The contract that assumes a crystal ball
A top-bottom spread contract (TBx) settles on the gap between a day's most and least expensive hours: TB4 pays the spread between the top four and bottom four priced hours, on a nominated capacity, whatever the asset actually does. Developers take these contracts because they turn merchant volatility into firm revenue a lender can finance against. The catch is structural: the index ranks hours after the day is over, so it prices a battery with perfect foresight, and no real battery has it. The gap between the index and what an asset actually earns is therefore the central number in structuring one of these contracts, and it has been surprisingly hard to see. The modelling that exists mostly comes from platforms, advisers and counterparties with a transaction to win.
This index makes the gap public. It is computed entirely from primary AEMO data: every bidirectional battery unit in the National Electricity Market, its actual five-minute dispatch, and the regional reference prices it faced, for the full quarter.
The index, defined in one paragraph
For each region and calendar day (NEM time, period-ending), the hourly price is the mean of the hour's twelve five-minute regional reference prices. The TBx daily settlement per nominated megawatt is one hour times the sum of the top x hourly prices minus the sum of the bottom x hourly prices; the quarterly index is the sum over complete days. An asset's actual revenue is the sum over five-minute intervals of its dispatch (megawatts, negative when charging) times the price, times one-twelfth of an hour: energy-only, before FCAS and other streams. The capture ratio is actual revenue divided by the index at the asset's registered capacity, matched to its duration (TB1 under 1.5 hours, TB2 to 3 hours, TB4 above). Anyone with the AEMO archive can reproduce every number.
| Region | TB1 A$/MW/quarter | TB2 A$/MW/quarter | TB4 A$/MW/quarter | Fleet actual A$/MW |
|---|---|---|---|---|
| South Australia | 53,387 | 92,598 | 133,721 | 34,040 |
| New South Wales | 20,135 | 32,872 | 47,804 | 17,738 |
| Victoria | 11,682 | 21,078 | 36,561 | 9,612 |
| Queensland | 12,317 | 21,765 | 35,965 | 9,580 |
The Solar Analytica TBx index per region, Q1 2026, 90 complete days each, alongside the actual energy-only revenue per MW of each region's full-quarter battery fleet. Computed from AEMO five-minute regional reference prices and unit dispatch; index methodology as defined above.
Two structural facts sit in that table. South Australia's spread is another market entirely: its TB4 index ran 3.7 times Queensland's, which is why the same contract is worth radically different firm revenue in different regions. And in every region the perfect-foresight index ran 2.7 to 3.8 times what the full-quarter fleet actually earned from wholesale energy. A TB4 swap written at nameplate on the average real asset would have owed the offtaker several times the asset's energy revenue this quarter, exactly the shortfall mechanism the market's own commentary describes, now measured across a fleet.
The capture ledger
Per asset, the picture is a wide distribution, and duration is its backbone:
| Unit | MW / duration | Active days | RT ratio | Actual A$ | Matched index | Capture |
|---|---|---|---|---|---|---|
| WDBESS1, QLD | 272 / 1.5h | 90 | 0.85 | 3,464,764 | TB1 | 1.03 |
| BRNDBES1, QLD | 215 / 1.9h | 90 | 0.85 | 3,198,055 | TB2 | 0.68 |
| BLYTHB1, SA | 281 / 1.4h | 90 | 0.82 | 9,976,381 | TB1 | 0.67 |
| MREHA3, VIC | 215 / 3.7h | 91 | 0.89 | 4,905,999 | TB4 | 0.62 |
| TEMPB1, SA | 138 / 2.1h | 90 | 0.81 | 7,613,884 | TB2 | 0.60 |
| TIB1, SA | 250 / 1.0h | 90 | 0.77 | 7,254,981 | TB1 | 0.54 |
| ERB01, NSW | 614 / 3.3h | 90 | 0.87 | 13,502,531 | TB4 | 0.46 |
| HPR1 (Hornsdale), SA | 150 / 1.3h | 91 | 0.76 | 3,486,400 | TB1 | 0.44 |
| TARBESS1, QLD | 393 / 1.5h | 89 | 0.85 | 3,158,038 | TB2 | 0.37 |
| VBB1 (Victorian Big Battery) | 360 / 1.3h | 91 | 0.75 | 978,001 | TB1 | 0.23 |
Selected full-quarter units from the 60-unit dataset (40 ran the full quarter; 20 commissioning units are flagged partial and excluded from all statistics). RT ratio is the observed AC discharge-to-charge energy ratio at the connection point, auxiliaries included. Units are identified by their AEMO dispatch identifiers; full ledger with every unit, both capture bases and sources in the public dataset.
Three findings carry the quarter. First, the median full-quarter unit captured 46% of its duration-matched index: perfect foresight, imperfect dispatch, round-trip losses and competing value streams cost the average asset more than half the index. Second, one asset beat its index: WDBESS1 in Queensland returned 1.03 times TB1, evidence that five-minute volatility capture and second cycles can out-earn a daily top-bottom-one-hour ranking, the retained upside that makes these contracts survivable. Third, the largest asset in the country, the 1,095 MW unit at Waratah, sat commissioning through the quarter with net negative energy revenue, a reminder that 20 of 60 units, over a third of registered capacity, is still ramping: the 45 GW connection pipeline is arriving faster than it can be energised.
Two physical numbers validate the whole ledger from inside. The median observed round-trip ratio of the full-quarter fleet is 0.81 to 0.83 in every region independently, exactly where lithium-ion plant efficiency sits. And Hornsdale, the market's oldest big battery, shows both the lowest round-trip ratio among the veterans and a capture ratio well below the new fleet's median, consistent with an asset that earns much of its living in frequency services this index deliberately excludes.
What a developer should do with this
The index turns three structuring intuitions into checkable numbers. Nominate against duration, not nameplate: a 1.5-hour asset facing a TB4 settlement is short the index by construction, and this quarter that mismatch alone separates a 0.35 capture from a 0.62 one. Size the foresight haircut empirically: the market's real capture distribution, median 0.46 with an upper decile near 0.7 for energy-focused assets, is a better basis for a swap's nominated quantity than any single optimised model. And price the region before the structure: the same TB4 contract that would have paid A$36,000 per MW in Queensland paid A$133,721 in South Australia this quarter; the choice of region moved nearly four times more money than the choice between TB2 and TB4 anywhere outside it. None of this says TBx contracts are mispriced. It says the gap they are priced across is now observable, quarterly, from public data. The residential half of the same storage wave is read in the behind-the-meter data and the lithium-ion decade.
Method and limits
The fleet is every unit registered bidirectional in AEMO's registration data effective in the quarter, 60 units, 11.2 GW and 20.4 GWh; pumped hydro and paired wind-farm load units are excluded by construction. Dispatch is AEMO MMSDM DISPATCH_UNIT_SCADA (five-minute, as-generated at the connection point); prices are AEMO five-minute regional reference prices. Five limits matter. Revenue here is wholesale energy only: FCAS and other ancillary streams, often the larger share for early assets like Hornsdale, are excluded, so capture ratios understate total asset economics by design. SCADA revenue is unadjusted for marginal loss factors and any settlement nuances, making it an estimate of energy market receipts, not a settlement statement. The TBx index is Solar Analytica's own transparently defined benchmark; real contracts differ in definitions, floors and windows, and no figure here describes any actual contract or any party's financial position. Commissioning units are flagged and excluded from statistics rather than silently dropped. And one quarter is one quarter: Q1 carries summer volatility, and the index is built to be republished each quarter so the record, not a single season, does the talking. Every input is public, the methodology is published above in full, and the per-unit dataset is queryable.