accrual option (§ 19 para. 3b EEG, § 21 para. 1–4 EnFG) will in future make it possible for a storage facility to absorb both green and grey electricity components — and still receive proportional EEG funding for the green component. The Federal Network Agency is shaping the specific implementation in the ongoing MiSpeL-determination procedure (ref. no.: 618-25-02); the determination must enter into force by June 30, 2026.

What that means in practice: A BESS combined with a RES plant and featuring a clean metering concept for the 15-minute differentiation of green and grey electricity shares will in future be neither a purely green nor a purely grey electricity storage system — but rather a mixed-flow storage pump, which flexibly assumes both roles. The terms green and gray electricity storage then merely describe the current operating mode, no longer the system architecture.

The decisive factor will therefore not be the origin of the charging current, but:

• the Measurement concept — calibration-compliant recording in 15-minute intervals
• the accounting cutoff — mathematically unique allocation of green and grey electricity shares
• the EMS Intelligence — optimal real-time control between operating modes

Regulatory Framework

Market Participant Registry (MaStR)

Every grey electricity storage system with an output of 1 kW or more is in the MaStR to register — as an independent entity, regardless of whether a renewable energy system is present or not.

Grid fees: Section 118 Paragraph 6 EnWG

Newly constructed storage facilities that are commissioned by August 4, 2029, are exempt from grid fees for electricity drawn for storage and fed back in for a period of 20 years. With the amendment to the Energy Industry Act (EnWG) of November 2025, this exemption now also applies proportionately to hybrid storage facilities — in other words, to the quantities of electricity that are actually fed back into the grid. Pure grey electricity storage facilities in front-of-the-meter (FTM) operation thus also fully benefit from the regulation. The Federal Network Agency has in this regard AgNes Interim Report announced protection of legitimate expectations for the full exemption from grid fees until August 2029.

EEG funding for grey electricity storage systems in combination with a renewable energy plant

A purely gray electricity storage system without renewable energy coupling is not eligible for EEG funding. However, if it is combined with a renewable energy system and features the required metering concept, it can receive a proportionate EEG subsidy for the fed-in green share via the demarcation option (§ 19 para. 3b EEG) once the MiSpeL determination enters into force — and thus be economically equated with a green electricity storage system.

C&I storage: the commercial and industrial grey electricity storage system

C&I (Commercial & Industrial) storage refers to battery storage systems designed for commercial and industrial customers — typically for the direct optimization of their own energy costs. The term describes the customer segment, not necessarily the exclusion of FTM capability.

From a system size of at least 1 MW / 2 MWh Does CUBE CONCEPTS also prepare C&I storage systems for FTM operation by default—including metering concepts, metrological delineation, and EMS preparation for grid balancing and trading? For this system size, this is economically sensible and is often explicitly requested by clients: during the day, the storage system primarily optimizes self-consumption and peak loads—and generates FTM revenues at night or during periods of low load.

The only case in which CUBE CONCEPTS dispenses with FTM preparation is a deliberate decision by the client for pure BTM operation — for example, if there is no interest in market revenues, the grid connection does not allow for feed-in, or the budget is clearly focused on BTM economic viability. This reduces system costs, but limits subsequent expandability.

Profitability & Revenue Structure

Generally, a distinction must also be made between two financing methods for the gray electricity storage system.

Purchase (traditional investment model)

For companies that pursue clear BTM goals and whose economic efficiency can be directly demonstrated via peak shaving, load shifting, or grid fee optimization, the Battery storage purchase the direct route. On the cost side, there are CAPEX (system costs, planning, grid connection, installation) and OPEX (maintenance, insurance, monitoring, operation). Depending on the system size and configuration, the specific system costs for lithium-ion BESS today typically range from 200–400 €/kWh (System price, indicative).

Contracting or CUBE Profit Flex Solution®

For companies for which the purchase does not yet pay off immediately, CUBE CONCEPTS offers the CPFS- Model: The storage system is provided with zero CAPEX and zero OPEX. The company benefits from a profit share in the FTM revenues (trading, balancing power), which typically pays for the storage system within about two years. After that, the BESS is available entirely for the company's own BTM or hybrid operation.

CUBE CONCEPTS prepares the storage system for both operating modes right from the start — including the measurement concept for a clear distinction between green and grey electricity shares. This is the prerequisite for both the MiSpeL separation option and a later combination with a renewable energy system.

Grey electricity storage vs. green electricity storage

With the MiSpeL differentiation option, the boundary between the two storage types is increasingly blurring. What remains is the question of the measurement concept and the operating strategy — no longer that of the system type.

Frequently Asked Questions