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Machine review · Oct 14, 2026 · 5 min read
Antminer S21+ review: a low entry price for 16.5 J/TH, and Nigeria is where it pays best
Bitmain's air-cooled S21+ makes bitcoin at 16.5 J/TH from a 3.9 kW box. It clears its power at 6¢ with room today, but a halved hashprice sits under that line. Cheap power is where it belongs, and one step up there are cheaper coins.
Bitmain Antminer S21+ (235 TH/s)
The bottom line
The S21+ is Bitmain's air-cooled step between the S21 and the S21 Pro: 235 TH/s from 3,877 W in the bin we list, 16.5 J/TH. In Argentina, at 6¢, it makes a bitcoin for about $52,400 of electricity and returns roughly 40% in its first year at today's price. At Nigeria's 3.9¢ the same coin costs about $34,100 and the first year returns 48%.
Bought once its price settled, in April 2025, it has mined and kept 0.0606 BTC for $3,194 of power: $1,757 of cashflow against a $4,870 landed cost, 36% of the box so far, which is what the first half of a four-year machine looks like. Between now and the 2028 halving it has another 0.061 BTC in it, worth about $7,617 with bitcoin back at its previous high: roughly 1.9× its hardware by the halving, still running afterwards.
What you are buying is a settled price per terahash on a mature Bitmain board. What you are not buying is the cheapest coin: the S21 Pro makes one for about $47,600 and the S21 XP for about $42,900 in the same room, against this machine's $52,400.
Scores
| Criterion | Score | Why |
|---|---|---|
| Cashflow | 6/10 | Makes a bitcoin for about $52,400 of electricity at 6¢ and returns roughly 40% in year one at today's price. It has covered 36% of its hardware so far and is on track for about 1.9× by the 2028 halving. The S21 Pro returns about 43%. |
| Longevity | 6.5/10 | At 16.5 J/TH its line is $23.76 a petahash a day at 6¢ and $15.44 at 3.9¢. Hashprice halved sits between the two, so after the halving it is a Nigeria machine, which is the move every machine makes once. |
| Efficiency | 5.5/10 | Mid-table for air: the S21 Pro runs 15 J/TH and the S21 XP 13.5 J/TH on the same kind of supply, and the water-cooled S23 Hyd 9.5. |
| Quality control | 5/10 | Scored as unknown rather than as good. We found no operator reports of the S21+ boards in the field that we could cite. The S21 family's reported weak points are solder cracking at domain handovers and connector wear, from a repair shop's guide, not a failure rate. |
| Serviceability | 7/10 | The S21 boards are among the best-understood in mining, with published repair guides and a deep parts market. Hosted with us, a fault is a repair queue and a line item. |
Weighted on what an owner gets back per dollar over the next four years. A low entry price and a comfortable margin today carry it; 16.5 J/TH and a reliability record we could not find hold it below the top.
Buy it if
- You are racking in Argentina at 6¢ and want an air-cooled Bitmain unit at $6.72 a terahash.
- Your power is at or below 6¢. At 6¢ it makes a coin for about $52,400; at 3.9¢, about $34,100.
- You expect to cross to Nigeria once, when the figures say so. At 16.5 J/TH its line there is $15.44 a petahash a day, well under today's hashprice and under half of it.
- You want hashrate in small steps: one 3.9 kW unit at a time, on ordinary 220 to 277 V single-phase power, with no coolant loop.
Walk away if
- You are planning past 2028 at 6¢. Its line there is $23.76 a petahash a day, and today's hashprice halved sits just under that, so those years lean on bitcoin recovering.
- You want the cheapest coin. The S21 Pro returns about 43% in year one against this machine's 40%, and the S21 XP makes a coin for about $42,900.
- Your power sits between 3.9¢ and 6¢ and is not going down. Between those two lines the machine works today and goes marginal after the halving, so price the later years before the first.
- You want a public failure record. We found no operator reports of the S21+ boards we could cite, and we score that as unknown.
What Bitmain shipped
Bitmain announced the air-cooled S21+ at Bitcoin Amsterdam in October 2024 at 216 TH/s and 16.5 J/TH, shipping in the first half of 2025, according to ForkLog's report of 10 October 2024. Bitmain also offered an exchange of hashrate on a 1:1 basis. Our store lists bins at 216, 225 and 235 TH/s, all at 16.5 J/TH.
The figures in this review are the 235 TH/s bin, 3,877 W, in a chassis 400 mm by 195 mm by 290 mm weighing 22.9 kg, with a noise rating of 76 dB. It takes 220 to 277 V and is rated for −20 to 45 °C. That is our store record: we could not open Bitmain's own product page for it, and ForkLog gives no wattage. It sits above the S21 at 17.5 J/TH and below the S21 Pro at 15 J/TH, the machines on either side of it in the range.
Priced like a settled machine
The first price we hold for the 235 TH/s unit gave a purchase of $3,208 in April 2025. It is $1,579 today, $6.72 a terahash, for the same 235 TH/s at the same 16.5 J/TH. The hashrate did not change; only the entry price did, and a proven machine at a settled price is what the entry looks like now.
Against its neighbours, price per terahash tells the story. The S21 Pro is $7.67 a terahash at 15 J/TH. The S21 XP is $11.55 at 13.5 J/TH. The S21+ asks the least per terahash of the three and the most electricity per terahash, and the guide to what a miner costs all-in shows what sits on top of the box.
What a coin costs it
At 6¢ in Argentina it makes a bitcoin for about $52,400 of electricity today, against a bitcoin price of $83,306. At Nigeria's 3.9¢ the same machine makes one for about $34,100. The S21 Pro makes one in Argentina for about $47,600, and the S21 XP for about $42,900.
In its first year the S21+ returns about 40% at 6¢ and 48% in Nigeria. The S21 Pro returns 43% in Argentina and 47% in Nigeria, and the S21 XP 37% and 38%. The S21+ sits close to the S21 Pro on both, which is what 16.5 J/TH at the lowest entry price should do.
Over its run since April 2025, electricity has averaged about $52,700 a coin. That is a lifetime average over 552 days; the figure to plan on is today's.
Where the halving puts it
A machine's line is the hashprice at which it stops covering its own power. At 16.5 J/TH a petahash uses 396 kWh a day: $23.76 at 6¢, and $15.44 at Nigeria's 3.9¢. Network hashprice was $39.73 a petahash a day on 14 October on our hashprice chart, so the S21+ clears the 6¢ line by about two thirds. Its line was also below the year's low of $27.74 on 6 June, by 17%.
The 2028 halving halves the coins a petahash earns. At today's price and difficulty that puts hashprice near $19.87, a little under the 6¢ line and well above the Nigerian one. That is today's conditions held flat, not a forecast: it is why every projection we publish assumes the old high returns, and the forward figures here do the same.
So the machine is an Argentina machine now and a Nigeria machine after the halving, and the move is made once. The guide to crossing once explains why, and the guide to efficiency and halvings shows where the line sits at every J/TH.
What a fault costs you here
We looked for operator threads and repair write-ups on the S21+ and found nothing specific to it that we could stand behind. What exists is for the family: D-Central's S21 hashboard repair guide, last reviewed in August 2026, lists the reported weak points of the S21 boards as solder cracking at the domain handovers, dead chips that break a board's chain, boost-converter failures and worn connectors, and says these are reported patterns from public sources, not a failure rate. It does not mention the S21+ by name.
What a fault costs is the part that matters. Hosted with us in Argentina, a repair is billed at the cost of the parts and nothing else: a failed hashboard is a line item, not a catastrophe. Board-level repair of an S21-family machine is something every bench knows how to do.
We found no operator reports of this machine's boards we could cite. We score that as unknown, not as good.
Our two cents
The S21+ is a sound machine at a settled price. At 6¢ it clears its power by a wide margin today, it carries to the halving, and after that the cheaper power in Nigeria is where its coins are made for the least. A buyer who wants hashrate in a small air-cooled box at the lowest entry price in the range has a sound reason to pick it.
Most buyers chasing the cheapest coin should look one step up: the S21 Pro makes it for about $47,600 in Argentina, and the figure below ranks everything that beats this machine on cost per coin and then year-one return, in stock today. A coin that costs less to make is a coin you keep more of.
- MicroBT Whatsminer M74S+ (332 TH/s) — 12 J/TH, In stock, $38,107 of electricity per bitcoin, 34% year one
- MicroBT Whatsminer M73S (560 TH/s) — 13 J/TH, In stock, $41,283 of electricity per bitcoin, 40% year one
- MicroBT Whatsminer M7DS (672 TH/s) — 13.5 J/TH, In stock, $42,870 of electricity per bitcoin, 42% year one
Sources and further reading
- Bitmain expands Antminer S21 series with two new bitcoin miners — ForkLog, 10 Oct 2024
- Antminer S21 family hashboard repair — D-Central, 2026
Which machines this covers
Disclosure
Firsthand Bitcoin sells and hosts mining hardware, including this machine. No manufacturer, distributor or affiliate programme paid for or reviewed this page and we take no commission on the links above. Historical figures are computed from daily bitcoin price and network hashprice, each day valued at its own prices. Nothing here is investment advice.