Battery Storage: Why Not Every Project Pays Off
A large battery doesn't pay for itself on its own. The site, the grid connection, and the revenue assumption decide. Too much of that gets checked too late.
One battery serves one market.Five revenue paths a cluster opens.
A single battery serves only one market per time window. The pool serves several at once, because different units run different windows. I show how the cluster is set up for this – and where the minimum bid sizes of the balancing markets sit.
- EPEX Spotfrom 1 unit
- FCR – primary control reservemin. bid 1 MW
- aFRR – secondary reservemin. bid 1 MW
- mFRR – minute reservemin. bid 1 MW
- § 118 (6) EnWGno minimum size
FCR, aFRR and mFRR each require a minimum bid of 1 MW – since decision BK6-21-042 of the Bundesnetzagentur, dated 28 April 2022, which replaced the earlier 5 MW figure for aFRR and mFRR. A 100 kW battery reaches one tenth of that. In a cluster the threshold falls – with one contract and one point of contact.
Ten units make a threshold.The hour decides what they do.
On the left you set the size of the cluster — it decides which markets are open at all. On the right you pick the time window — it decides what the cluster is doing right now. One unit equals 100 kW.
3units|300kWcluster capacity
- EPEX Spotfrom 1 unit
- FCR – primary reservemin. bid 1 MW
- aFRR – secondary reservemin. bid 1 MW
- mFRR – minute reservemin. bid 1 MW
- § 118 (6) EnWGno minimum size
Charging
EPEX Spot – procurement
At midday solar pushes the exchange price below zero. Whoever takes power now is paid for taking it instead of paying for it. The cluster recharges in this window — it is the charge between the morning and evening peaks. On days with a long negative phase, two hours are enough for the entire capacity.
A battery has only one capacity window per time slot. Charging, discharging and holding capacity exclude one another — blended revenue comes from the best daily mix, not from the sum of all markets. And capacity is finite: a storage unit will not give you much more than one and a half charges a day, however attractive the next window looks. The 1 MW threshold has applied equally to FCR, aFRR and mFRR since Bundesnetzagentur decision BK6-21-042 of 28 April 2022.
The Challenge with BESS Projects
A storage project is hardware, a grid connection, and a set of numbers that have to fit together. Site, connection offer, permit, and revenue assumption run side by side. Each piece pulls on the others.
If you only look at the equipment and underestimate the rules, you get a project that works on paper and stalls in practice.
Why BESS Projects Fail
The usual reasons: revenue taken from old market data, no grid capacity at the chosen site, and a permit that takes longer than anyone planned. Many investors use a standard spreadsheet and check the place too late.
The connection offer is the most common stop. And often the last thing anyone checks.
Mixed operation and grid charges
Since the MiSpeL ruling of 1 October 2026, you no longer have to meter solar electricity and grid supply separately on the same storage unit. Mixed operation runs through the delineation option. For PV up to 30 kWp there is also a flat-rate option. Until 30 September 2027 you need the consent of the grid operator and the metering operator. The flat-rate option also needs EU state-aid approval. This applies to the market premium and direct marketing, not the fixed feed-in tariff.
Twenty years without a grid charge under § 118 (6) EnWG is a different question. In the AgNeS draft, status 7 October 2026 and not yet adopted, it remains only with an investment decision before the planned publication on 1 January 2027 and commissioning by 4 August 2029.
In-depth articles
Multi-use marketing →
How large-scale storage becomes profitable through sequential marketing across balancing reserve, intraday and arbitrage. The most valuable market per time window, not the sum.
MiSpeL: storage and charge points →
Delineation and flat-rate options under the ruling of 1 October 2026. Transition until 30 September 2027, state-aid caveat, direct marketing only.
AgNeS and grid charges →
Determination draft, not a ruling. Under the draft the 20-year exemption remains only with FID before 1 January 2027.
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