The Antminer S21 XP breaks even on electricity at 8.85 pence per kilowatt hour. The cheapest half-hour on Octopus Agile in Eastern England today is 28.47 pence, more than three times that. It is the most efficient air-cooled bitcoin miner Bitmain sells, and in a UK home it loses money in every half-hour of the day.
The machine
Bitmain lists one air-cooled bin: 270 terahashes per second at 3,645 watts, or 13.5 joules per terahash, rated at 25°C with a tolerance of three per cent on hashrate and five on power. It takes 220 to 277 volts single phase, draws up to 20 amps, weighs 18.7 kilograms and moves 550 cubic feet of air a minute at 76 decibels.
Mind the suffix. The S21 XP Hydro, at 473 TH/s for 5,676 watts, and the XP+ Hydro are different machines on three-phase 380 to 415 volt supplies that no house has. Every figure here is the 270 TH/s air unit at 3,645 watts, which is also the bin every quoted price refers to.
Why the numbers look like this
Bitcoin difficulty implies a network of 950.3 EH/s. The last 144 blocks paid 3.1385 BTC each, fees only 0.43 per cent of that, and with bitcoin at £60,900 a terahash earns 2.867 pence a day after a one per cent pool fee. The S21 XP therefore makes £7.74 a day and burns 87.48 kilowatt hours doing it. Divide one by the other and you get 8.85 pence.
Octopus had published 46 of the UK day’s 48 half-hourly Agile rates when we fetched, all including five per cent VAT. The median is 33.46 pence and not one slot falls below break-even, so running only in the cheap hours earns nothing either. The Ofgem cap is 26.11 pence until 30 September, and 26.32 pence from 1 October — a figure with no VAT in it, because none is charged on domestic electricity from 1 October 2026 to 31 March 2027.
| Gross revenue per day | £7.74 net of pool fee |
| Consumes per day | 87.48 kWh |
| Break-even electricity | 8.85 p/kWh |
| Cheapest Agile half-hour today | 28.47 p/kWh |
| Agile median today | 33.46 p/kWh |
| Ofgem cap, from 1 October | 26.32 p/kWh |
| Loss per day at that cap | −£15.28 |
| Revenue per megawatt per day | £2,124 |
| Bitmain price, 270 TH/s bin | $4,590 (£3,440) |
A megawatt of S21 XP earns £2,124 of bitcoin a day and costs £6,317 of capped electricity to run.
The case against, in practical terms
Payback does not exist at any UK retail price, because daily cash flow is negative before the purchase is counted. At the October cap the machine loses about £465 a month. Even with free electricity it would take 444 days to earn back Bitmain’s £3,440, or 533 once 20 per cent import VAT is added, and every difficulty rise stretches that.
Then the ordinary problems. At 230 volts it draws 15.8 amps continuously, beyond a 13 amp plug and close to the limit of a 16 amp commando socket, so it needs a dedicated circuit fitted by an electrician. It puts 3.6 kilowatts of heat and 76 decibels into whatever room it is in. Bitmain’s warranty is 365 days from shipping, for the original purchaser only, excluding machines whose original software has been replaced or modified, and the nearest repair centre is in Aalsmeer in the Netherlands, with the owner paying insured freight out.
The case for
It is a good machine on the wrong grid. At 13.5 J/TH a megawatt of it earns about six times the £360 a day that the IceRiver KS3 managed last week, and in a hosting contract at around three pence, or on curtailed generation, it is cash positive. Power at 3.46 pence or less would repay Bitmain’s price in two years at today’s hashprice, before VAT.
It is also cheaper elsewhere. European resellers list the same 270 TH/s bin at €3,349 to €3,575 before VAT, and a UK reseller at £3,540 before VAT with duty and delivery included. None of that changes the verdict for a home installation, which is that 8.85 pence is not a price any UK household pays.
Sources, all retrieved 21 September 2026 between 06:07 and 06:50 UTC: shop.bitmain.com for specifications, variants, price, stock and warranty; mempool.space for difficulty, hashrate and the last 144 blocks’ rewards including fees; CoinGecko for bitcoin at £60,900; Frankfurter (ECB) for GBP/USD at 1.3344, which is Friday 18 September’s rate because the ECB does not publish at weekends; the Octopus Agile Eastern England API; Ofgem for the price cap; the UK Trade Tariff for 0 per cent duty and 20 per cent VAT under 8471; 21energy, Ultrabit, Cryptohall24 and The Mining Shop for reseller prices. Every profitability figure was derived here in python3 from network data, never from a mining calculator; standing charges are excluded. Contested: difficulty implies 950.3 EH/s where mempool’s recent-block estimate is 945.7; Ofgem does not state whether its July to September figure includes VAT; the next difficulty adjustment is estimated at −1.3 per cent with the epoch 14 per cent complete, too early to rely on; whether HMRC classifies ASIC miners under 8471 could not be verified, and The Mining Shop’s price was read through a summarising tool rather than directly. Not a recommendation to buy anything.