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Antminer S21+ Underclock

The honest answer to “S21+ underclock settings”: set a watt target of 2,440 W — the Low Power floor, which also posts the set’s best efficiency (14.1 J/TH), with flat curves across all three modes. One honesty note before the numbers: this set is status Supported (catalog data whose provenance is untraced to the firmware extraction), and the page treats it as planning bands, not verified points.

Quick answer

The Antminer S21+ tunes across a 2,640–4,690 W envelope (Normal mode: 176–313 TH/s). Best catalog efficiency is 14.9 J/TH, and the sweet spot — everything within 5% of it — spans 2,640–4,690 W. The floor is 2,440 W — 68% of the 3,564 W factory rating. There are no frequency/voltage settings to copy: you set a watt target and the firmware autotuner derives the rest for your unit. Status: Supported.

The efficiency curve is essentially flat across the envelope, so the watt target is a heat, noise and circuit decision — underclocking costs almost nothing per terahash.

Antminer S21+ — Normal mode Supported

2,640 W – 4,690 Wwatt envelope
176 TH/s – 313 TH/shashrate span
14.9 J/THbest efficiency @ 2,890 W–3,970 W
2,640 W – 4,690 Wsweet spot (≤5% off best)
Flatcurve shape
9catalog points

Floor exceeds both 120 V budgets (1,440 / 1,920 W) — this is 200–240 V territory. Budgets are the NEC 80% continuous-load rule; circuit fit only — stock Bitmain APW-series PSUs require 200–240 V input, so any 120 V build needs a PSU change regardless of watt target. At the floor this mode dissipates about 9,008 BTU/h (watts × 3.412).

Antminer S21+ — Performance mode Supported

3,975 W – 7,347 Wwatt envelope
265 TH/s – 448 TH/shashrate span
15.0 J/THbest efficiency @ 3,975 W–5,940 W
3,975 W – 6,382 Wsweet spot (≤5% off best)
Knee (high)curve shape
9catalog points

Floor exceeds both 120 V budgets (1,440 / 1,920 W) — this is 200–240 V territory. Budgets are the NEC 80% continuous-load rule; circuit fit only — stock Bitmain APW-series PSUs require 200–240 V input, so any 120 V build needs a PSU change regardless of watt target. At the floor this mode dissipates about 13,563 BTU/h (watts × 3.412).

Antminer S21+ — Low Power mode Supported

2,440 W – 3,650 Wwatt envelope
170 TH/s – 248 TH/shashrate span
14.1 J/THbest efficiency @ 2,910 W
2,440 W – 3,650 Wsweet spot (≤5% off best)
Flatcurve shape
9catalog points

Floor exceeds both 120 V budgets (1,440 / 1,920 W) — this is 200–240 V territory. Budgets are the NEC 80% continuous-load rule; circuit fit only — stock Bitmain APW-series PSUs require 200–240 V input, so any 120 V build needs a PSU change regardless of watt target. At the floor this mode dissipates about 8,325 BTU/h (watts × 3.412).

Source: Theme catalog set, three modes; provenance untraced to the extraction (Supported, not Verified). Treat as planning bands.

How to set a power target on this model

Aftermarket firmware exposes underclocking as a power target (sometimes "power limit" or a preset picker), not as frequency/voltage fields. Pick a watt figure inside the envelope above; the autotuner benchmarks your boards and settles onto it over a tuning period. Braiins OS+, VNish and LuxOS each expose this in their own UI, and DCENT_OS uses watt-target autotuning on its supported models; the undervolting guide walks through the firmware side step by step, and the firmware comparison covers which firmware supports which model. Verify the result at the wall with a meter — the target is an input, the wall draw is the fact.

Frequency and voltage are calculated at runtime by the firmware autotuner — never fixed presets; per-unit silicon quality, ambient temperature and cooling shift real results.

These envelopes are derived aggregates (bounds, bands, counts) — the row-level operating points, the interactive picker and the CSV/JSON of the source data live in the ASIC Power Profiles Database, the single source dataset for every number on this page.

Antminer S21+ full specifications → · Row-level operating points (source dataset) → · Overclock headroom by cooling → · All tuning envelopes →

Related products, repair, and setup paths

Last reviewed August 13, 2026.