Antminer S21+ Undervolting — Efficiency Tuning
Undervolt the Antminer S21+ for better J/TH: custom firmware options, dev fees, safe voltage steps, and what it actually saves on a solo mining rig.
The stock Antminer S21+ runs at 235 TH/s and 16.5 J/TH. With undervolting on custom firmware you can reach roughly 14–15 J/TH at the same hashrate, cutting power by about 10% — worth around $250–300 a year in electricity per machine at $0.085/kWh, plus cooler, longer-lived chips. The honest catch: every major custom firmware charges a dev fee when you mine on SoloFury, so the net improvement in cost per PH/s/day is real but modest. This guide explains why undervolting works, compares the firmware with current 2026 fees, gives safe profiles, and does the real math.
Key Takeaways
- Undervolting cuts power ~10–15% at the same hashrate, improving J/TH and saving about $250–300/year per S21+ at $0.085/kWh.
- Every major custom firmware charges a ~2–2.8% dev fee on SoloFury. None is free here (the fee is only waived on the maker’s own pool); stock firmware is fee-free but cannot undervolt.
- The net cost-per-PH/s/day improvement is modest (~5%) after the dev fee — worthwhile out of warranty, not the “free extra miner” some guides claim.
- AsicBoost is already active and is not a separate stacking gain — efficiency comes from undervolting and per-board tuning.
- Start conservative, watch the hardware-error rate, and tune per board — silicon quality varies between the three hashboards.
Why does undervolting work?
ASIC chips ship with a voltage safety margin: the stock voltage is set high enough that even the weakest chip from the silicon lottery runs reliably at the rated hashrate. Most chips are better than worst-case and can hit the same hashrate at lower voltage.
That matters because dynamic power scales with the square of voltage (roughly P ∝ V²·f), while hashrate scales linearly with frequency. So lowering voltage cuts power faster than it costs hashrate — the result is lower power at similar hashrate and better J/TH. The limits: some chips produce hardware errors when pushed too far, and going too low causes instability. Stock Bitmain firmware does not expose voltage controls, so undervolting requires custom firmware. In practice, S21+ silicon is generally good for a 5–15% efficiency improvement depending on chip quality and cooling.
Which custom firmware should you use?
Three custom firmwares dominate Antminer tuning in 2026 — and the most important 2026 fact is that all three charge a dev fee when you mine on a pool other than their own affiliated pool, which includes SoloFury. The “free” options you may read about only apply when mining on the firmware maker’s pool.
| Firmware | Dev fee on SoloFury | Notable strengths |
|---|---|---|
| Braiins OS+ | ~2–2.5% (0% only on Braiins Pool) | Power-target autotuning, partial open-source, native Stratum V2 |
| Vnish | ~2–2.8% (no free tier, any pool) | Widest model support, aggressive manual tuning |
| LuxOS | ~2.8% (0% only on Luxor Pool) | 120V PSU bypass, sub-5-second curtailment, SOC 2 certified |
| Stock Bitmain | none | No undervolting — fee-free but inefficient |
For a solo miner on SoloFury, a hashrate-based dev fee means roughly 2–2.8% of your hashing time works for the firmware maker — so about that share of your expected reward is redirected, on top of SoloFury’s 1%. Practically: Braiins OS+ is the easiest set-and-forget choice (set a power target, let it autotune), Vnish gives the most manual control and the widest hardware support, and LuxOS is the pick if you need 120V household power or fast curtailment for demand-response. The efficiency gain usually outweighs the dev fee at most electricity rates, but it is a real cost — not zero.
Pre-flight checks before flashing
- Back up the factory firmware from a trusted source so you can restore it.
- Record the serial number in case you need warranty later.
- Confirm cooling headroom — ambient under 30°C, exhaust well-routed.
- Check all chips report online — the S21+ has three hashboards; all should be healthy before flashing.
- Run a baseline benchmark — record stock hashrate, wall power (a plug meter), and J/TH to compare against after tuning.
- Use wired Ethernet, never Wi-Fi — a flash interrupted by a dropped connection can brick the machine.
- Check for a firmware lock — units shipped after March 2024 may need an unlocking step before a third-party firmware will install.
How do you flash custom firmware?
The process is similar across all three firmwares. In outline: download the firmware image for your exact S21+ model from the official source and verify its checksum; open the miner’s web interface at its IP and log in; go to System → Firmware Upgrade; upload the image and start the flash, without interrupting power or network (it takes several minutes and may reboot two or three times); then log into the new interface and apply any license or pool settings. If a flash fails, each firmware documents a recovery procedure (usually an SD-card reflash or a serial-console connection).
What are safe voltage profiles for the S21+?
After flashing, the S21+ runs at stock voltage until you tune it. Custom firmware exposes chip voltage, frequency, and per-board adjustment. Start conservative, test, and never jump to aggressive numbers without monitoring.
Profile A — efficient at stock hashrate (recommended start)
| Parameter | Stock | Profile A |
|---|---|---|
| Chip voltage | ~14V | 13.0–13.5V |
| Frequency | stock | -2 to -3% |
| Hashrate | 235 TH/s | 232–236 TH/s |
| Power | 3,877 W | 3,400–3,550 W |
| Efficiency | 16.5 J/TH | 14.5–15.0 J/TH |
The safest starting point: same hashrate, about 10% less power. Run it for 48–72 hours to confirm stability before pushing further.
Profile B — more hashrate at higher power
| Parameter | Stock | Profile B |
|---|---|---|
| Chip voltage | ~14V | 14.0V (stock) |
| Frequency | stock | +5 to +8% |
| Hashrate | 235 TH/s | 245–255 TH/s |
| Power | 3,877 W | 4,100–4,300 W |
| Efficiency | 16.5 J/TH | 16.5–17.0 J/TH |
This trades more power for more hashrate via per-chip autotuning. It is not the goal for most solo miners — efficiency is — but it helps if your electricity is very cheap and you want maximum hashrate per unit.
Profile C — aggressive efficiency (only after weeks of stability)
| Parameter | Stock | Profile C |
|---|---|---|
| Chip voltage | ~14V | 12.5–13.0V |
| Frequency | stock | -5 to -7% |
| Hashrate | 235 TH/s | 220–230 TH/s |
| Power | 3,877 W | 3,000–3,200 W |
| Efficiency | 16.5 J/TH | 13.5–14.5 J/TH |
Only attempt this after at least two weeks of stable Profile A operation. The hardware-error rate often climbs on aggressive undervolting, and the savings can be offset by rejected shares.
How do you tune per board?
Because the three hashboards are not identical — silicon lottery means one may undervolt further than another — custom firmware lets you set voltage per board. Start all three at Profile A, watch the per-board error rate for 48 hours, push the cleanest boards (error rate under 0.3%) a little further, and ease boards above 1% back toward stock. Iterating to each board’s sweet spot typically gains 1–2 J/TH over uniform tuning.
What about AsicBoost?
AsicBoost is already active — it is not an extra efficiency you stack on top of undervolting. Overt version-rolling is automatic on the S21+ and already included in its 16.5 J/TH rating, and SoloFury negotiates it on all five coins. Custom firmware does not extract “more” AsicBoost; the efficiency gains here come from undervolting and per-board tuning. If you want to confirm version-rolling is negotiated, see the AsicBoost guide — but there is nothing to add to your undervolt profile for it.
Monitoring after tuning
After applying any profile, monitor for 48–72 hours before calling it stable.
| Metric | Where | Target |
|---|---|---|
| Hashrate (firmware) | ASIC dashboard | Within ±3% of the profile target |
| Hashrate (SoloFury) | miner page | Should match the ASIC reading |
| Hardware error rate | ASIC dashboard, per board | Under 1% per board |
| Reject rate (SoloFury) | worker stats | Under 0.5% baseline, under 2% acceptable |
| Stale rate | worker stats | Under 1% |
| Chip temperature | ASIC dashboard | 60–80°C, never above 85°C |
| Fan speed | ASIC dashboard | Under 90% — if pegged, cooling is inadequate |
If anything is off-target, the Reading Your Worker Stats guide and the Miner Health Check guide cover diagnosis.
What is the real dollar improvement?
Translate the efficiency gain into money — honestly, including the dev fee. Setup: one S21+ at $0.085/kWh, 720 hours/month, with depreciation of about $78/month (a $2,800 machine over 36 months) included so this lines up with the Owned vs Rental Cost Comparison.
Stock baseline: 235 TH/s @ 3,877 W → 2,791 kWh/month → $237/month electricity → about $44.7 per PH/s/day including depreciation.
Profile A undervolt: 234 TH/s @ 3,475 W → 2,502 kWh/month → $213/month electricity. That is $24/month, about $288/year, saved on electricity per machine — a real, guaranteed reduction in your power bill.
Net cost per PH/s/day, including the dev fee: a ~2–2.8% dev fee redirects a slice of hashrate, so your effective hashrate is roughly 228 TH/s. Monthly cost is $213 electricity plus $78 depreciation = $291, giving about $42.5 per PH/s/day — versus $44.7 stock. That is roughly a 5% improvement, not the double-digit figures some guides quote.
How do you roll back to stock firmware?
If custom firmware is not for you, download the official Bitmain image for your exact S21+ model, go to System → Firmware Upgrade in the custom firmware, upload the Bitmain image, and let it flash and reboot. The miner returns to factory defaults, so you will need to re-enter your SoloFury pool URLs (and re-apply TLS endpoints if you use them — see the TLS Stratum Mining guide). You lose any tuning profiles, so save them if you might return later.
Next Steps
- After tuning, verify clean shares with the Reading Your Worker Stats guide.
- For chip-level issues, run the Miner Health Check guide.
- Confirm version-rolling with the AsicBoost guide.
- Recalculate your economics with your new efficiency in the Owned vs Rental Cost Comparison.
- For first-time hardware setup, see the Antminer S21+ Setup Guide.
Frequently Asked Questions
Is undervolting worth it for solo mining?
For a machine out of warranty running 24/7, usually yes: you save about $250–300/year in electricity per S21+ at $0.085/kWh and the chips run cooler. But every custom firmware charges a ~2–2.8% dev fee on SoloFury, so the net cost-per-PH/s/day improvement is modest (around 5%). Undervolt for the power bill and longevity, not a big odds boost.
Which firmware is free on SoloFury?
None of the three big options. Braiins OS+, Vnish, and LuxOS all charge roughly 2–2.8% when you mine on a pool other than their own (which SoloFury is). Only stock Bitmain firmware is fee-free, and it cannot undervolt.
Does undervolting reduce my hashrate?
Not at Profile A — the goal there is the same hashrate at lower power. Aggressive profiles (Profile C) trade a little hashrate for much better efficiency. The dev fee, separately, redirects about 2–2.8% of your hashrate to the firmware maker.
Will custom firmware give me extra AsicBoost?
No. Version-rolling AsicBoost is already active and included in your rated efficiency. Custom firmware improves efficiency through undervolting and per-board tuning, not by adding AsicBoost.
How aggressive an undervolt can the S21+ take?
Typically a 5–15% efficiency improvement depending on chip quality and cooling. Start at Profile A (about 10% less power at stock hashrate), prove stability for a few days, and only move toward Profile C after weeks of clean operation while watching the hardware-error rate.
Does flashing custom firmware void my warranty?
Yes. It voids the Bitmain warranty (commonly 6–12 months). Most miners wait until warranty expires. Units made after March 2024 may also need an unlocking step before a third-party firmware will install.
What's the single most important thing to watch while tuning?
The per-board hardware-error rate. Under 1% is fine; above 2% means you are losing hashrate to invalid shares and should raise voltage or frequency. It is the earliest signal that an undervolt is too aggressive.
Can I undervolt without custom firmware?
No. Stock Bitmain firmware does not expose the voltage and per-board controls undervolting needs, so a custom firmware (with its dev fee) is required.