The commissioning date of a solar park: which date fixes the tariff, the deadlines and the depreciation

Every solar park prospectus contains a date that matters more than the purchase price: the commissioning date (Inbetriebnahme). From that day the tariff applies, from that day the 20 years run, and the deadlines of an award from the tender and the registration duty in the Marktstammdatenregister hang on it. Yet the term is often used loosely in offers: sometimes for the day the grid operator releases the connection, sometimes for the acceptance, sometimes for the first electricity.

For a direct investment there is a second layer. Tax law does not ask when the plant was commissioned under the EEG but when it was acquired, and that is the moment beneficial ownership passes. The two dates can fall in different years. Anyone who has used the investment deduction should know the difference before December runs out.

What counts as commissioning of a solar park under the EEG?

A solar installation is commissioned as soon as it generates electricity from sunlight for the first time after its technical operational readiness has been established. Sec. 3 no. 30 EEG defines it in three sentences: commissioning is the first putting into operation of the installation exclusively with renewable energy after its technical operational readiness has been established; the installation is ready when it has been installed permanently at the place intended for lasting operation and permanently with the accessories required to generate alternating current; and the later replacement of components does not change the date.

The Clearingstelle EEG|KWKG, the body that settles disputes between plant operators and grid operators, translates this into three conditions. First, an installation must exist, and under Sec. 3 no. 1 EEG that is every single module in the case of photovoltaics. Second, the module must be permanently mounted at its final location and permanently connected to an inverter; a loaned device for the test is not enough. Third, the operator must, by an active act, have the installation generate electricity for the first time that is converted or consumed outside the installation. Expressly not required are the grid connection, a metering device and the registration in the Marktstammdatenregister.

This leads to the most practical sentence in this article: a solar park can be commissioned under the EEG before it has fed a single kilowatt-hour into the grid. Whoever has the modules on the racks and the inverter permanently wired can secure the tariff of a support period even if the grid operator only releases the connection months later. What the grid operator then does is called Inbetriebsetzung in its paperwork: the release for feed-in under the connection rules and the plant certificate described in Grid connection, easement, building rights: how to tell whether a direct investment has truly secured its site. The two terms sound alike but denote different days, and prospectuses like to pass off the later one as the earlier.

The proof runs through the installer's commissioning protocol (Inbetriebnahmeprotokoll). The Federal Network Agency refers to it expressly in its register guidance: the date to be entered in the Marktstammdatenregister can be taken from the installer's commissioning protocol. Such a protocol is not a statutory form, but it is the document that the grid operator, the register and later the tax auditor want to see.

Which deadlines and amounts depend on the commissioning date?

The commissioning date governs the tariff, the duration of payment, the applicable version of the law, three deadlines for awarded plants and a one-month deadline for the register. The table sorts them with the respective provision; the paragraphs after it explain the points that move most for a park section from an investor's perspective.

The tariff only depends on the date if the park takes its applicable value from the statute, that is, if it is a ground-mounted plant of up to and including one megawatt (Sec. 22 (3) EEG). For these the base value is 7.00 cents per kilowatt-hour (Sec. 48 (1) EEG) and has fallen by one percent every six months since 1 February 2024, each time for the plants commissioned afterwards (Sec. 49 EEG). For commissioning from 1 August 2026 the Federal Network Agency lists 6.59 cents, for the period before 6.66 cents; the next step follows on 1 February 2027. A park operated with an award from the tender, by contrast, already has its value: the applicable value is the award value (Sec. 38b (1) EEG), and no commissioning month changes that. How the value turns into the market premium is explained in Direct marketing and the market premium: how a solar park earns its money.

The duration runs from commissioning, not from the grid connection. Sec. 25 (1) EEG grants the market premium and the feed-in tariff for 20 years and names the time of commissioning as the start. For plants with a statutory value, sentence 2 extends the period to 31 December of the twentieth year of payment, that is, to the end of the year in which the 20 years expire. For a park commissioned on 31 January that is up to eleven extra months; for one starting on 15 December, two weeks. Awarded plants do not get this extension: for them the 20 years end to the day. What comes afterwards is described in A solar park after 20 years: continued operation, repowering or decommissioning?.

The applicable version of the law is also decided by the date. Under Sec. 100 (1) of the government draft of the EEG 2027, plants remain under existing law if they were commissioned before 1 January 2027 or if their value was determined in a bid round before that day. For an awarded park the bid date therefore counts, for a park below one megawatt without a tender only the commissioning. The details of the transition, in particular the refinancing contribution for new plants, are covered in EEG 2027: what the refinancing contribution means for a solar park direct investment.

For awarded plants the date acts through three deadlines, all of which run from the public announcement of the award. The award lapses to the extent that the plant was not commissioned within 24 months or the payment entitlement (Zahlungsberechtigung) was not applied for, admissibly and with grounds, at the latest 26 months after the announcement (Sec. 37e EEG); the 26 months are a substantive cut-off, and the statute provides no extension for solar. The Federal Network Agency issues the payment entitlement only if the plant was commissioned after the award and before the application and all details are in the register (Sec. 38a (1) nos. 1 and 2 EEG). And whoever applies only after the 18th calendar month loses 0.3 cents per kilowatt-hour of the award value, for the entire term (Sec. 54 (1) EEG); what counts is receipt by the authority. Once issued, the payment entitlement is permanently assigned to the plant and is not transferable; it travels with the park, not with the bidder.

Registration in the Marktstammdatenregister is not a prerequisite of commissioning but a consequence: the operator must register the plant within one month of commissioning (Sec. 5 MaStRV). Until that is done, the market premium and the feed-in tariff do not fall due, nor do advance payments (Sec. 23 (1) MaStRV); the claim is not lost, the money simply arrives later. A sanction under Sec. 52 EEG has, since the EEG 2023, only applied to a double breach, that is, when the registration is missing and the operator has also failed to report the plant data to the grid operator under Sec. 71 EEG: then 10 euros per kilowatt and month, retroactively reduced to 2 euros once the duty is fulfilled. Under the EEG 2017 and 2021 a simple breach already cost 20 percent of the remuneration; for plants of those vintages it is worth checking at purchase whether the registration was on time.

What does a month of delay cost?

Per megawatt-peak, a missed degression step moves around 14,000 euros over the term, an application for the payment entitlement that is a few weeks late around 60,000 euros, and the calendar remainder under Sec. 25 EEG around 20,000 euros. The table compares the cases of our own calculation; the assumptions are deliberately simple: one megawatt-peak, 1,000 kilowatt-hours per kilowatt-peak and year, and for the market premium at the end of the period the 2025 annual market value for solar of 4.508 cents derived in Solar market value: why solar power is worth less and less on the exchange. All amounts are nominal, without discounting, curtailment or negative hours.

Two things stand out. First, the most expensive mistakes are not the best known: offers talk a lot about a degression step and rarely about the 18-month deadline of Sec. 54 EEG, although it moves four times as much. Second, the clock for awarded plants is a different one from that for plants with a statutory value. An awarded park gains not a cent on the tariff from early commissioning; its reason for haste is the 24 and 26 months. A small park without an award gains on the tariff and on the calendar remainder, and for it waiting for January or not missing July can pay.

This leads to a trade-off that offers almost never work out: should the park be commissioned before the grid connection? Commissioning without the grid secures the step but starts the 20 years while no electricity is being sold yet. In our example, 14,000 euros of secured step stand against around 10,800 euros of market premium missing for six months at the end of the period; the electricity itself remains saleable after the 20 years, just without the premium. With a step of one percent it is close, with a turn of the year into a new law it is clear-cut. For awarded plants the trade-off comes out differently: there is no step to secure, and early commissioning only costs term.

When is the solar park acquired for tax purposes, and why is that a different date?

For tax purposes a solar park is acquired when it is delivered, and it is delivered when beneficial ownership passes, not with the first electricity. Sec. 9a EStDV defines the year of acquisition as the year of delivery, and the income tax guidelines place delivery, for a purchase contract that also covers installation by the seller, only at the completion of the installation (R 7.4 (1) EStR). For technical plants accepted after a trial run the Federal Fiscal Court (BFH) made this precise using the example of a wind turbine: beneficial ownership passes only with the transfer of risk, that is, on acceptance, and neither full payment of the price nor the inflow of revenue during the trial run changes that (BFH, judgment of 22 September 2016, IV R 1/14). A solar park delivered turnkey with a trial run and acceptance follows the same logic.

So there are two clocks. The EEG clock starts with the first electricity from permanently mounted modules, often before the grid connection. The tax clock starts with acceptance, usually after the grid connection and the trial run. Weeks can lie in between, months with a grid operator in arrears, and if the turn of the year falls in between, four things move into the next year: the start of straight-line depreciation, which in the year of acquisition is in any case only calculated pro rata by month (Sec. 7 (1) sentence 4 EStG); the special depreciation of up to 40 percent, which is tied to the year of acquisition and the four following years (Sec. 7g (5) EStG); declining-balance depreciation, which is only open for acquisitions after 30 June 2025 and before 1 January 2028 (Sec. 7 (2) EStG); and the addition of the investment deduction, which takes place in the financial year of acquisition (Sec. 7g (2) sentence 1 EStG).

The last point is the most expensive. An investment deduction must be reversed if the asset was not acquired by the end of the third financial year following the year of deduction (Sec. 7g (3) sentence 1 EStG); the deadlines in detail are shown in Forming the IAB now: which tax deadlines and time windows count in 2026/2027. A deduction from 2023 therefore requires acquisition by 31 December 2026. A commissioning protocol dated 15 December 2026 does not help if the acceptance is only signed on 20 January 2027: under the EEG the park has the tariff and the legal version of 2026, under the Income Tax Act it is acquired in 2027, the deduction is reversed retroactively and the 2023 tax assessment is amended with interest. The table shows both cases side by side.

One side effect of the second clock is welcome: because the special depreciation under Sec. 7g (5) EStG allows up to 40 percent in total in the year of acquisition and the four following years, a business that acquires in December can still place the full amount in that year, provided the conditions of subsection 6 are met, among them use in the year of acquisition and the following year. How special depreciation and declining-balance depreciation combine is shown in Sonder-AfA §7g (5) vs. declining-balance AfA §7 (2): which combination, when?.

What applies to tranches, partial commissioning and used modules?

A solar park has as many commissioning dates as it has construction sections, because every module is a separate installation within the meaning of Sec. 3 no. 1 EEG. In practice, strings or sub-fields are completed and put into operation one after another, and the sections then carry different dates. The register allows modules that the same operator commissions at the same site at the same time to be registered summarily as one unit (Sec. 5 (1) sentence 2 MaStRV); sections with different dates are to be recorded separately. For the buyer of a section this means: the date that applies to their part is in the protocol of their part, not in the prospectus of the whole park.

Different dates do not create different installations when thresholds are concerned. Sec. 24 (1) EEG aggregates installations on the same plot or in immediate spatial proximity that were commissioned within twelve consecutive calendar months for determining the claim and the plant size. For the question whether a ground-mounted plant exceeds the one-megawatt limit and therefore needs a payment entitlement, subsection 2 goes further still: ground-mounted plants in the same municipality that were commissioned within 24 months and at a distance of up to two kilometres as the crow flies count as one installation. A three-megawatt park therefore cannot be taken out of the tender by dividing it into three sections with three dates. Anyone offered a section of less than one megawatt without an award should ask what was built in the neighbourhood in the 24 months before and after.

Used modules keep their date. The Federal Network Agency compares the date of first commissioning to a date of birth: if a unit is dismantled and re-erected elsewhere, the date does not change, the register merely records the new site as additional information. A park made of modules that have already generated electricity somewhere therefore carries their old date and their old tariff. An exception applies to replacement at the same site: modules that replace other modules there count, up to the previously installed capacity, as commissioned at the time of the replaced installations (Sec. 38b (2) EEG), and according to the Clearingstelle this replacement has, since the Solar Package I, been possible for building-mounted plants regardless of the reason. That a swap of modules or inverters does not shift the date already follows from Sec. 3 no. 30 sentence 3 EEG; the consequences for ageing and replacement are described in Module degradation and inverter replacement: what ages in a solar park and what it means for returns.

Which documents prove the commissioning date?

The date is proven by a chain of six documents that have to match one another; each on its own is open to challenge. In the order in which they arise in a project:

• The installer's commissioning protocol: date and time, number and capacity of modules per section, serial numbers of the inverters, description of the first generation, photos, signatures. It is the document from which the register is populated.
• The registration confirmation of the Marktstammdatenregister: register numbers of the units, the commissioning date entered, the operator entered. The date must match the protocol, the operator the party that actually uses the plant.
• The Federal Network Agency's payment entitlement for awarded plants: award number, allocated bid volume, date of the application. The application date shows whether the 18 months were kept.
• The grid operator's Inbetriebsetzung protocol and plant certificate: the day of the feed-in release. It usually lies after commissioning and proves the start of revenues, not the start of support.
• The acceptance protocol from the purchase contract: date of the transfer of risk, reservations for defects, partial acceptances. That is the acquisition date for tax.
• The direct marketing contract: start of marketing and the first statement from the direct marketer; it shows from when electricity was actually sold.

If protocol, register and payment entitlement do not agree, that is not a formality but a revenue risk: the grid operator pays according to the register data, the Federal Network Agency rejects an application with incorrect details, and the tax auditor reads the acquisition date from the acceptance, not from the prospectus.

Which questions should you ask the provider?

• Does the park take its applicable value from an award or from the statute, and in which document is the value stated?
• If awarded: when was it publicly announced, how much of the 24 and 26 months has been used up, and was the payment entitlement applied for within 18 months?
• Which commissioning date applies to exactly the section I am acquiring, and is there a separate protocol for it?
• Who was registered as operator at the time of commissioning, and when will the change of operator be registered in the Marktstammdatenregister?
• Was the registration made within one month, and for older parks: did the 20 percent reduction still apply at the time?
• Which ground-mounted plants were commissioned in the same municipality within two kilometres and within 24 months, and has Sec. 24 (2) EEG been checked?
• Are the modules new, or have they already generated electricity at an earlier site?
• When does risk pass under the purchase contract, are there partial acceptances, and does that date fall safely in the year my tax planning assumes?
• How far apart are commissioning and the grid operator's feed-in release, and who bears the lost revenue in between?

A provider who answers these questions with documents has the project under control. If instead the answer is that the date is in the prospectus, a second look is worthwhile; further warning signs are collected in How to tell a trustworthy provider of energy direct investments, and the other points to check on a park are shown in Investing in solar parks: revenues, costs and tax leverage at a glance.

How we check the commissioning date in our project review

We do not read the date from the prospectus but from the four documents that carry it: commissioning protocol, register extract, payment entitlement and acceptance protocol. If they diverge, that is an open point for us before we talk about revenues. For awarded plants we recalculate the deadlines from the announcement and check the receipt of the application against the 18 months; for plants with a statutory value we assign the date to its degression period and compare it with the tariff in the model. We check the tax side against the purchase contract: transfer of risk, acceptance, partial acceptances, and whether the date fits the deadline set by an investor's investment deduction. We do not make return promises. Anyone who would like a specific offer reviewed can bring it to a non-binding initial consultation.