ASIC Mining Glossary
What is energy efficiency in mining?
Energy efficiency tells you how much electricity a miner needs to produce its hashrate. For Bitcoin ASICs it is expressed in joules per terahash (J/TH): the lower the number, the less electricity each terahash costs.
Efficiency is the second figure to read on a miner, right after its hashrate. Two machines can earn the same revenue, yet one can keep a much larger part of it because it wastes less power as heat.
How to calculate J/TH
Formula
Efficiency (J/TH) = power draw (W) ÷ hashrate (TH/s)
A watt is one joule per second, so dividing watts by terahashes per second gives joules per terahash. A Bitmain Antminer S21 XP draws 3,645 W for 270 TH/s: 3,645 ÷ 270 = 13.5 J/TH.
Efficiency of current Bitcoin miners
| Miner | Hashrate | Power | Efficiency | Cooling |
|---|---|---|---|---|
| Bitmain Antminer S21 | 151 TH/s | 2,643 W | 17.5 J/TH | Air |
| MicroBT Whatsminer M61S+ | 230 TH/s | 3,910 W | 17 J/TH | Air |
| Bitmain Antminer S21+ Hydro | 395 TH/s | 5,925 W | 15 J/TH | Hydro |
| Bitmain Antminer S21 XP | 270 TH/s | 3,645 W | 13.5 J/TH | Air |
Each new ASIC generation has lowered this figure. Machines above 25 to 30 J/TH are generally considered older generations for Bitcoin mining.
What efficiency means for your electricity bill
Multiply J/TH by 24 hours and divide by 1,000 to get the kilowatt-hours needed per terahash per day. At 13.5 J/TH, one terahash uses 0.324 kWh per day; at 17.5 J/TH it uses 0.42 kWh.
Worked example: break-even electricity price
On September 29, 2026 the hashprice was about $40 per PH/s per day (OMS calculator data), or $0.040 per TH/s per day. A miner stops covering its electricity when the power for one terahash costs more than that:
- S21 XP at 13.5 J/TH: $0.040 ÷ 0.324 kWh = break-even at about $0.12 per kWh.
- S21 at 17.5 J/TH: $0.040 ÷ 0.42 kWh = break-even at about $0.095 per kWh.
At $0.08 per kWh both machines are profitable before pool fees, but the S21 XP keeps about $0.014 per terahash per day against $0.006 for the S21.
Hashprice changes every day with the Bitcoin price, fees and network difficulty. Check a current estimate with the profitability calculator.
What changes a miner's real efficiency
- Measured at the wall: manufacturers rate efficiency under test conditions. Power measured at the outlet includes power supply losses, so expect a few percent more.
- Temperature: hot intake air makes fans spin faster and chips run less efficiently. Hydro and immersion cooling keep chips at a stable temperature.
- Firmware tuning: underclocking lowers hashrate but usually improves J/TH; overclocking does the opposite. Custom firmware can change heat, noise and warranty terms.
- Other algorithms: units change with the coin: J/MH or J/GH for Scrypt miners (Litecoin, Dogecoin), J/GH for X11, J/KSol for Equihash. Only compare machines that mine the same algorithm.
Air, hydro and immersion cooling
Cooling has a direct effect on efficiency. Air-cooled miners are the simplest to install but depend on the temperature of the room and spend part of their power on fans. Hydro miners, such as the Antminer S21+ Hydro, move heat through a liquid loop to a dry cooler or heat exchanger: chips stay at a stable temperature, noise drops sharply and the heat can be reused. Immersion cooling places standard miners in a dielectric fluid and often allows them to run harder without overheating.
Hydro and immersion setups cost more to build, so their better stability and efficiency pay off mainly on larger sites or where noise and heat reuse matter.
How to measure your own efficiency
- Measure the power draw at the outlet with a metered PDU or a clamp meter, once the miner has warmed up.
- Read the 24-hour average hashrate on your mining pool dashboard.
- Divide the measured watts by the average TH/s. Compare the result with the rated J/TH: a large gap often points to dust, heat or a failing hashboard.
Frequently asked questions
Is a lower J/TH always better?
For running costs, yes. The most efficient machines also cost more to buy, so compare efficiency together with the purchase price and the payback period.
Does efficiency matter if my electricity is cheap?
Less, but it still matters over time. Revenue per terahash falls as network difficulty rises and after each halving; efficient machines stay profitable longer.
What is W/TH?
W/TH is the same number as J/TH. Both describe the power needed for one terahash per second.