The S21 Pro is the machine most people land on when they start pricing Bitcoin mining seriously. It is Bitmain's efficiency flagship, it is available, and the specifications are genuinely impressive. It is also, on a UK domestic electricity supply, a way to convert money into heat at a loss.
Both things are true at once, and the gap between them is the interesting part.
| Hashrate | 234 TH/s |
| Power draw | 3,510 W |
| Efficiency | 15 J/TH |
| Revenue at current difficulty | £5.43 per day |
| Break-even electricity price | 6.45p per kWh |
| Profitable half-hours today (Agile, Eastern England) | 0 of 48 |
Reading those numbers properly
Bitcoin's network is running at roughly 892 EH/s. The S21 Pro contributes 234 TH/s, which is about 0.000026% of global hashrate — and that share, multiplied by daily issuance and the bitcoin price, is £5.43 a day gross.
To keep that revenue you need to buy 84 kWh. At 6.45p per kWh you break even exactly. Today's cheapest half-hour on Octopus Agile in Eastern England was 13.77p, more than double. The median was 26.47p. Run this machine continuously on a standard domestic tariff and you are losing somewhere around £16 a day.
The S21 Pro has the best break-even of any machine in our calculator — better than every GPU by a wide margin. It still misses the cheapest UK electricity of the day by more than half.
This is why industrial Bitcoin mining lives in Texas, Paraguay and Ethiopia rather than Hertfordshire. Those operations buy power at 3–5p equivalent, often on interruptible contracts that pay them to shut down during grid stress. At those prices the same machine clears comfortably. The hardware is not the variable that decides this. The electricity contract is.
The two things spec sheets never mention
It will not plug into your wall. A UK 13A socket delivers around 3kW. This machine draws 3,510W continuously, which is not a marginal overshoot — it is a fire risk on an extension lead and a tripped breaker at best. You need a dedicated circuit installed by a qualified electrician, and if you are running more than one, you are past the 7.36kW capacity of a typical 32A ring final and into supply-upgrade territory. Budget for that before you budget for the machine.
It is genuinely loud. Around 75 decibels, continuously, forever. That is a vacuum cleaner running in the next room around the clock. There is no spare bedroom in Britain where this is acceptable, and immersion cooling or a purpose-built shed is a real project rather than an accessory purchase.
Where it does make sense
Three cases, and they are narrow.
Solar with poor export rates. If you are exporting at 4–5p and could instead run a miner on that generation, the arithmetic flips decisively. This is the strongest UK case and the one worth modelling carefully.
Winter heat displacement in a building you heat anyway. 3.5kW of heat is a serious contribution to a workshop or outbuilding. If it replaces heating you would have bought, the effective power cost falls sharply. It only works if the heat lands somewhere you want it, and only for about five months of the year.
A commercial supply with genuinely cheap power. Under about 8p and this becomes a real business rather than a hobby that loses money.
The verdict
Excellent hardware, wrong country. If you want Bitcoin exposure and you are in the UK, buying bitcoin gives you the same exposure without the electrician, the noise, the depreciation on a single-purpose machine, or the daily loss. If you want to mine because you find mining interesting — which is a perfectly good reason — go in knowing it is a hobby with running costs, not an income.
Run your own numbers with our mining calculator, which prices this machine against your actual regional tariff by the half hour.
Revenue calculated 14 August 2026 from live Bitcoin network hashrate (mempool.space), the GBP bitcoin price (CoinGecko) and live Octopus Agile rates for Eastern England, net of a 1% pool fee. Excludes hardware cost, cooling, electrical work and tax. Manufacturer specifications are nominal and vary with firmware, ambient temperature and tuning.