At 18.5 J/TH, the Antminer S21 Immersion is the machine the 2028 halving will test

Bought in January 2025 and racked in the UAE, it has paid its power bill every month and covered a third of its hardware. It is also an 18.5 J/TH machine, which makes its next two years the clearest test of the forward figures we publish.

An Antminer S21 Immersion against a deep oxblood studio backdrop

Bitmain Antminer S21 Immersion (227 TH/s)

Bought in January 2025, running in the UAE ever since

A 227 TH/s S21 Immersion at 18.5 J/TH, bought at $3,814 and landed at $5,495, racked in the UAE at 6¢ in January 2025. Our Nigeria site runs air and hydro, so an immersion machine's home is the UAE and it has been there for all 633 days.

It has mined and kept 0.0695 BTC, worth about $5,802 today, for $3,992 of electricity: about $57,400 a coin over its run. That is $1,811 of cashflow, or 33% of its landed cost, on a machine a little under half way through a four-year life.

Chart: Coins kept since January 2025, the electricity that made them, and where the forward figures put it by the April 2028 halving.

A coin costs it more than a third above the S21 XP Immersion's

At 6¢ it makes a bitcoin for about $58,700 of electricity today. The S21 XP Immersion, which is also immersion-cooled and runs at 13.5 J/TH, does it for about $42,800.

The arithmetic behind that gap is simple. A machine's line is the hashprice at which it stops covering its power, and at 18.5 J/TH a petahash uses 444 kWh a day: $26.64 at 6¢. Network hashprice was $39.77 per petahash per day on 30 September on our hashprice chart, so it clears the line with room: about half as much again.

Room is what it has today; distance is what it lacks for 2028. The gap is more than a third in electricity per coin, and that is the whole case for the newer machine.

At 18.5 J/TH a petahash burns 444 kWh a day. That is $26.64 at 6¢, against a hashprice of $39.77.

Chart: What a coin costs it in electricity against bitcoin's price. The gap between the lines is the cashflow.

The halving is the test

The 2028 halving cuts the coins a petahash earns in half. At today's price and difficulty that would put hashprice near $19.90, under this machine's $26.64 line at 6¢. It is today's conditions held flat, not a forecast, and it is why our forward figures assume bitcoin returns to its previous high, as every projection on the site does.

On that assumption, with the price back at $124,493, it adds another 0.061 BTC by April 2028, worth about $7,581: roughly 1.7× its landed cost by the halving. The same arithmetic with bitcoin where it is today does not clear the line. That is what separates this machine from a 13.5 J/TH one, and why efficiency decides how many halvings a machine survives.

It is the machine where the assumption matters most. It is also an assumption that has held after every previous halving.

The best entry price it has ever had

The first price we hold for the 227 TH/s unit, on 7 January 2025, was $16.80 a terahash. It is $6.51 today, $1,478 a unit, for the same machine at the same 18.5 J/TH. A buyer now pays less than half what we did for identical output.

A low entry price does a lot of work for a machine like this: at today's price it returns about 30% in its first year. It does not change the efficiency, which is the part that meets the halving.

Chart: What it has cost, from the first price we hold. The machine and its output are unchanged; only the entry price has moved.

Who should buy one, and who should not

Buy it as a short, cheap position in hashrate: you are racking in the UAE, you want immersion because that is the room you have, and you are comfortable with a return that leans on the 2028 recovery.

Look at the S21 XP Immersion instead if you are planning past the halving. At $8.30 a terahash it costs more per terahash, and returns about 48% in year one against this machine's 30%. It makes a coin for about $42,800, which is where the runway comes from.

Bitmain Antminer S21 XP Immersion (300 TH/s) — 13.5 J/TH, In stock, $42,806 of electricity per bitcoin, 48% year one
Bitmain Antminer S23 Immersion (442 TH/s) — 12 J/TH, Pre-order, $38,050 of electricity per bitcoin, 39% year one
MicroBT Whatsminer M66S++ (348 TH/s) — 15.5 J/TH, Sold out, $49,148 of electricity per bitcoin, 42% year one

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

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.