Antminer S21 XP Hydro Tuning

The honest answer to “S21 XP Hydro underclock settings”: run at or below the knee. The efficiency king of the dataset holds 12.0 J/TH from its 4,620 W floor up to roughly 5,676 W, then efficiency starts paying for hashrate. The knee position is the number to know, and it is below.

Quick answer

The Antminer S21 XP Hydro tunes across a 4,620–7,588 W envelope (Normal mode: 385–562 TH/s). Best catalog efficiency is 12.0 J/TH, and the sweet spot — everything within 5% of it — spans 4,620–6,539 W. The floor is 4,620 W — 81% of the 5,676 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).

Efficiency holds near its best up to a knee in the curve, then degrades — run at or below the knee unless you need the extra hashrate.

Antminer S21 XP Hydro — Normal mode Verified (extraction)

4,620 W – 7,588 Wwatt envelope

385 TH/s – 562 TH/shashrate span

12.0 J/THbest efficiency @ 4,620 W–5,676 W

4,620 W – 6,539 Wsweet spot (≤5% off best)

Knee (mid)curve shape

13catalog 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 15,763 BTU/h (watts × 3.412).

Source: Firmware-extraction set.

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 XP 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.