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Machine review · Oct 31, 2026 · 6 min read

Antminer S21 review: 17.5 J/TH on air at a low box price, and Nigeria is where it pays best

Bitmain's S21 is an air-cooled Antminer from 2023, rated 17.5 J/TH. Today it clears Argentina's power with room to spare, and its box costs less per terahash than the newer air machines. Halve the coins a petahash earns and hold everything else flat, and only Nigeria's 3.9¢ still covers it.

A Bitmain Antminer S21 photographed whole in front of a softly blurred air-cooled mining hall with racked machines and a polished concrete floor

Bitmain Antminer S21 (151 TH/s)

The bottom line

The S21 is Bitmain's air-cooled Antminer at 17.5 J/TH: 151 TH/s from 2,643 W in the bin we stock, and a 200 TH/s unit at the same efficiency that has sold out. In Argentina, at 6¢, it makes a bitcoin for about $60,700 of electricity and returns roughly 21% in its first year at today's price. At Nigeria's 3.9¢ the same coin costs about $39,500, with Nigeria's lower uptime already counted.

Bought in July 2026, once we held a price for it, it has mined and kept 0.0089 BTC for $485 of power in 123 days: $207 of cashflow against a $1,660 landed cost, 12% of the box so far. Between now and the 2028 halving it has another 0.025 BTC in it, worth about $3,172 with bitcoin back at its previous high of $124,493: roughly 2.0× its hardware by the halving.

What you are buying is a cheap box per terahash and a coin that costs more than the newer air machines make. The S21 XP makes one in Argentina for about $46,800, the S21 Pro for about $52,100 and the M74S+ for about $41,600, against this machine's $60,700. Its line holds today and, held flat to 2028, holds only on Nigeria's power.

Scores

CriterionScoreWhy
Cashflow5/10Makes a bitcoin for about $60,700 of electricity at 6¢ and about $39,500 at 3.9¢, and returns roughly 21% in year one in Argentina. It has covered 12% of its hardware in 123 days and is on track for about 2.0× by the 2028 halving.
Longevity4/10At 17.5 J/TH its line is $25.20 a petahash a day at 6¢ and $16.38 at 3.9¢. Half of today's hashprice, $16.61, is under the first and above the second, so after the halving it clears at Nigeria's power by 23 cents and not at Argentina's.
Efficiency4/1017.5 J/TH. The air machines we stock go to 13.5 for the S21 XP and 12 for the M74S+; the water machines go below 10.
Quality control5/10Operators report a spread: one on Bitcointalk warns to expect at least 5% failures in the first year, another counts six warranty repairs on ten units in eight months. We found no failure rate we could cite beyond those posts, so it is scored as mixed rather than as good.
Serviceability6/10Air-cooled, so it lives in Argentina, where we repair on site and bill the cost of the parts only. We could not open a maker page with warranty terms, so we score on our own repair terms.

Efficiency and longevity count most, because they decide how many halvings a machine clears. Reliability is scored from public operator reports, which are mixed.

Buy it if

Walk away if

TL;DR

17.5 J/TH on air, in a 16 kg box

The S21 is an air-cooled Antminer that Bitmain announced in 2023, rated 17.5 J/TH. Our store lists the 151 TH/s bin from 2,643 W, 400 × 195 × 290 mm and 16 kg, and a 200 TH/s unit at the same 17.5 J/TH that is sold out. This review uses the 151 TH/s bin. Its figures are our store record: we could not open a maker spec page for the machine, so we leave out any figure the record does not hold.

It is air-cooled, which decides where it lives. A machine on air starts in Argentina, at 6¢ and 98.7% uptime, and crosses once to Nigeria's 3.9¢ when a year of figures says its power pays better there. The 200 TH/s unit's two and a half years are in its field report, and the newer step up is the S21 XP at 13.5 J/TH.

Chart: Coins kept since the first settled price we hold for it, the electricity that made them, and where it lands by the 2028 halving.

The box is the cheap part

A purchase once the price settled, in July 2026, cost $888. It is $888 today, $5.88 a terahash. Since February 2026 the market price has sat between $5.25 and $6.17 a terahash, so the box has moved by under a dollar in nine months.

Against the air machines it sits beside, it asks the least. The S21+ is $6.72 a terahash at 16.5 J/TH, the S21 Pro $8.61 at 15, the S21 XP $11.55 at 13.5, the M60S++ $9.45 at 15.5 and the M74S+ $14.86 at 12. A lower price per terahash buys more hashrate for the money and a dearer coin; the guide to what a miner costs all-in shows what sits on top of the box.

Chart: What the 151 TH/s unit has cost since the first price we hold for it, in February 2026.

A dearer coin than the newer air machines, and a first year to match

In Argentina, at 6¢, the S21 makes a bitcoin for about $60,700 of electricity today, against a bitcoin price of $77,392. At Nigeria's 3.9¢ it makes one for about $39,500. The S21 XP makes one in Argentina for about $46,800, the S21 Pro for about $52,100, the S21+ for about $57,300 and the M74S+ for about $41,600.

In its first year it returns about 21% in Argentina and 34% in Nigeria at today's price and difficulty. The S21 Pro returns 28% in Argentina and the S21 XP 27%. Efficiency decides the cost of a coin and the S21 is the least efficient of these, so it is dearest.

Over its run since July 2026, electricity has averaged about $54,600 a coin, a lifetime average over 123 days at 6¢. The figure to plan on is today's.

Chart: One bought today at today's price and racked in Argentina: year one at today's difficulty, and what it returns by the April 2028 halving with bitcoin back at its previous high.

Where the halving puts it

A machine's line is the hashprice at which it stops covering its own power. At 17.5 J/TH a petahash uses 420 kWh a day: $25.20 at 6¢, and $16.38 at Nigeria's 3.9¢. On our hashprice chart, network hashprice closed at $33.21 a petahash a day on 30 October, so the S21 clears the 6¢ line with $8.01 to spare and Nigeria's with $16.83.

Neither line has been crossed since the April 2024 halving. Hashprice reached its low of $27.74 on 6 June 2026, $2.54 above the 6¢ line for this machine, and that is the closest it came in the 925 days of data we hold since the halving. The count for a 19 J/TH machine shows how a dearer line has fared.

The 2028 halving halves the coins a petahash earns. At today's price and difficulty that puts hashprice near $16.61, which is $8.59 under the 6¢ line and 23 cents over Nigeria's. That is today's conditions held flat, not a forecast: it is why every projection we publish assumes the old high returns. The guide to efficiency and halvings shows where the line sits at every J/TH. At 17.5 J/TH, about 4¢ is the highest power rate that still clears across the halving, and a machine on Nigeria's power is already there.

Half of today's hashprice still covers Nigeria's power for an S21, by 23 cents. It does not cover Argentina's.

Chart: Network hashprice per petahash per day. The S21 needs about $25 of it at 6¢ and about $16 at 3.9¢; half of today's level is about $17. Open the chart

What a fault costs you here

Operators report a spread. On the Bitcointalk thread on S21-generation reliability, mikeywith writes that you "should be prepared for at least 5% failure rate for the first year" and that most Antminer issues are hashboards; CryptoPig333, running ten 200 TH/s units since March 2024, counts six warranty repairs through his hosting provider. These are two owners, not a sample, and we found no failure rate we could cite beyond them.

What a fault costs is the part that matters. In Argentina we repair on site and bill the cost of the parts only, and an air machine is serviced where it hangs: a failed hashboard is a line item, not a catastrophe. The guide to cooling explains what air changes.

Chart: Scored on what costs an owner money over a four-year life. Reliability reflects mixed operator reports.

Our two cents

The S21 is a cheap box on a proven design. It clears its power at 6¢ today with $8 a petahash to spare, it fits the Argentina site we already run, and its one move, to Nigeria, takes a coin from about $60,700 to about $39,500 and, on today's conditions held flat, keeps it covering its power through the 2028 halving.

If the aim is a cheaper coin on air, the S21 XP is the step up at 13.5 J/TH, and the M74S+ goes to 12. If the aim is hashrate for the least money into a rack, and the power is Nigeria's, this is a machine that does the job. The figure below ranks what beats it on cost per coin and then on first-year return, in stock today.

The S21 is a cheap box on a proven design. It is not the cheapest way to make a coin.

What beats it on cost per coin and on first-year return, in stock today.

What people are saying online

I would say however that you should be prepared for at least 5% failure rate for the first year

An experienced forum operator's planning figure for hashboards, the part he says fails most.

My fleet right now is 10x Antminer S21 (200T) which has been running since March 2024.

A ten-unit owner who then counted six warranty repairs through his hosting provider by late November 2024.

Sources and further reading

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.