Underclocking lowers an ASIC miner’s chip frequency; undervolting lowers the voltage supplied to those chips. Used together, they can reduce wall power and improve joules per terahash (J/TH), but only if the miner remains stable and the lost hashrate, rejected shares, heat and downtime do not cost more than the electricity saved. The safest method is to establish a measured baseline, use the least-invasive power control your model supports, change one target at a time, and validate results at the wall for long enough to expose errors and restarts.
There is no universal “best” voltage or clock. As Braiins puts it, “Every ASIC is unique”: manufacturing variation means two units of the same model may need different settings. Treat published efficiency figures as reference points, not guarantees for your machine.
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What underclocking and undervolting change
An ASIC miner contains hashing chips, control electronics, fans and a power supply. The chips perform more work per second when their clock frequency rises, but power consumption and heat also increase. Underclocking reduces that frequency, so the miner generally produces fewer terahashes per second (TH/s) while drawing fewer watts.
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Undervolting reduces the electrical potential supplied to the hashing chips. Dynamic power is strongly affected by voltage, so a modest voltage reduction can save substantial energy. Too little voltage, however, prevents chips from switching reliably. The result can be hardware errors, rejected shares, missing hash boards or repeated reboots.
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Firmware commonly couples frequency and voltage through a power target or tuning profile. You may select a wattage target rather than type a separate voltage for every chip. Advanced firmware can then search for a stable frequency-voltage combination for each chip; stock firmware often offers only discrete operating modes.
Why J/TH is the key efficiency measure
J/TH is joules consumed per terahash produced. Lower is better. Calculate it from the same observation period:
J/TH = (average wall watts × 1,000) ÷ accepted hashrate in TH/s
For example, 3,500 W at 200 TH/s is 17.5 J/TH. A setting that draws 2,500 W but delivers only 120 TH/s is 20.8 J/TH, so the lower watt number is not automatically an efficiency improvement. Use accepted or effective hashrate, not only the dashboard’s instantaneous estimate.
What the published specifications show
Bitmain’s 2025 Antminer S21 product manual lists multiple operating points. Its headline specification is 200 TH/s at 3,500 W (17.5 J/TH); the same manual lists 175 TH/s at 3,062 W and 151 TH/s at 2,642 W. Those figures demonstrate the trade-off between output and consumption, but they are manufacturer values under stated test conditions, not a promise that every unit will reproduce them after tuning.
Bitmain Support reported 13.88 TH/s at 1,258 W for an S9 in Low Power Mode in 2018. The support guidance also says to disable Low Power Mode or flash back to the original firmware if the miner becomes unstable or its hashrate drops. That older S9 result cannot be applied to an S19, S21 or another model.
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Braiins reported example efficiency changes from 34 to 26 J/TH on an S19 and from 17.5 to 15.4 J/TH on an S21. These are Braiins-reported comparisons, not guaranteed outcomes for every ASIC. Temperature, chip quality, power supply losses, firmware version and the particular unit all affect the result.
Before changing a setting: identify the machine and its limits
Compatibility is model- and firmware-specific. Record the following before touching a power control:
- Exact miner model, board count and nominal hashrate.
- Power-supply model, input voltage, circuit rating and connector condition.
- Firmware name and version, current operating mode and any custom profile.
- Warranty or hosting rules that prohibit third-party firmware or altered operation.
- Ambient temperature, ventilation arrangement, filter condition and fan status.
Confirm that the circuit and PSU can safely carry the miner’s maximum rated load. Never defeat thermal, over-current or protective shutdowns to maintain a target wattage. Inspect connectors for discoloration and use appropriately rated cabling and breakers.
Capture a useful baseline
Run the miner in its normal mode until temperatures and hashrate have settled. A short dashboard snapshot is not enough; record a stable period long enough to include pool-side hashrate and normal share variance. Use a properly rated true-power meter at the wall, a facility power monitor, or another calibrated measurement suitable for the miner’s voltage and current. Firmware watt estimates are useful telemetry but should not replace wall measurement.
Log:
- Average wall watts and, if available, power factor.
- Accepted or effective pool hashrate and the miner’s local hashrate.
- Rejected shares, stale shares and hardware-error counters.
- Temperature for each board or chip domain, fan speed and inlet/outlet temperature.
- Ambient temperature, humidity if relevant, and any restarts or dropped boards.
Keep the baseline record with the date, firmware version and pool. Without it, you cannot tell whether a later improvement came from your setting or simply cooler weather or normal pool variance.
Choose the least-invasive control first
Stock Bitmain firmware
On supported Bitmain models, begin with the documented Low-power or Sleep mode rather than manually forcing an undocumented voltage. Firmware revisions differ, so the exact label and available modes may not match another model’s web interface. Apply the mode, save or restart as the interface requires, and wait for the miner to ramp and settle.
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Bitmain’s API documentation exposes fields including miner mode, frequency level, wattage and J/T. Those fields are useful for monitoring and automation, but an API value does not mean every model accepts arbitrary per-chip voltage control. Stay within values exposed by your firmware and model documentation.
Braiins OS or other autotuning firmware
On a compatible miner, Braiins OS can use an arbitrary power target, chip-level frequency and voltage autotuning, and dynamic performance scaling. Its tuner ramps toward a target and searches for a profile suited to the individual chips. Braiins documents that temperature differences during tuning materially affect the outcome, so allow the system to reach normal thermal conditions before judging a profile.
Installing alternate firmware may affect warranty, support, licensing or hosting compliance. Verify exact model support and recovery instructions before flashing. Keep a copy of the original firmware and your configuration so you can return to the manufacturer profile if necessary.
Step-by-step underclocking and undervolting workflow
- Write down the baseline. Save wall watts, accepted hashrate, J/TH, temperatures, fan speed, rejected shares, hardware errors and ambient conditions.
- Apply one conservative change. Select the manufacturer’s Low-power or Sleep mode, or set a modest power target in compatible autotuning firmware. Do not copy another miner’s voltage.
- Allow ramping to finish. Autotuners may test chips at several frequencies. Wait until board temperatures, fan speed and reported hashrate have stabilized before comparing results.
- Observe for a meaningful interval. Record wall watts and pool results continuously or at regular intervals. Include enough time to reveal intermittent errors and at least one normal pool reporting cycle.
- Compare four primary metrics. Check wall watts, accepted hashrate, J/TH and stability together. Add temperature and noise when the miner is near people or in a constrained room.
- Move in small increments. If the profile is stable and savings justify a further reduction, lower the frequency or power target slightly and repeat. Changing several values at once prevents you from identifying the cause of a failure.
- Stop at the stability boundary. Reboots, dropped boards, rapidly increasing hardware errors, rejected shares, overheating or an unacceptable hashrate loss mean the target is too aggressive. Raise it slightly or restore the previous profile.
- Save the final profile. Record the exact target, firmware version, measured watts, accepted TH/s, J/TH, temperatures, fan behavior and ambient conditions. Export or photograph the configuration before firmware updates.
How to decide whether a lower-power profile pays
Electricity savings must be compared with the value of the hashrate you give up. For a simple estimate:
daily electricity cost = average watts ÷ 1,000 × 24 × electricity price per kWh
Then compare the change in mining revenue at the same pool and coin conditions. Pool payouts vary with network difficulty, fees, luck and coin price, so use a conservative period rather than a single payout. Include the value of downtime: a profile that saves 300 W but reboots several times a day may earn less than a stable profile using more power.
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| Operating point | Published output | Published power | Calculated efficiency | What it demonstrates |
|---|---|---|---|---|
| Antminer S21 reference point | 200 TH/s | 3,500 W | 17.5 J/TH | Bitmain 2025 manual’s headline operating point |
| Antminer S21 lower point | 175 TH/s | 3,062 W | 17.5 J/TH | Lower output without a stated J/TH change in the listed figures |
| Antminer S21 lower point | 151 TH/s | 2,642 W | 17.5 J/TH | Another manufacturer-listed operating point |
| Antminer S9 Low Power Mode | 13.88 TH/s | 1,258 W | About 90.6 J/TH | Bitmain Support example from 2018; model-specific |
| Braiins S19 example | Not stated | Not stated | 34 to 26 J/TH | Braiins-reported comparison, not a guarantee |
| Braiins S21 example | Not stated | Not stated | 17.5 to 15.4 J/TH | Braiins-reported comparison, not a guarantee |
The table’s figures come from the cited manufacturer or firmware vendor examples. They are not interchangeable test results: the S9 measurement is from 2018, the S21 values are from a 2025 manual, and the Braiins ranges are vendor-reported comparisons.
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Temperature, airflow and noise are part of the setting
Lower power usually reduces heat and fan demand, but a profile tuned in a cool room may fail when inlet air warms. Autotuners can select a different profile as chip temperature changes, and dynamic performance scaling can reduce power when temperatures rise. Keep intake paths clear, maintain filters, and verify that hot exhaust is not recirculating into the intake.
Record inlet and outlet temperatures rather than relying on a single chip reading. A board that repeatedly reaches a thermal limit, loses hashing or drives fans to maximum is not operating efficiently even if its wall wattage looks attractive. In a home, include acoustic impact: a slightly higher power target may be preferable if it avoids unstable fan cycling, while a lower target may make continuous operation tolerable.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common failure symptoms and recovery
Frequent reboots or a board disappearing
Raise the power target or voltage within the firmware’s supported range and return to the last stable profile. Check PSU capacity, input voltage, connectors and airflow. If the issue began immediately after alternate-firmware installation, use the documented recovery path and original firmware rather than repeatedly forcing restarts.
Hardware errors or rejected shares increasing
Reduce the aggressiveness of the clock or power target. Distinguish hardware errors from ordinary pool variance: a persistent rise that follows the setting change indicates marginal chip stability. Confirm temperatures and allow the miner to settle before making another change.
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Check whether the tuner is still ramping, whether a board has dropped, and whether the pool is reporting stale or delayed data. Compare local and accepted hashrate. If the loss persists, restore the previous profile; an underclock that does not reduce wall watts has not achieved its purpose.
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Overheating after a “low-power” change
Inspect fan operation, blocked filters, ambient temperature and exhaust recirculation. Do not bypass protective shutdowns. Increase airflow or reduce the target until temperatures remain within the manufacturer’s limits.
Instability after a seasonal change
Re-tune at representative warm and cool conditions, or choose a conservative profile that remains stable across the expected range. Save separate documented profiles only when your firmware supports reliable switching and you can monitor the transition.
When manual tuning is inappropriate
- The model or firmware does not document the control you want to change.
- Your circuit, PSU, connectors or cooling system are already near their rated limits.
- A warranty, hosting contract or insurer prohibits modified firmware or operation.
- You cannot measure wall power and pool-side results reliably.
- The expected electricity saving is smaller than the value of lost hashrate, maintenance time or downtime.
In these situations, use the manufacturer’s supported mode, improve airflow and power delivery, or operate the miner at its standard profile. A repeatable, stable result is more valuable than a record-low watt reading.
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Firmware updates can reset frequencies, power targets or autotuner data. Before updating, export settings and photograph the current dashboard. Afterward, confirm the model, mode and target, then repeat a shortened baseline check before returning the miner to unattended operation.
Review the profile whenever ambient temperature, supply voltage, pool, firmware or hardware changes. Keep a dated log with wall watts, accepted TH/s, J/TH, temperatures, errors and restarts. That record lets you identify gradual fan or filter problems and proves whether a new setting actually improved efficiency.
The practical decision rule
Select the lowest-power profile that delivers acceptable accepted hashrate, the same or better J/TH, low rejected-share and hardware-error rates, safe temperatures and reliable uptime. If any of those conditions fails, move back toward the last stable target. Underclocking and undervolting are optimization tools, not invitations to run outside the miner, PSU or facility’s electrical and thermal limits.
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