Bitmain Antminer S21 Pro review

Once a shelf of Bitaxes stops being enough, the next thing you end up looking at is a farm-class machine like the Antminer S21 Pro, and whether one could live in the garage. The S21 Pro is rated at 234 TH/s and 15 J/TH, and in return it wants a 220-277V supply, somewhere for 3510W of heat to go and a building you don’t mind filling with 76 dB of fan noise.

The straight take

If you’re weighing it against the standard S21, the Pro’s BM1370 chip buys you more hashrate and a better efficiency figure than the S21’s 200 TH/s at 17.5 J/TH on the BM1368. J/TH is joules per terahash, so lower is more efficient. The S21 XP is rated higher, at 270 TH/s and 13.5 J/TH. Bitmain’s current list goes as low as 8.9 J/TH on the hydro-cooled S23 XP Hyd., and the Pro isn’t on it any more, although retailers still had new ones in stock at the time of writing.

At home, the fan noise and the heat off 3510W put it in a garage, an outbuilding or a basement with the right feed, and keep it out of any room you sit, sleep or work in.

What the numbers mean

Check the listing before you buy, because Mineshop sells a 234 TH/s and a 245 TH/s version side by side. The S21 Pro comes rated at 220, 234 or 245 TH/s, all at 15 J/TH. Bitmain’s spec also lets the actual hashrate sit 3% either side of the rating, and the power 5% either side.

Run flat out, 3510W is about 84 kWh a day. At the Ofgem price cap for October to December 2026 that’s about £22 a day, and at the US residential average for August 2026 it’s about $15 a day. Home mining almost never pays, and at those rates the S21 Pro has a lot of electricity to earn back every single day. The same power arrives as heat, about 12,000 BTU/h, which only helps if you’ve got a space that wants warming.

Bitmain’s support page gives the 76 dB at 25C, and D-Central reports the manual quoting it at 30C with the fans at maximum speed.

Then there’s the network, which stood at about 942 EH/s on 12 November 2026. Solo mining means hashing on your own for a whole block reward, where a pool shares the work and pays each miner for their part of it. Mining solo at that day’s difficulty, 234 TH/s can expect about one block in 86 years, and the figure drifts with every difficulty adjustment.

Power and siting

Start with the supply, because the PSU takes 220-277V AC. UK and EU mains at 230V sit inside that range, and a 110V US socket sits outside it. If you’re in the US and the house hasn’t got a circuit in that range, you’ll need one fitted. On paper 3510W at 220V is about 16A, but Bitmain rates the input current at 20A. That higher figure is the one to hand to a qualified electrician, who sizes the circuit for you.

The machine itself is 450 x 219 x 293mm and weighs 20 kg, or 22 kg boxed. Bitmain gives the operating range as -20 to 45C and quotes its rated figures at 25C. In a closed garage in summer all of that power ends up as heat around the machine, so you’ll need a way to vent the hot air.

Firmware

The S21 Pro runs at Bitmain’s rated figures on its stock firmware, and it can be left there. If you want to tune it, Braiins OS, Vnish and LuxOS all list it as supported.

Braiins publishes a performance table for it, and at the stock 3510W limit the table claims 250 TH/s at 14.1 J/TH. With the power turned down by 500W it claims 227 TH/s at 13.2 J/TH, only 3% less hashrate by its own count. Turn the power up by 500W instead and it lists 267 TH/s at 15 J/TH, the same as Bitmain’s stock figure, though it also notes that overclocking is restricted by the manufacturer. For a home miner paying retail electricity the underclock row is the one to look at, since it’s the efficient end of the table.

Bitmain’s terms exclude unauthorised firmware changes made with a third party’s product, so flashing any of the three costs you the warranty.

Who should get one, and who shouldn’t

The S21 Pro makes sense if you’ve got a garage, outbuilding or basement where the fan noise bothers nobody, and a 220-277V circuit to plug it into, or the budget for an electrician to fit one. Some appetite for tuning helps too, because Braiins’ underclock row claims 13.2 J/TH against the stock 15. It makes more sense again if the heat has a job, such as keeping a workshop or a cold outbuilding warm through winter.

Skip it if the only place for it is a room you use, because 76 dB of fan noise is too much to live alongside. Skip it too if the house only has 110V sockets and new wiring isn’t on the cards. And if you pay more per kWh than the Ofgem cap or the US average, think hard. Running it then costs you more than £22 or $15 a day, whatever the machine earns.