Most Bitcoin heaters are marketed the wrong way round. The pitch is usually "heat your home and earn Bitcoin," which invites you to compare the mining income against zero and conclude it is free money. It is not, and 21energy — to their credit — are one of the few manufacturers who mostly avoid that framing.
The correct comparison is not mining income versus nothing. It is a Bitcoin heater versus the electric heater you would otherwise have bought. Framed that way the numbers are modest, real, and easy to check. This page does the arithmetic on the current Ofen range at European electricity prices, using our own earnings model and 21energy's published specifications. Figures checked 9 September 2026.
If you want the general theory of how mining hardware works as a heater — coefficient of performance, where to put it, what it cannot replace — that is in our Bitcoin heater guide. This page is specifically about 21energy's machines.
Who 21energy are
21energy is an Austrian company that builds Bitcoin-mining space heaters and sells them across the EU. Two things distinguish them from the general run of "heater miner" brands:
- They ship from an EU warehouse in Austria, with free shipping over €200 and typical delivery of three to five working days. For a European buyer that removes the two biggest headaches of buying mining hardware — customs and a six-week wait from Shenzhen.
- They sell a 30-day money-back option. Almost nobody in mining hardware does this. It matters more than it sounds, because a heater is a product you can only really evaluate in your actual room.
They also resell conventional miners from Bitmain, MicroBT and Canaan alongside their own products, and stock solo-lottery devices like the Bitaxe and NerdAxe. So the company is part manufacturer, part European reseller.
The Ofen range
| Model | Hashrate | Power | Efficiency | Noise | Our price |
|---|---|---|---|---|---|
| Ofen 2 | 40 Th/s | 1,000 W | 25 J/TH | 35 dB | $1,634 |
| Ofen 2 Pro | 60 Th/s | 1,100 W | 18.3 J/TH | ~35 dB | $1,713 |
Two observations before the money.
The Pro is the better machine by a wide margin and costs $79 more. It produces 50% more hashrate for 10% more power. In efficiency terms that is 18.3 J/TH against 25 J/TH — a 27% improvement. If you are choosing between them and the price gap is anything like this, there is no argument for the standard Ofen 2.
35 dB is the number that makes these usable indoors. A full-size Antminer S21 XP runs at 75 dB — a thousand times the sound intensity and roughly eight times as loud to the ear. Germany's Nachtruhe limit is 35 dB(A) measured in your neighbour's room between 22:00 and 06:00, so the Ofen sits right at the line rather than absurdly over it. Our quiet mining guide covers the acoustics properly, and the apartment guide covers the legal side.
Both units are also unremarkable electrically: at 1,000–1,100 W they draw 4.3–4.8 A and run from any ordinary Schuko or BS 1363 socket with room to spare.
The arithmetic, done honestly
At the current Bitcoin hashprice of roughly $0.052 per Th/s per day:
| Model | Mining revenue/day | Electricity used/day | Break-even $/kWh |
|---|---|---|---|
| Ofen 2 | $2.08 | 24 kWh | $0.087 |
| Ofen 2 Pro | $3.12 | 26.4 kWh | $0.118 |
Now read those break-even figures against reality. Eurostat puts the EU household electricity average at €0.2896/kWh — about $0.337. Both machines are three to four times over the line. As pure mining investments, at European domestic prices, they lose money every hour they run. The Ofen 2 loses about $6.01 a day; the Pro about $5.78.
Which is exactly why the heater comparison is the only one that makes sense.
Versus a normal electric heater
A resistive electric heater converts essentially 100% of its electricity into heat. So does a Bitcoin miner. Watt for watt, in the room they occupy, they are equivalent heat sources — the miner simply does arithmetic on the way through.
So if you were going to run a 1 kW electric heater anyway, the question becomes: what does the mining income knock off the bill?
| Over one 180-day heating season | Ofen 2 | Ofen 2 Pro |
|---|---|---|
| Electricity consumed | 4,320 kWh | 4,752 kWh |
| Cost at €0.2896/kWh ($0.337) | $1,456 | $1,601 |
| Bitcoin mined | $374 | $562 |
| Net heating cost | $1,082 | $1,039 |
| Reduction vs a dumb heater | 26% | 35% |
| Seasons to repay the purchase price | 4.4 | 3.0 |
That is the real proposition: a quarter to a third off your electric heating bill, and a machine that pays for itself in three to four winters. It is a decent result for a heater. It is not an investment, and anyone presenting it as one is misleading you.
The three conditions that have to hold
The maths above only works if all three of these are true. Check them honestly before buying.
1. You were going to heat that space with electricity anyway. If your alternative is a gas boiler or a heat pump, the comparison collapses. A modern air-source heat pump delivers roughly 3 to 4 kWh of heat per kWh of electricity. A Bitcoin heater delivers 1. The mining income does not come close to closing a 300% efficiency gap — you would need the machine to earn about two-thirds of your electricity cost back, and it earns a quarter to a third. Against a heat pump, a Bitcoin heater loses.
2. You actually want the heat, all the time it is running. The revenue figures assume 24/7 operation. A heater you switch off in April is a miner you switch off in April, and it mines nothing for six months. If you run it through summer for the mining income, you are back to losing $6 a day. Seasonal operation roughly halves everything in the table above and pushes payback past six seasons.
3. You are comfortable with the price and difficulty risk. Every figure here moves with the Bitcoin price and with network difficulty, which has risen roughly fivefold since 2021 and keeps climbing. The subsidy halves again in April 2028. A machine bought today will earn less each year it runs — our guide on how long ASIC miners last covers the realistic working life.
How the Ofen compares to other home miners
21energy are not the only option, and they are not the most efficient.
| Machine | Hashrate | Power | Efficiency | Break-even $/kWh |
|---|---|---|---|---|
| 21energy Ofen 2 Pro | 60 Th/s | 1,100 W | 18.3 J/TH | $0.118 |
| 21energy Ofen 2 | 40 Th/s | 1,000 W | 25 J/TH | $0.087 |
| Canaan Avalon Q | 90 Th/s | 1,674 W | 18.6 J/TH | $0.116 |
| Fluminer T3 | 115 Th/s | 1,700 W | 14.8 J/TH | $0.146 |
On raw mining economics the Fluminer T3 and the Avalon Q both beat the Ofen. What the Ofen buys you instead is heat you can actually place in a living room — a designed enclosure, an Austrian warranty, EU shipping and a return window. Whether that premium is worth it depends on whether you are buying a heater or a miner. Be clear with yourself about which.
If it is really a miner you want, the honest answer is that domestic electricity is the problem and no home machine solves it. Our home versus hosted comparison runs both cases, and hosted power is roughly an order of magnitude cheaper than any European domestic tariff.
Practical notes before you buy
- Size it as a heater first. Domestic heating is sized at roughly 25–45 W per cubic metre. A 1 kW Ofen suits a well-insulated room of about 22–40 m³ — say 9 to 16 m² with a standard ceiling. Put it in a bigger room and it will not keep up; put it in a smaller one and you will be opening windows in January, which wastes the entire point.
- It runs constantly or it earns nothing. Unlike a thermostatic heater that cycles, a miner only mines while powered. If you need thermostatic control you will be trading heat comfort against mining income directly.
- Set up the pool properly. Small machines take a long time to reach payout thresholds — a 60 Th/s unit needs weeks to clear a 0.001 BTC minimum. Choose a low-threshold pool or enable Lightning payouts. Our setup guide lists real thresholds by pool, and the pool guide covers payout schemes.
- Check the noise where it matters. 35 dB is measured at the machine under test conditions. In a hard-floored room with the unit against a shared wall, what your neighbour hears is what counts.
So should you buy one?
Yes, if you heat a room with electricity for several months a year, you want that heat anyway, and a 26–35% reduction in that room's heating bill plus a small BTC balance appeals to you. The Ofen 2 Pro is the one to buy.
No, if you have gas or a heat pump, if you would run it for the mining income rather than the heat, or if you are hoping for a return on capital. At $0.337/kWh the mining side of this is deeply unprofitable and only the heat displacement rescues it.
Run your own electricity rate and your own heating season through the profitability model before deciding — at $0.15/kWh the picture is much better than at $0.34, and the answer genuinely flips depending on where you live.
Sources: 21energy published specifications and shop terms; Kryptex hardware database (Ofen 2: 40 Th/s, 1,000 W, 25 J/TH, 35 dB, 5–45 °C ambient), read 9 September 2026; Eurostat household electricity prices H2 2025 (dataset nrg_pc_205); Bull Miners earnings model at $0.052 per Th/s per day. Heating-season figures assume 180 days of continuous operation and are illustrative. Mining income is never guaranteed and moves with Bitcoin price and network difficulty.