The Antminer T21 breaks even on electricity at 6.46 pence a kilowatt hour. The cheapest half-hour on Octopus Agile in eastern England today was 25.36 pence including VAT, and the Ofgem cap unit rate in force this week is 26.11 pence. Across every half-hour published today there was no point at which the machine paid for its own power.
That is the whole review. The arithmetic is worth setting out, because the same sum disqualifies every miner sold today.
The machine
Bitmain's own specification gives the T21 two operating modes rather than two models. In normal mode it is 190 terahash a second at 3,610 watts, which is 19.0 joules per terahash. In high-energy mode it is 233 terahash at 5,126 watts, or 22.0 joules — more hashrate bought at worse efficiency. Everything below uses the 190 terahash bin at 3,610 watts, the bin the price is quoted against.
The specification that matters most for a British buyer is not on the hashrate line. The T21 takes 380 to 415 volts, three phase. A UK home has 230 volts, single phase. Running one at home is not a heavier cable; it is a distribution network operator supply upgrade, measured in thousands of pounds and months. Third-party conversions drop the machine to about 170 terahash on single phase, at the cost of the warranty and a firmware developer fee.
The arithmetic
Difficulty this morning was 132.76 trillion, implying a network hashrate of 950.3 exahash. Across the last 144 blocks the average reward was 3.139 bitcoin including fees, and fees were 0.45 per cent of that — no fee cushion at present. With bitcoin at £62,555 and a one per cent pool fee deducted, a terahash of work is worth 2.9459 pence a day.
So 190 terahash earns £5.60 a day gross, drawing 86.64 kilowatt hours to do it. Divide revenue by consumption and the break-even price is 6.46 pence. At the cap rate the power costs £22.62 against £5.60 of production: a loss of £17.02 a day, or £6,214 a year. Paying today's cheapest half-hourly rate around the clock would still lose £16.37 a day.
| Gross revenue | £5.60/day |
| Consumption / efficiency | 86.64 kWh/day · 19.0 J/TH |
| Break-even electricity price | 6.46 p/kWh |
| Agile cheapest half-hour today | 25.36 p/kWh |
| Ofgem cap, 1 Jul–30 Sep 2026 | 26.11 p/kWh |
| Loss at the cap rate | £17.02/day |
| Revenue per megawatt | £1,550/day |
| Agile median today | 33.86 p/kWh |
| Break-even, Antminer S21 XP | 9.09 p/kWh |
It is not just this machine
Run the same sum on Bitmain's current flagship. The S21 XP does 270 terahash at 3,645 watts, 13.5 joules per terahash, 29 per cent less energy for the same work. Its break-even is 9.09 pence — better by about two-fifths, and still just over a third of the cheapest half-hour today.
That is the finding to take away. The T21 is a generation behind and the deficit is real, but it is not why the machine loses money in Britain. The cap rate is four times the T21's break-even price, and still nearly three times that of the best machine on sale.
Efficiency changes how fast you lose, not whether.
Price, warranty and the rest
A UK seller lists the 186 terahash bin at £2,500 excluding VAT, delivered and duty paid, but out of stock. That is £13.44 per terahash before VAT, £16.13 after it, which is cheap hashrate either way. Bitmain's own listing appears sold out, though we could not confirm it on the page, and two US resellers are out of stock too. This is a clearance and secondary-market machine.
Bitmain warrants bitcoin miners for 365 days from the date of shipment, not the date you buy. The T21 shipped from late 2023, so any unit you find now is out of warranty whoever sells it. Whether cover transfers on resale is not addressed in Bitmain's terms; the clock is date-based, and it has run out. The nearest repair centre is in the Netherlands, and the buyer pays freight and customs both ways.
It runs at 76 decibels and emits 3.61 kilowatts of heat continuously, more than an immersion heater. Both facts point the same way: an outbuilding, not a house.
Who it is for
One buyer only: somebody who already has three-phase power, electricity priced like an industrial contract rather than a household tariff, and a use for 3.6 kilowatts of heat. Heat recovery is a real offset in a British winter, and the three-phase requirement that kills domestic demand is what holds the price near £13 a terahash before VAT. If your power is metered at the cap, buy nothing.
Calculated and retrieved 28 September 2026. Difficulty, network hashrate and the 144-block reward including fees from mempool.space; bitcoin price from CoinGecko, £62,555 at retrieval. Electricity from the Octopus Agile eastern England tariff, of which 44 of 48 half-hourly periods were published at retrieval, covering 00:00 to 22:00 UTC, quoted including VAT at 1.05 times the excluding-VAT value; cap rate from Ofgem. Agile is priced regionally, so these figures are one region on one day. Specifications, warranty terms and repair locations from Bitmain's own support pages; prices from reseller listings, unnamed here because none could be independently confirmed as in stock. All economics derived from those inputs, not from a profitability calculator. Contested: mempool.space reports a network hashrate of 946.8 exahash while the difficulty implies 950.3, and we have used the latter; Bitmain's shop status is unconfirmed on the page; neither a return courier quote nor a three-phase upgrade quote was obtained, so neither cost is stated. Not a recommendation to buy, sell or hold anything.