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Machine review · Oct 10, 2026 · 6 min read
Antminer S21+ Hyd review: 395 TH/s in one box, at the best entry price it has had
Bitmain's 15 J/TH water-cooled machine packs the most hashrate per box of any we list at its efficiency. It makes a coin dearer than the 13 J/TH machines above it, so its case is the entry price and a site with cheap power.
Bitmain Antminer S21+ Hyd (395 TH/s)
The bottom line
The S21+ Hyd is Bitmain's mid-range water-cooled machine: 395 TH/s from 5,925 W in the bin we list, 15 J/TH. In the UAE, at 6¢, it makes a bitcoin for about $49,200 of electricity and returns roughly 49% in its first year at today's price. At Nigeria's 3.9¢ the coin costs about $32,000 and year one is 49%.
Bought once its price settled, in April 2025, it has mined and kept 0.1008 BTC for $5,003 of power: $3,233 of cashflow against a $7,583 landed cost. That is 544 days into a four-year machine. Between now and the 2028 halving it has another 0.104 BTC in it, worth about $12,991 with bitcoin back at its previous high, roughly 2.1× its hardware by the halving.
What you are buying is a lot of hashrate for a small first cheque: $2,737 for 395 TH/s, $6.93 a terahash. What you pay for it is electricity: a coin costs about $42,700 to make on the 13 J/TH S21e XP Hyd and about $49,200 here, at the same site. The entry price is the case; the efficiency is not.
Scores
| Criterion | Score | Why |
|---|---|---|
| Cashflow | 6.5/10 | Makes a bitcoin for about $49,200 of electricity in the UAE and returns roughly 49% in year one. It has covered 43% of its hardware so far and is on track for about 2.1× by the 2028 halving. The low entry price does the work. |
| Longevity | 5.5/10 | At 15 J/TH its line is $21.60 a petahash a day at 6¢, above the roughly $19.50 that hashprice would fall to after a halving at today’s conditions. At Nigeria’s 3.9¢ the line is $14.04, which clears it. |
| Efficiency | 5/10 | 15 J/TH is a middling figure for water-cooled hardware today: the S21e XP Hyd runs at 13 J/TH and the S23 Hyd at 9.5. |
| Quality control | 5/10 | Scored as unknown rather than as good. We found no operator reports of this machine’s boards that we could cite, so there is no record to score. |
| Serviceability | 6.5/10 | Hosted in the UAE, an in-warranty unit goes direct to the manufacturer’s warranty centre close to the facility. A 13.8 kg unit is simple to handle. |
Weighted on what an owner gets back per dollar over four years. The entry price and the single big box lift cashflow; 15 J/TH, a halving line above 6¢ and an unproven failure record hold the other scores down.
Buy it if
- Your power is at or below about 5.4¢ and you can run a water loop. At that rate the machine still clears its power after a halving of today’s hashprice, with conditions held flat.
- You want a lot of water-cooled hashrate from one box for the least money down. At $6.93 a terahash and 395 TH/s a unit, the freight, duty and setup that come with every machine are spread over more hashrate than on a 225 TH/s air-cooled one.
- You are hosting at Nigeria’s 3.9¢, where its line is $14.04 a petahash a day, well under half of today’s hashprice.
- You are building a fleet and want a low-cost position that can be replaced out of what it earns, as the [roof-and-building guide](/blog/machines-are-the-roof-not-the-building) describes.
Walk away if
- Your power is above about 10.8¢. At that rate the machine does not clear its electricity at today’s hashprice.
- Your power sits between 5.4¢ and 10.8¢. It clears today, but with conditions held flat it would stop clearing after the 2028 halving: you would be leaning on bitcoin recovering.
- You want the cheapest coin. The [S21e XP Hyd](/blog/bitmain-antminer-s21e-xp-hyd-review) makes one for about $42,700 in the UAE against about $49,200 here, and the [S23 Hyd](/blog/bitmain-antminer-s23-hyd-review) for about $31,300.
- You have no water loop and no plan for one. Hydro hardware needs a coolant circuit, which is why it lives in the UAE with us rather than in an air-cooled hall.
What Bitmain shipped
The S21+ Hyd is the water-cooled member of Bitmain’s S21+ line. Bitmain announced it on 9 October 2024 at 319 TH/s and 15 J/TH, with shipping from the first to the second quarter of 2025, as Forklog reported. It has since grown into bins of 319, 338, 358 and 395 TH/s at the same 15 J/TH, drawing 4,785 W, 5,070 W, 5,370 W and 5,925 W in our store record. The 395 TH/s unit is the one the figures in this review follow.
In our store record it measures 410 by 170 by 209 mm, weighs 13.8 kg, takes 380 to 415 V and runs in an ambient range of 20 to 50 °C. Bitmain’s own product page could not be opened, so these are the store record’s figures, and anything that record does not hold is left out.
It sits one tier below the S21e XP Hyd and well below the S23 Hyd: less efficient, and far cheaper to buy. We hold a look at what ours has banked in a progress report.
The best entry price it has had
The first price we hold for the 395 TH/s unit, on 20 February 2025, was $7,383, which is $18.69 a terahash. It is $2,737 today, $6.93 a terahash, for the same hashrate. The hashrate in the box has not changed; what it costs to get one has. That is the best entry price we have on record for it, and it is why this machine is in the conversation at all.
Among water-cooled machines at 17 J/TH or better it is one of the cheapest ways in per terahash. The S21e XP Hyd costs $8.82 a terahash, the S21J XP Hyd $11.55 and the S23 Hyd $23.52. The guide to what a miner costs all-in shows what sits on top of the box before it is running.
395 TH/s in one box, at $6.93 a terahash.
What a coin costs it
At 6¢ in the UAE it makes a bitcoin for about $49,200 of electricity today, against a bitcoin price of $82,785. At Nigeria’s 3.9¢ the same coin costs about $32,000. The S21e XP Hyd makes one for about $42,700 in the UAE, the S21J XP Hyd for about $39,300 and the S23 Hyd for about $31,300.
The return per dollar tells the other half. In its first year the S21+ Hyd returns about 49% in the UAE. The S21e XP Hyd returns 51%, the S21J XP Hyd 46% and the S23 Hyd 31%. A lower price per terahash buys more hashrate for the money, and in the first year that can outweigh a dearer coin. Efficiency pays later.
Over its run since April 2025, electricity has averaged about $49,600 a coin. That is a lifetime average; 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 15 J/TH a petahash uses 360 kWh a day: $21.60 at 6¢, and $14.04 at Nigeria’s 3.9¢. Network hashprice was $38.94 per petahash per day on 9 October on our hashprice chart, so the S21+ Hyd clears the 6¢ line today by about four-fifths. The most it can pay for power and still clear today is about 10.8¢ a kilowatt-hour.
The 2028 halving halves the coins a petahash earns. At today’s price and difficulty that puts hashprice near $19.50, below the 6¢ line and 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. With bitcoin back at its previous high of $124,493 the same halved hashprice would be about $30, and the machine would clear 6¢ comfortably.
This is the cost of 15 J/TH. A 13 J/TH machine has a line of $18.72 at 6¢, under the flat-conditions level; the guide to efficiency and halvings shows where the line sits for every J/TH figure. It is the profile of a machine that crosses to cheap power once: it starts where the water plant is, and moves to Nigeria when a full year of figures says the cheaper site is decisively better.
What owning one is like
We looked for operator threads and repair write-ups on the S21+ Hyd and found nothing specific to it that we could stand behind, so none are quoted here. The machine has been on sale since the spring of 2025, and a reliability score needs a record that has had time to build.
What a fault costs is the part that matters. Every ASIC develops faults, and in the UAE an in-warranty unit goes direct to the manufacturer’s warranty centre close to the facility; it is a line item, not a catastrophe. It is also a water-cooled unit, which is why it is hosted where the water plant is. The guide to air, hydro and immersion covers what the loop means for an owner.
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+ Hyd is a good machine at a very low price for the hashrate. Today it clears its power at 6¢ with room, it clears it at Nigeria’s 3.9¢ even after a halving, and a buyer building a fleet on a small first cheque has a sound reason to pick it.
A buyer who wants the cheapest coin should look one step up: the S21e XP Hyd makes it for about $42,700 in the UAE against about $49,200 here, at a first-year return of 51%. The figure below ranks what beats it on cost per coin and then on year-one return, in stock today. A coin that costs less to make is a coin you keep more of.
- Bitmain Antminer S23 Hyd (563 TH/s) — 9.5 J/TH, In stock, $31,266 of electricity per bitcoin, 26% year one
- Bitmain Antminer S21j XP Hyd (495 TH/s) — 12 J/TH, In stock, $39,348 of electricity per bitcoin, 40% year one
- Bitdeer SealMiner A3 Pro Hyd (666 TH/s) — 12.5 J/TH, In stock, $40,605 of electricity per bitcoin, 32% year one
Sources and further reading
- Bitmain expands Antminer S21 series with two new bitcoin miners — Forklog, 10 Oct 2024
Which machines this covers
- Bitmain Antminer S21+ Hyd (395 TH/s)
- Bitmain Antminer S21+ Hyd (358 TH/s)
- Bitmain Antminer S21+ Hyd (338 TH/s)
- Bitmain Antminer S21+ Hyd (319 TH/s)
The other side of this machine
The S21+ Hyd is the best-returning machine on our floor — 395 TH/s from one box at a settled entry price, and one of the strongest first-year returns we host. Seventeen months in, here is what it has banked and what is left in it.
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.