The intermediate storage of renewable energy and the grid-stabilizing properties of electrical storage systems are key building blocks on the path toward a carbon-neutral electricity system. Decentralized home storage systems and electric vehicles already collectively provide considerable storage capacity, yet this remains largely untapped for system-serving purposes. A contributing factor has been that charging a storage from the grid incurred taxes, levies, and grid fees that were not reimbursed upon subsequent feed in, rendering intermediate storage economically unviable. German legislation has since addressed this by introducing partly reimbursement mechanisms for these charges.

Where a renewable generation system is present behind the grid connection point, a distinction must be made between intermediately stored grid electricity, stored green electricity, and direct PV feed-in since each is subject to different regulatory treatment. Once both green and grey electricity are held within the same storage system, an exact metrological separation is no longer possible. This paper examines these challenges and analyses the German Federal Network Agency’s proposals for the categorisation and valuation of these different energy quantities in the context of the market integration of storage systems (MiSpeL). Two billing approaches are presented, the “Pauschaloption” (flat-rate option) and the “Abgrenzungsoption” (delineation option). Under the “Pauschaloption”, the first 500 kWh per kWp and year fed into the grid are classified as green electricity , while any feed-in exceeding this threshold is assumed to constitute intermediately stored grid-sourced electricity. This approach requires only a single meter. The “Abgrenzungsoption” uses a two-meter concept to track the green and grey shares charged into storage throughout the year, applying the resulting ratio to classify fed-in electricity accordingly. These shares are not allocated on a 15-minute basis, rather the resulting energy mix is determined over the entire calendar year. At the end of the year, the same annual ratio is applied to the electricity fed back into the grid in order to classify the corresponding feed-in quantities. The effects of both approaches are investigated through simulations for stationary battery storage and bidirectionally charging electric vehicles in a residential context across a broad range of scenarios.

The “Pauschaloption” benefits users with low self-consumption ratios, enabling cost savings of up to 30%, but weakens the incentive for local PV utilization, as conventional incentive mechanisms for on-site consumption are effectively undermined by assuming a uniform self-consumption rate of 50% for all customers. High self-consumption households face barriers to energy trading, as feed-in thresholds must be exceeded before surcharge exemptions apply. The “Abgrenzungsoption” introduces a central problem since allocation is based on annual energy balances, PV electricity can be offset against previously consumed grid electricity, even if no corresponding trading activity has actually taken place, potentially resulting in unjustified reimbursement of levies. Both approaches increase the operational complexity for Home Energy Management Systems (HEMS), as forecast-based strategies are required. A complementary approach to reduce abusive exploitation of the reimbursement mechanism is additionally proposed.