BESS (Battery Energy Storage System)
A grid-connected battery installation that absorbs electricity when prices are low or surplus power is available, and discharges it when needed. In Estonia, BESS projects are built primarily for frequency-reserve revenue (FCR, mFRR) plus spot-arbitrage stacking.
A Battery Energy Storage System (BESS) is a grid-connected installation that absorbs electricity when it is cheap or surplus and discharges it when it is expensive or scarce. The core product is the time-shifting of energy on a sub-hourly to multi-hour horizon. Modern grid-scale BESS is overwhelmingly lithium-ion (LFP chemistry dominating new builds since 2023) and arrives as containerised systems with battery racks, climate control, fire suppression, a battery management system (BMS), and a bidirectional inverter.
How BESS is sized
Two numbers: power (MW) and energy (MWh). A "1 MW / 2 MWh" battery can deliver one megawatt of charge or discharge for two hours straight. The ratio MWh ÷ MW is the duration; "1C" means one-hour duration, "0.5C" means two-hour. For Estonian frequency-reserve markets, 1C–2C systems fit best — mFRR activations rarely outlast two hours, so longer durations buy capacity you can't monetise. Round-trip efficiency on new Li-ion BESS is 85–92%; that loss plus calendar/cycle degradation is the gross margin you have to clear.
Where BESS makes money in Estonia
In 2026 Estonian conditions, mFRR capacity is the largest single revenue line for grid-scale batteries — see mFRR explained. FCR pays steady but smaller; aFRR is intermediate. Pure spot-price arbitrage in EE/FI bidding zones contributes 5–15% on top, depending on the year. Stacking — bidding into multiple markets simultaneously and re-allocating the same megawatt-hour where it earns most — is what separates a 12% IRR from a 22% IRR.
Aggregation
Small BESS (< 1 MW) typically cannot prequalify for balancing markets alone — the technical thresholds were designed around centralised assets. An aggregator (a BSP) pools many small batteries into a single virtual unit and bids them as one. This is the legal structure behind most home battery flexibility programmes in 2026.
Usein kysyttyä
- What is a Battery Energy Storage System (BESS)?
- A BESS is a grid-connected battery installation — typically lithium-ion — that absorbs electricity when prices are low or excess power is available, and discharges it when prices spike or when called on by the TSO for frequency-reserve services. Modern utility-scale BESS projects in the Baltics are 1–50 MW and stack multiple revenue streams.
- How does a BESS make money in Estonia?
- Three stacking revenue streams: (1) FCR/aFRR/mFRR balancing capacity payments via BBCM, (2) balancing energy activations via MARI/PICASSO, (3) day-ahead and intraday spot arbitrage on Nord Pool. mFRR has been the dominant revenue source since 2024 because of the asymmetric up-vs-down spread on the Baltic CoBA. As a BSP, the operator must be Elering-certified.
- How big are typical Estonian BESS projects?
- Most operating projects in 2026 are 1–10 MW; the largest in development phase are 50–100 MW. The minimum bid size for BBCM is 1 MW, which sets the practical floor for direct participation. Smaller assets pool through aggregators like Volton to reach the threshold.
- What duration is needed for FCR / aFRR / mFRR?
- FCR is symmetric and short-burst — the asset must hold full power for at least 15 minutes per Estonian rules. aFRR requires sustained delivery over the 5-minute Full Activation Time. mFRR can run for hours when activations stack — Estonian operators typically size BESS at 2–4 hours of duration to stack mFRR with day-ahead arbitrage. Capacity factor matters less than the price spread.