Retrofitting battery storage at a solar park: what does the MiSpeL ruling change?

Many solar parks that now have an offer for a battery on the table were planned without one. The park feeds in at midday, when electricity is worth least; in 2025 almost a quarter of a new park's output fell into hours with a negative price. A battery at the same grid connection is meant to change that. Until now it came with a hard limit: it was only allowed to charge the park's solar power. A single kilowatt-hour from the grid within the calendar year put the support for the stored electricity at risk.

On 1 October 2026 the Federal Network Agency (Bundesnetzagentur) lifted that limit. Since then storage forecasts read differently: the battery charges wind power at night, works in winter too and is supposed to earn correspondingly more. We have recalculated how much of that depends on grid electricity, what the switch costs an older park and which consents are missing while the transition period runs.

What does the MiSpeL ruling of 1 October 2026 regulate?

It governs how solar and grid electricity in a shared battery are separated in the accounts. The ruling's full name is “market integration of storage and charging points” (Marktintegration von Speichern und Ladepunkten, file no. 618-25-02); it was issued by the Federal Network Agency and has been effective since its publication. For electricity that is stored before being fed in, the Renewable Energy Sources Act (EEG) knows three routes (§ 19 (3)): the exclusivity option, which already existed, plus the demarcation option and the flat-rate option. The two new ones have been in the statute since the Solar Peak Act of February 2025, but under § 100 (34) EEG they only became applicable once the agency had laid down the details. That has now happened.

For a ground-mounted park only the demarcation option matters. It applies to direct marketing with the market premium, not to the fixed feed-in tariff. Support continues for the solar power, whether fed in directly or stored first. For grid electricity that the battery charges and later feeds back, there is no market premium; in return, the levies on it fall away (§ 21 EnFG). Which kilowatt-hour counts as what is metered per quarter-hour and settled per calendar month. The draft of autumn 2025 had still provided for the calendar year.

Two allocation rules are set by statute and shape every settlement (§ 21 (4) EnFG). If the connection draws electricity from the grid in a quarter-hour while the battery is charging, the grid electricity counts first as consumed in the battery. If the connection feeds in while the battery is discharging, the battery's electricity counts first as fed in. For electricity fed in despite negative prices, the levy advantage falls away and, where the EEG provides for it for the plant, so does the support. The incentive not to feed in during such hours therefore remains.

Why was a battery at a solar park not allowed to charge from the grid until now?

Because the EEG only supports electricity from renewable sources, and grid electricity in a battery cannot be told apart from solar power. The statute's answer so far was an either-or: a battery only belongs to the supported plant if it takes in nothing but renewable electricity throughout the calendar year (§ 3 no. 1 and § 19 (3a) EEG). For operation that meant: the battery charges when the park delivers, otherwise not. At night, on dull days and in winter it stands idle, although price differences on the power market can be large at exactly those times. Balancing power, where a battery also has to charge from the grid on call, did not fit.

The demarcation option replaces the either-or with bookkeeping. How a battery at a park differs in principle from a battery without one is covered in Co-located vs. stand-alone; this article is about the park that already exists.

What does a battery add to a solar park with and without grid electricity?

In our calculation the battery earns the larger part of its additional revenue with solar power alone; grid electricity mainly fills the winter. We modelled a one-megawatt park near Hanover with a battery of 0.5 megawatts and one megawatt-hour at the same connection, hour by hour with the day-ahead prices of 2025. The park belongs to the latest generation: when the price is negative, its applicable value drops to zero and it curtails.

Three things can be read off. First, the battery works on 292 days of the year even without grid electricity. The 73 days without a cycle all lie between October and February, when the park does not fill it; that is just under half of the 151 days in those months. Second, in solar-only operation it recovers 82 of the 232 megawatt-hours that the park would curtail in negative-price hours without a battery, a good third. Eligible feed-in rises from 716 to 756 megawatt-hours as a result. Third, grid electricity adds 30 percent for the battery, and 49 percent with a second cycle per day. Of the total additional revenue, 23 to 33 percent therefore depends on grid electricity.

What stands out is what happens to the solar power once grid electricity is allowed: the battery then charges two thirds from the grid, at midday too. In hours with a negative price it is paid for drawing power, while the park's solar output earns nothing in the same hour. From the negative-price hours it takes in only 26 instead of 82 megawatt-hours of solar power, the park curtails more again, and the eligible volume falls to 724 megawatt-hours. For the battery that is the right calculation. Anyone who sees it as a rescue for the park's solar power should know: with grid electricity the battery becomes a business of its own at the park's connection.

The order of magnitude holds when the assumptions are changed. With 2024 prices the index stands at 136 and 155, with a battery twice the size at two megawatt-hours at 145 and 161, with 80 percent efficiency at 131 and 146. What the calculation leaves out weighs more: balancing power and additional cycles in intraday trading are missing entirely, as is the cost of import capacity at the grid connection. And no battery knows the day's prices in advance; the figures are an upper bound for the day-ahead market, not an expectation. How strongly storage duration shapes the result is shown in 2-hour or 4-hour battery.

One meter or two?

Where only storage and generation sit behind the connection, the ruling allows a simplification with a single meter. It has a price: eligible is then the total feed-in minus the total grid import. The storage losses on grid electricity therefore come off the supported volume. In our calculation that is 34 megawatt-hours a year, or 4.7 percent of eligible feed-in. At a market premium of 0.3 cents that is around 100 euros, at a premium of four cents and the same dispatch around 1,400 euros a year. The second meter pays off sooner, the higher the park's tariff. The formula set is chosen for the whole calendar year as a rule.

What changes in the park's payment if it chooses the demarcation option?

Nothing for parks with award and commissioning from 2023, the calculation of the market premium for older parks. Plants that were commissioned before 1 January 2023 or received their award before that date get the market premium month by month as the difference between the applicable value and the monthly market value for solar. If the market value in a month is above the applicable value, the premium is zero; the higher market revenue stays with the park all the same. With the demarcation option the annual market value applies to these parks (Annex 1 no. 2 sentence 3 EEG): a single difference for the whole year. The expensive winter months, in which the park earns more than its applicable value under the monthly calculation, are then offset against the cheap summer months. How both market values are derived is explained in Solar market value.

The lower the applicable value, the more expensive the switch. A park from the October 2018 tender with 4.69 cents would have received no premium in seven of twelve months of 2025 under the monthly calculation, because the market value was higher, and a correspondingly higher one in the other five: 1.13 cents per kilowatt-hour over the year. Under the annual market value it would have been 0.18 cents. The difference corresponds to around 9,000 euros per megawatt and year. A 2014 park with 9.23 cents hardly notices the switch, because its value is almost always above the market value.

For comparison: in our storage calculation grid electricity adds around 8,000 euros a year on the 2025 day-ahead market, just under 13,000 euros with two cycles, before all costs and with full knowledge of prices. For a park with a low award value, switching to the demarcation option can therefore cost more than grid electricity earns the battery on this market. Whether balancing power closes the gap is a question for the provider's forecast. Parks with an award from an innovation tender cannot choose the option at all; their batteries must continue to charge only electricity from their own plant combination.

In the consultation, numerous associations and companies asked that existing parks keep the monthly market value. The Federal Network Agency shares the concern and considers the purpose of the provision largely obsolete since settlement became monthly; but only the legislator can change it, it says. The government draft of the EEG 2027 does not do so for now. It carries the switch forward and adds a lock: anyone who leaves the option again stays on the annual market value until the end of the following calendar year (§ 100 (6) of the draft). A short trial therefore does not come free. What else the draft provides for solar parks is covered in the article on the EEG 2027.

Who has to agree until 30 September 2027?

The grid operator and the metering point operator, both of them. The ruling has applied since 1 October 2026, but until 30 September 2027 it can only be used if settlement and the park's assignment to the demarcation option take place with the consent of both (operative part no. 9 a). The Federal Network Agency justifies this with the time grid and metering point operators need for their settlement processes. During this period the park operator has no entitlement to the option; afterwards both must make it possible (operative part no. 7).

For a forecast this means: revenue from grid electricity before October 2027 is only as reliable as the written consent behind it. A grid operator's commitment to connect the battery is not yet consent to settlement under the demarcation option. Then there is the assignment itself: the park is assigned to direct marketing with the addition of the demarcation option, under the rules for changing the form of sale (§ 21b and § 21c EEG). Its electricity must be kept in a separate balancing group (§ 20 sentence 2 EEG). Both run through the direct marketer, whose contract has to be adapted for this.

What has to fit at the grid connection, in the lease and with the bank?

More than for a battery that only charges solar power, because grid electricity brings a second direction into play. Four points need checking.

• Grid connection: the park was connected as a feed-in plant. If the battery is to charge from the grid, it needs an agreed import capacity. A battery that also takes in grid electricity does not count as a plant within the meaning of the EEG (§ 3 no. 1); its connection is governed by the Energy Industry Act (§ 17 EnWG). For the import the grid operator can demand a construction cost contribution (Baukostenzuschuss); the Federal Court of Justice approved this for battery storage on 15 July 2025 (EnVR 1/24). This item is missing from our calculation.
• Grid fees: the exemption of stored electricity from grid fees (§ 118 (6) EnWG) refers to the levy rules and, in the Federal Network Agency's understanding, thereby also to the new demarcation. What grid fees for storage will look like from 2029 is being settled in a separate procedure; the MiSpeL ruling says nothing on this. A forecast beyond 2028 needs a stated assumption for it. The current position is described in Grid fees for battery storage.
• Lease, easement and permit: the purpose of use and the easement of many parks name photovoltaic installations including ancillary facilities. Whether a battery that trades grid electricity falls under that is a matter of interpretation and should be settled in writing with the landowner before the container is in place. The same applies to the building permit and the development plan. What matters in the contract is covered in The lease from the investor's perspective.
• Bank and insurance: the loan agreement of a financed park is based on its revenue model. A battery and a different calculation of the market premium change it; the bank's consent therefore belongs before the order. The insurance must include the battery with fire risk and business interruption.

For tax purposes the battery is an investment of its own with its own depreciation. Whether and to what extent the investment deduction (Investitionsabzugsbetrag) and special depreciation come into question for it depends on the design and the individual case. That is a question for the tax adviser, not for the prospectus.

Which questions belong to the provider when a forecast includes grid electricity?

Seven. Each can be answered with a document or a number.

1. Which option is used for settlement, exclusivity or demarcation? From which month?
2. Are the consents of the grid operator and the metering point operator available in writing? If not: does the forecast only assume grid electricity from October 2027?
3. Which metering concept is planned, one meter or two? How much eligible volume does the simplification cost?
4. Did the park receive its award or was it commissioned before 2023? If so: where does the forecast show the switch to the annual market value? Which market values is it based on?
5. What share of the storage revenue comes from solar power, what share from grid electricity, what share from balancing power? What does the result look like without grid electricity?
6. Which import capacity has been agreed with the grid operator? What does it cost (construction cost contribution, grid fees after 2028)?
7. Do the lease, easement, permit, insurance and loan agreement cover the battery and the way it is operated?

The fifth question is the most important. A forecast that does not hold without grid electricity depends on two third-party consents until October 2027, and after that on markets that change with every additional battery. Where a battery's revenue comes from in detail is shown in Direct marketing explained.

What speaks against retrofitting?

Four points that rarely sit at the front of an offer.

• The battery shares the connection with the park. When the park feeds in at full power at midday, the battery cannot discharge at the same time. In our calculation that matters little because it discharges in the evening; with more tightly sized connections it matters more.
• Storage revenue is market revenue without a support commitment. The market premium secures the solar power, not the price differences the battery lives on. The more batteries are connected, the smaller these differences tend to become.
• The battery ages faster than the park and has its own replacement cycle; see Battery storage lifetime.
• For older parks with a low award value, the annual market value can eat up the advantage of grid electricity. Anyone retrofitting there can stay with the exclusivity option: the battery charges solar power only, the park keeps the monthly market value, and in our calculation the battery still earns the larger part of what the day-ahead market has to offer.

How we assess parks with storage

We run every storage forecast twice: once as presented, once without grid electricity. If the project holds in the second version, grid electricity is an opportunity. If it only holds in the first, it is a condition; then we want to see the consents, the metering concept and the agreed import capacity. For parks from before 2023 we put the switch to the annual market value next to it, using official market values. What remains after this review we discuss in a non-binding initial call. We give no return promises in the process.