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Underclocking reduces hashing-chip frequency; undervolting reduces the voltage supplied to those chips. Most modern ASIC firmware couples both controls through a power target or tuning mode. The practical goal is not the lowest wattage number: it is the best stable balance of wall power, accepted hashrate, joules per terahash (J/TH), temperature and uptime.
Make one small change at a time, measure power at the wall, and stop when errors, board dropouts, temperature or lost hashrate outweigh the electricity saved. A profile that works on one miner is not automatically safe on another because chip quality and cooling differ.
What underclocking and undervolting change
Underclocking
Underclocking lowers the frequency at which the hashing chips operate. Hashrate generally falls with frequency, while chip power and heat usually fall as well. The relationship is not perfectly linear because efficiency, fan power, pool behavior and the quality of individual chips also matter.
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Undervolting lowers the electrical potential supplied to the chips. Lower voltage can reduce power substantially, but it also reduces the operating margin that keeps chips switching reliably. If voltage is pushed too low for a given frequency, hardware errors, rejected shares, board dropouts or restarts can appear.
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Why firmware usually links them
ASIC firmware often exposes a power target or a small set of performance modes instead of independent voltage controls. The firmware then selects frequency and voltage combinations that fit that target. Stock firmware may offer only discrete Normal, Sleep and Low-power modes. More advanced firmware can tune individual chips and adjust performance dynamically.
Braiins describes the reason for per-chip tuning succinctly: “Every ASIC is unique.” Manufacturing variation means two miners of the same model may need different voltage and frequency values. Its autotuner searches for a suitable setting for each chip rather than copying one global value to every board.
Use J/TH, not watts alone
Efficiency is calculated as:
J/TH = wall watts ÷ accepted terahash per second
For example, a miner drawing 2,000 W and delivering 100 TH/s uses 20 J/TH. A 1,700 W setting that delivers only 75 TH/s uses 22.7 J/TH, so it saves power but performs less efficiently. Compare all of the following together:
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- Wall watts measured with a properly rated meter or facility monitor.
- Accepted hashrate, preferably averaged over a stable interval rather than read from a momentary dashboard value.
- J/TH calculated from the same interval.
- Rejected shares and hardware-error counters.
- Chip and board temperatures, fan speed and ambient temperature.
- Uptime, restarts and whether every hash board remains online.
Firmware watt estimates are useful for observing trends, but they do not replace a wall measurement. PSU losses, fans and control electronics are included in the wall figure and can change the result materially.
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Published operating points and what they show
The following figures are vendor-published examples, not promises for every unit. The J/TH values shown in parentheses are calculated from the listed watts and hashrate.
| Source and miner | Operating point | Power | Efficiency | What the point demonstrates |
|---|---|---|---|---|
| Bitmain Antminer S21 product manual, 2025 | 200 TH/s | 3,500 W | 17.5 J/TH | Rated high-performance point |
| Bitmain Antminer S21 product manual, 2025 | 175 TH/s | 3,062 W | About 17.5 J/TH | Lower operating point; about 438 W below the 200 TH/s listing |
| Bitmain Antminer S21 product manual, 2025 | 151 TH/s | 2,642 W | About 17.5 J/TH | Further reduced output; about 858 W below the 200 TH/s listing |
| Bitmain Support, S9 Low Power Mode, 2018 | 13.88 TH/s | 1,258 W | About 90.6 J/TH | Low-power mode can trade a large amount of efficiency for lower absolute draw on an older design |
| Braiins vendor example, 2025 | S19 efficiency range | Power varies by profile | 34 to 26 J/TH | Reported improvement with tuned operation; not guaranteed for every S19 |
| Braiins vendor example, 2025 | S21 efficiency range | Power varies by profile | 17.5 to 15.4 J/TH | Reported improvement with tuned operation; not guaranteed for every S21 |
The S21 manual’s three listed points illustrate that a lower target can reduce wall draw while retaining roughly the same rated efficiency. Your miner may behave differently once ambient temperature, PSU efficiency, fan speed and chip variation are included.
Prepare before changing a setting
Identify the exact hardware and software
Write down the miner model, number of hash boards, PSU model and rating, firmware version, current mode and any warranty restrictions. Compatibility is model- and firmware-specific. Do not assume a mode or power target available on one generation exists on another.
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Check electrical and cooling headroom
- Use the miner and PSU manufacturer’s electrical ratings; never exceed the circuit, connector or PSU limits.
- Maintain clean filters, unobstructed intake and exhaust, and adequate airflow.
- Record room temperature. A profile tuned in a cool room can fail when seasonal heat raises chip temperature.
- Keep protective shutdowns enabled. Do not bypass thermal or electrical safeguards to hold a target.
Capture a baseline
Let the miner run in its current profile until readings are stable. Record wall watts, accepted hashrate, pool-reported hashrate, rejected shares, hardware errors, each board’s status, chip temperatures, fan speed and ambient temperature. Save the firmware page or configuration export if the software provides one.
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Step-by-step underclocking and undervolting workflow
- Select the least invasive control. On supported stock firmware, begin with the documented Low-power or Sleep mode rather than entering arbitrary voltage values. On compatible Braiins OS installations, begin with a conservative power target and let the tuner generate the chip profile.
- Change one target at a time. Lower the power target or frequency in a small increment. Avoid changing frequency, voltage, fan limits and temperature limits simultaneously; otherwise you cannot identify which change caused an error.
- Allow ramping and autotuning to settle. Controlled ramping may take time to reach the target. Autotuning can test chips individually. Judge the profile only after the miner has reached a steady state and pool statistics cover a meaningful interval.
- Measure at the wall. Read the external meter or facility monitor, then note accepted TH/s, J/TH, temperatures, rejected shares and hardware errors. Record the ambient temperature beside every result.
- Compare against the baseline. Keep the new setting only if the watt reduction and J/TH improvement justify the accepted-hashrate change and the miner remains stable.
- Test the stability boundary. If the profile is clean, make another small reduction and repeat the settling and measurement period. Stop when errors rise, a board disappears, restarts occur, temperatures become unsafe, or the revenue lost from lower hashrate exceeds the electricity saving.
- Back off from the boundary. Raise the target slightly or restore the previous profile. A setting that survives a short test but produces intermittent errors over hours is not a successful profile.
- Save and document the result. Record the exact firmware, mode or target, wall watts, accepted hashrate, J/TH, temperatures, fan behavior, rejected-share rate and ambient conditions. Keep the prior known-good profile so you can recover after an update or a weather change.
Choosing between stock modes and advanced autotuning
Stock firmware modes
Stock firmware is the lowest-change option and generally preserves the manufacturer’s support path. Depending on model and firmware revision, it may expose Normal, Sleep and Low-power modes plus a frequency level or wattage field. Bitmain’s API documentation includes miner mode, frequency level, wattage and J/T fields, which can be useful for monitoring or automation. The exact controls and allowed ranges vary by model.
Bitmain’s 2018 S9 Low Power Mode guidance reports 13.88 TH/s at 1,258 W. It also warns that instability or hashrate drops are reasons to disable Low Power Mode or flash back to the original firmware. Treat that warning as an operating rule: a named low-power mode is not a guarantee of stable operation on every unit.
Firmware with chip-level autotuning
Compatible Braiins OS versions can accept an arbitrary power target, tune frequency and voltage per chip, and apply dynamic performance scaling. Braiins documents reducing power as temperatures rise, which can help maintain operation in changing conditions. The trade-off is added configuration complexity and the need to verify model support, firmware compatibility and warranty implications before installation.
Autotuning does not remove the need for measurement. The tuner searches for a profile; you still decide whether its wall watts, accepted hashrate, J/TH and long-run stability fit your electricity price and uptime requirements.
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How to decide whether a profile is worthwhile
| Observation after a stable test | Interpretation | Action |
|---|---|---|
| Watts fall, accepted hashrate is nearly unchanged, J/TH improves, errors stay near baseline | Useful efficiency gain | Continue a longer test and save the profile if stable |
| Watts fall in proportion to hashrate, J/TH is unchanged | Lower output without an efficiency gain | Choose based on electricity price, required revenue and noise or heat limits |
| Watts fall, but J/TH worsens sharply | The operating point is less efficient even though absolute draw is lower | Raise the target or compare the revenue and power costs explicitly |
| Hardware errors or rejected shares increase | Voltage or frequency margin is insufficient, or cooling is inadequate | Raise the target, improve cooling or revert |
| A board drops, the miner restarts or temperatures trigger protection | The profile is outside the reliable operating envelope | Restore the last stable profile immediately and inspect airflow, PSU and connections |
For a simple cost comparison, multiply the measured watt reduction by operating hours and your electricity rate. Then account for the hashrate-related revenue change and any cost of downtime. A lower bill is not a gain if the miner is offline or submitting substantially fewer accepted shares.
Temperature, noise and facility constraints
Lowering chip power usually reduces heat, but fan behavior can complicate the result. A miner may increase fan speed to hold a temperature limit, reducing part of the expected wall-power saving and increasing noise. Track fan speed and inlet temperature rather than assuming a lower chip-power target automatically means a quieter machine.
In a home or other constrained space, compare profiles on sound, exhaust heat and circuit loading as well as J/TH. In a managed facility, the most valuable setting may be the one that stays inside a predictable thermal and electrical envelope during the hottest operating period, even if a cooler-day profile is marginally more efficient.
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The miner reboots after lowering the target
Restore the last stable target, verify PSU capacity and connectors, and check whether the reboot coincides with a board temperature or fan alarm. If the reboot disappears at the previous setting, the new voltage or frequency combination was too aggressive.
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One hash board disappears
Return to the prior profile and inspect board status, power leads, airflow and temperature. A single marginal board can make a global target look unstable. Do not keep retrying a setting that repeatedly drops the same board.
Hardware errors climb while the dashboard still shows hashrate
Use accepted pool hashrate and rejected-share data, not only the local instantaneous figure. Rising hardware errors indicate that chips are producing invalid work. Raise the target slightly, allow tuning to settle, and retest.
Pool hashrate is lower than local hashrate
Allow for normal reporting delay, then compare accepted shares over a longer interval. If the gap persists alongside rejects or errors, the profile may be unstable. Revert one step and verify the wall measurement and network connection before making another change.
The profile worked in winter but fails in summer
Retest at the warmer ambient temperature, clean filters and confirm that intake and exhaust paths are clear. Keep a seasonal profile with a higher power target or stronger cooling margin rather than forcing the cold-weather setting.
Keeping profiles reproducible
Store a dated record for each profile containing model, board count, PSU, firmware, mode or target, wall watts, accepted TH/s, J/TH, temperatures, fan speed, rejected shares, hardware errors and ambient temperature. Note whether readings came from a wall meter or firmware estimate. After firmware updates, PSU changes, board repairs or major weather changes, start from the last known-good profile and revalidate instead of assuming settings were preserved.
When several miners are deployed, do not clone voltage values blindly. Use the same measurement procedure, but tune each unit to its own stable boundary. Grouping miners by model and firmware helps with management; it does not eliminate chip-to-chip variation.
Quick Recap
Safety checklist
- Use a meter and wiring rated for the miner’s continuous load.
- Keep manufacturer electrical, thermal and protective limits active.
- Do not operate with damaged connectors, loose power leads or blocked airflow.
- Change one variable or power target at a time.
- Wait for ramping and autotuning to settle before judging a result.
- Watch accepted hashrate, J/TH, rejects, errors, temperatures and uptime together.
- Keep a rollback profile and document every successful setting.
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