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Machine review · Oct 5, 2026 · 5 min read
Bitmain Antminer S21 XP Immersion review: the air XP's 13.5 J/TH for less per terahash
The S21 XP Immersion makes 300 TH/s from 4,050 W, the same 13.5 J/TH as Bitmain's air-cooled S21 XP, for an immersion tank. It costs less per terahash than the air machine and returns more in its first year. Here is where it sits among the machines it will be weighed against.
Bitmain Antminer S21 XP Immersion (300 TH/s)
The bottom line
The S21 XP Immersion is Bitmain's immersion version of the S21 XP: 300 TH/s from 4,050 W, 13.5 J/TH, built to run submerged in a tank of dielectric fluid. At our UAE site, where immersion machines start, it makes a bitcoin for about $42,800 of electricity and returns roughly 51% in its first year at today's price. At Nigeria's rate the same machine would make a coin for about $27,800.
It is the same efficiency as the air-cooled S21 XP, so the coin costs the same at the same power rate. The difference is the price. A terahash of the immersion machine is $8.30 today, $2,489 for the unit, against $11.55 for the air-cooled one, and that is why it returns 51% in year one against that machine's 39%.
What you are buying is a mature 13.5 J/TH design at a low entry price, in a form that suits our UAE site. What you are not buying is the cheapest coin: the S23 generation makes one for much less. Weigh it against the S21e XP Hyd 3U, which returns 56% in year one, and the S23 Hyd.
Scores
| Criterion | Score | Why |
|---|---|---|
| Cashflow | 7/10 | Makes a bitcoin for about $42,800 of electricity at 6¢ and returns roughly 51% in year one at today's price, above the 39% of the A3 Hyd at the same efficiency. |
| Longevity | 6/10 | 13.5 J/TH keeps it above the line at 6¢ through the halving by a small margin, and well above it at 3.9¢. The S23 generation buys distance beyond that. |
| Efficiency | 6/10 | Good for last year and middling for this one: the S23 generation runs 8.9 to 10 J/TH, and the S21e XP Hyd 3U runs 13.0. It is a large step on the 18 J/TH immersion machines before it. |
| Quality control | 5/10 | Scored as unknown rather than as good. We found no operator reports of this machine's boards in the field that we could stand behind, so there is no record to score. |
| Serviceability | 7/10 | A Bitmain machine, hosted in the UAE, where Bitmain's warranty centre is near the site. In-warranty units go direct, which is the strongest repair route we have. |
Weighted toward cashflow: it returns more in year one than the other machines of its efficiency class we compare it with, apart from the S21e XP Hyd 3U, while durability is scored as unknown.
Buy it if
- You want 13.5 J/TH in an immersion form and a low price per terahash. At $8.30 it is among the cheapest ways into this efficiency class on our floor.
- Your power is at or below 6¢. At 6¢ it makes a coin for about $42,800; at 3.9¢, about $27,800.
- You are planning through the 2028 halving. Up to about 6¢ it clears its own power by a small margin at today's hashprice halved, and by a wide one at 3.9¢.
- You want the machine at its first-year return rather than at its distance from the halving: 51% in the UAE is among the higher first-year returns of the machines we compare it with.
Walk away if
- You want the cheapest coin. The S23 Hyd makes one for about $31,200 in the same room, and the S23 XP Hyd for about $19,000 at Nigeria's rate against this machine's $27,800.
- You are buying on first-year return alone. The S21e XP Hyd 3U returns about 56%, against this machine's 51%.
- You need a long public failure record for the machine. We found none we could stand behind, so we score that as unknown rather than as good.
- Your power is dearer than about 6¢. Between 6¢ and about 6.2¢ it clears today's hashprice halved with almost nothing to spare, and above 6.2¢ it does not clear it, so choose a more efficient machine.
What Bitmain shipped
Bitmain announced the S21 XP Immersion on 18 June 2024 and it has been on sale since 10 October 2024, according to the dates in our store record. It is the immersion-cooled member of the S21 XP family: 300 TH/s from 4,050 W, which is 13.5 J/TH, and a weight of 14.4 kg. Our store record lists its operating temperature as 0 to 40 °C.
Bitmain's own product page could not be opened when we wrote this, so the specifications above are from our store record, which retailers' listings agree with. Where the record is silent, this review is too: we have left out dimensions, voltage and fluid requirements rather than copy them from a listing.
The family it belongs to matters. The air-cooled S21 XP has the same silicon and the same 13.5 J/TH. The older immersion machines, the S21 Immersion that we follow in our 2028 halving piece and the Whatsminer M66S, run at 18 to 18.5 J/TH. This is the first immersion step on them.
Cheaper per terahash than the air-cooled version
The price is the machine's argument. $8.30 a terahash is $2,489 for the unit. The air-cooled S21 XP is $11.55 a terahash, for the same efficiency. A buyer who can rack immersion machines gets the same coin cost for less money up front, and that is the whole of the gap between a 51% first-year return and the air machine's 39%.
The comparison is each machine at its own home site. Air-cooled machines start in Argentina and immersion ones in the UAE, and both rates are 6¢, so the electricity is the same. The cooling is the difference, and so is what the room around the machine has to be.
For the price per terahash, the nearest rivals are not the S23 machines but the water-cooled S21e XP Hyd 3U, at $9.24 a terahash and 13.0 J/TH, and the A3 Hyd at $13.65. Both ask for a coolant loop rather than a tank.
The same 13.5 J/TH as the air-cooled XP, at a lower price per terahash.
What a coin costs it
At 6¢ in the UAE the S21 XP Immersion makes a bitcoin for about $42,800 of electricity today. At Nigeria's 3.9¢ the same machine would make one for about $27,800, and it would cross there once, later in its life, when the trailing year says cheap power pays better. Immersion machines start in the UAE, where the tanks, the 99% uptime and the warranty centre are.
The older immersion machine we hold the longest record for, the M66S, makes a coin for about $57,100 in the same room, because it runs at 18 J/TH. The S23 Hyd at 9.5 J/TH makes one for about $31,200. A coin costs this machine more electricity than the S23 generation, and less than the machines that came before it.
The rate is the lever. Every cent of power price changes the coin cost in proportion, which is why the same machine is cheaper to run at Nigeria's rate and why the home-site rule exists.
Does 13.5 J/TH last through the halving?
A machine's line is the hashprice at which it stops covering its own power. At 13.5 J/TH a petahash uses 324 kWh a day. That is $19.44 at 6¢ and $12.64 at 3.9¢.
Network hashprice was $40.38 per petahash per day on 4 October on our hashprice chart. The 2028 halving cuts the coins a petahash earns in half at an unchanged price and difficulty, which would put it near $20.19. That is above the line at 6¢ by 75 cents and well above it at 3.9¢. The break-even rate at that halved hashprice is about 6.2¢.
This is today's conditions held flat, not a forecast. It says the S21 XP Immersion is not at risk of being switched off at the halving at either of our rates, and that at 6¢ it carries to the halving with little to spare. The guide to efficiency and halvings has the long version.
What a fault costs you here
Every machine on this site develops faults eventually. This one is a Bitmain, and our UAE site sits near Bitmain's warranty centre, so an in-warranty unit goes direct and a fault is a repair queue rather than a shipment from your own address. It is a line item, not a catastrophe.
What is not known is how these boards behave over years in fluid. The machine has been on sale for two years, and we looked for operator reports of its boards and found none we could stand behind, so there are none in this review. We score reliability as unknown, not as good, and we say so plainly.
Where to go from here
If you want the cheapest coin, the S23 generation is a step up and the figure below ranks what we would send you to. If you want more first-year return from the same money, the S21e XP Hyd 3U is the one to look at.
The S21 XP Immersion is not the machine to choose on either of those. It is the machine for an owner who wants the XP's 13.5 J/TH at the lowest price per terahash we hold for it, in a form we already run.
- Bitmain Antminer S23 Hyd (563 TH/s) — 9.5 J/TH, In stock, $31,246 of electricity per bitcoin, 33% year one
- Bitmain Antminer S23 XP Hyd (600 TH/s) — 8.9 J/TH, Pre-order, $29,271 of electricity per bitcoin, 31% year one
- Bitmain Antminer S23e Hyd 2U (865 TH/s) — 10 J/TH, Pre-order, $32,435 of electricity per bitcoin, 61% year one
Our two cents
The S21 XP Immersion is a sound, cheap way into 13.5 J/TH for an owner who is happy to run an immersion machine in the UAE. It clears its power at both of our rates and it carries to the halving, narrowly at 6¢.
It is not the cheapest coin, and it is not the longest-proven machine. It is the same silicon as the air machine we already know, sold at a lower price per terahash, and that is the reason to buy it. Output is never guaranteed.
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.