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Antminer S21 Hydro Tuning

The honest answer to “S21 Hydro underclock settings”: set the watt target as low as your hashrate goal tolerates. From 1,630 W to 6,480 W — a four-fold power span — Normal-mode efficiency moves only from 17.7 to 18.0 J/TH. One of the most remarkable curves in the whole catalog: the efficiency penalty for underclocking is close to zero.

Quick answer

The Antminer S21 Hydro tunes across a 2,500–6,300 W envelope (Performance mode: 148–383 TH/s). Best catalog efficiency is 16.1 J/TH, and the sweet spot — everything within 5% of it — spans 2,500–6,300 W. The floor is 1,630 W — 30% of the 5,360 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: Verified (extraction).

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 Hydro — Performance mode Verified (extraction)

2,500 W – 6,300 Wwatt envelope
148 TH/s – 383 TH/shashrate span
16.1 J/THbest efficiency @ 4,370 W–4,980 W
2,500 W – 6,300 Wsweet spot (≤5% off best)
Flatcurve shape
11catalog 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 8,530 BTU/h (watts × 3.412).

Antminer S21 Hydro — Normal mode Verified (extraction)

1,630 W – 6,480 Wwatt envelope
92 TH/s – 360 TH/shashrate span
17.7 J/THbest efficiency @ 1,630 W
1,630 W – 6,480 Wsweet spot (≤5% off best)
Flatcurve shape
16catalog points

Floor fits a 120 V 20 A continuous-load budget (1,920 W) but not 15 A. 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 5,562 BTU/h (watts × 3.412).

Source: Firmware-extraction set, two modes.

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 Hydro 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.