Fixed price or day-ahead market? A biogas plant in H1 2025

Not on the RDN alone, but FIP with flexibility, yes. In H1 2025 the average RDN price was PLN 427.5/MWh (52% of FIT at PLN 828.4/MWh), 2× operation gives PLN 559.1/MWh (67% of FIT), 3× operation PLN 607.0/MWh (73% of FIT). FIP with 2× operation reaches PLN 845.3/MWh, with 3× operation — PLN 893.2/MWh, i.e. 102% and 108% of FIT. In the first full half-year after the change in the rules, the market delivered 251 hours with a negative price — the most in the whole series.

A reminder of the mechanism

The support rules (Articles 70a–70f of the Act on Renewable Energy Sources (ustawa o OZE)), the scenarios and the model assumptions are described in the first analysis of the series. You will find the results for the previous half-year in the previous analysis of the series (H2 2024).

Negative prices: the rule in practice

Since 28 December 2024, Article 93(4) of the Act on Renewable Energy Sources deducts from the quantity of energy covered by FIP settlement the energy generated in every hour in which the RDN price is below PLN 0/MWh (previously at least six consecutive hours were required). A generator under FIP therefore loses twice: it sells energy at a negative price, and receives no top-up for that energy. The model assumes that the biogas plant switches the engine off in such hours (the gas goes into the roof) and does not generate energy. The effect: revenue per MWh with switching off is PLN 458.4/MWh against PLN 427.5/MWh for continuous operation, i.e. +PLN 30.9/MWh. A biogas plant with a double-membrane roof has an advantage here: gas from negative hours can be burned in expensive hours. FIT (for installations smaller than 200 kW) does not have this problem, because the obligated seller pays the fixed price regardless of the market.

The reference prices remain those from the regulation of 8 November 2023: FIT PLN 828.4/MWh and FIP PLN 713.7/MWh; a 2019 installation after indexation (index for 2024: 103.6) has a FIT of PLN 906.4/MWh.

Figures for the half-year

The "2019 installation" price is 95% of the 2019 reference price (PLN 617.5/MWh) after cumulative indexation by the annual average CPI for 2019–2024 (Article 70e(3); GUS: 102.3; 103.4; 105.1; 114.4; 111.4; 103.6).

The FIT in the table is 95% of the reference price for the "up to 500 kW" class. Since 28/12/2024 only installations smaller than 200 kW can use FIT, so for a 0.5 MW installation it is a reference point, and the support actually available remains FIP (the 200 kW – 1 MW class).

The monthly averages fall from PLN 559.0/MWh in February to PLN 347.6/MWh in June. The deepest decline comes in spring: April PLN 366.8/MWh, June PLN 347.6/MWh.

The daily profile has a pronounced trough at midday (12:00: PLN 210.2/MWh) and a peak at 20:00 (PLN 697.9/MWh) — a spread of PLN 487.7/MWh. The premium for 2× operation (PLN 131.6/MWh) and 3× operation (PLN 179.5/MWh) is the largest component of the advantage of FIP + flexibility.

Where the FIP + 2× result comes from

The result of PLN 845.3/MWh consists of the captured price (PLN 559.1/MWh) and the top-up to the FIP fixed price (on average PLN 286.2/MWh), calculated from the average RDN price for the day, and therefore independent of which hours the generating set runs in. We also leave out investment aid, which could lower the fixed prices, and the costs of the additional generating set. Nor can the generator assume that the top-up is unlimited in time: its amount depends on the balance (Article 93(11)–(12)), and the support period ends 15 years after the first sale. A 2019 installation, after cumulative indexation, has a FIP fixed price of PLN 779.4/MWh (90% of PLN 590) and a FIP + 2× result of PLN 911.0/MWh, so the old declaration is even more favourable.

Is a double-membrane roof enough

In the model, "2×" operation means that a generating set with capacity increased to about 1.0 MW runs only in the 12 most expensive hours of the day, and "3×" that a set of about 1.5 MW runs in the 8 most expensive hours; the same daily energy (about 10.96 MWh) is therefore produced in a shorter time, at higher power. Biogas from fermentation flows throughout the day (in the model about 216 Nm³/h, resulting from a capacity of 0.5 MWel, an electrical efficiency of 40% and a methane content of 53%), so the rest of the day is storage in the double-membrane roof. The gas balance for this half-year gives on average 2,686 m³ of capacity for 2× (95th percentile: 3,629 m³) and 3,503 m³ for 3× (4,839 m³). According to DLG, a single double-membrane roof with a diameter of 26 m has a gas capacity of about 2500 m³ (we adopt this as an assumption for Polish installations too): in this half-year it accommodates the 2× demand on 45% of days, and the 3× demand on 3% of days. Two roofs, i.e. 5000 m³, cover all days of the half-year in the model in both variants (the maximum demand did not exceed 5000 m³).

How much can be spent on flexibility

The premium for operating in the expensive hours is in this half-year PLN 131.6/MWh for 2× and PLN 179.5/MWh for 3× compared with continuous operation. After annualisation (×2, an assumption) this gives an annual surplus of PLN 522 thousand for 2× and PLN 712 thousand for 3×; divided by the annuity factor (12-year depreciation, cost of capital 10% or 6%), this gives a budget for an additional generating set and gas store of PLN 3,558–4,378 thousand for 2× and PLN 4,853–5,971 thousand for 3× (the lower value at a cost of capital of 10%, the higher at 6%). The cost ranges are assumptions, not offers: €500–800/kW of additional capacity plus €20–70/m³ of missing storage, at an exchange rate of PLN 4.3/€, which gives PLN 1,091–1,776 thousand for 2× and PLN 2,236–3,742 thousand for 3×. The arithmetic conclusion: both variants (2× and 3×) pay back across the whole assumed cost range. The same benefit from the price profile occurs under FIP and in a pure RDN (under FIP it is sometimes slightly larger, because flexible operation avoids the hours excluded from settlement), because it stems from the market, not from support; FIT does not provide it, because it pays the same for every hour.

The third option: an energy store

A 0.5 MW / 1 MWh store, one cycle per day, 85% efficiency, adds PLN 42.8/MWh to continuous operation (revenue PLN 470.3/MWh). The average of the daily differences between the average price of the two most expensive and the two cheapest hours of the day is PLN 580.1/MWh. The annual surplus is PLN 170 thousand; with 15-year depreciation, servicing at 2.5% of capital expenditure per year and an average capacity degradation of 2% per year (assumptions), this allows spending PLN 932–1,140 thousand on the store, i.e. PLN 932–1,140 per kWh of capacity. The assumed range for the system cost is PLN 600–1,200/kWh, and for the additional grid connection PLN 50–400 thousand — these are assumptions, because there are no hard Polish prices for a scale like this. After deducting the connection (PLN 50–400 thousand) from the budget at a cost of capital of 6%, the permissible system cost is PLN 740 to 1,090/kWh; the store would pay back only in part of the range (a low connection cost, a low cost of capital, a cheaper offer). The model ignores distribution charges for off-take, the capacity charge and the balancing market, so the result is rather optimistic.

Conclusions for this half-year

• The RDN alone: PLN 559.1–607.0/MWh is 67%–73% of FIT; a gap of PLN 221.4–269.3/MWh.
• FIP + 2× (PLN 845.3/MWh) exceeds FIT, but only after spending on the generating set and gas store: the 2× variant pays back across the whole assumed cost range.
• The every-negative-hour rule makes continuous operation at spring middays unprofitable; switching off raises revenue by PLN 30.9/MWh.
• An energy store adds PLN 42.8/MWh and would pay back only in part of the range (a low connection cost, a low cost of capital, a cheaper offer).