Proof of Work is a consensus mechanism where computers compete to solve a difficult mathematical puzzle, and the winner earns the right to add the next block of transactions. The puzzle serves no purpose beyond being expensive to solve, and that expense is the security.
Since January 2009, Bitcoin has used it without its ledger ever being successfully rewritten. The cost of that record is real: the Cambridge Bitcoin Electricity Consumption Index estimated the network's power demand at roughly 20 gigawatts in early 2025, equating to around 175 terawatt-hours annualised.
The problem it solves
Without a central authority deciding, a distributed network still needs a way to agree on which transactions are valid.
Letting participants vote fails immediately. Anyone can create unlimited identities. Creating a million fake nodes costs nothing, so a vote by identity is a vote by whoever generates the most identities. This is called a Sybil attack.
Proof of Work ties influence to something that cannot be faked. Votes are weighted by computation. Computation costs electricity and hardware. Producing a million fake identities is trivial; producing a million times the computing power is not.
How mining actually works
Complicated as the puzzle sounds, it is conceptually simple.
Miners take the block they want to add, combine it with a changing number called a nonce, and run it through a hashing function. A hash is a one-way fingerprint: easy to compute from the input, effectively impossible to reverse.
Against a target the network sets, the resulting hash must fall below it. There is no method for calculating the right nonce, so miners guess, billions of times per second, until one produces a qualifying hash.
Whoever finds it broadcasts the block to the rest of the blockchain network. Every other node checks it in a fraction of a second. That asymmetry is the elegant part. Solving takes enormous work, verifying takes almost none.
Newly created coins plus the transaction fees in the block go to the winner. On Bitcoin that reward is 3.125 BTC following the April 2024 halving, and it halves again around April 2028.
Difficulty adjustment
Bitcoin targets one block roughly every ten minutes, regardless of how much computing power is pointed at it.
Every 2,016 blocks, about two weeks, the network measures how quickly those blocks arrived and adjusts the target. More miners joining makes the puzzle harder. Miners leaving makes it easier.
Thanks to that mechanism, the block interval has stayed near ten minutes since 2009 while the network's computing power has grown by orders of magnitude. It is also why energy consumption tracks the value of the reward rather than the number of transactions: miners spend up to the point where mining stops being profitable.
The 51% attack
Here is the theoretical vulnerability, alongside its practical limits.
Controlling more than half the network's computing power would let an attacker exclude transactions and reverse their own recent ones, spending coins and then rewriting history to un-spend them.
What they could not do is steal coins from other people's addresses, since that requires private keys rather than hashing power, or create coins outside the issuance schedule.
For Bitcoin, the hardware and electricity required make this economically irrational. Succeeding would likely destroy the value of the asset being attacked. Smaller Proof of Work chains are genuinely vulnerable, and several have been attacked this way, because renting enough hashing power to overwhelm a small network is affordable.
Proof of Work vs Proof of Stake
| Proof of Work | Proof of Stake | |
|---|---|---|
| Who adds blocks | Miners solving a puzzle | Validators selected partly at random, weighted by stake |
| Cost to participate | Hardware and electricity | Capital locked as stake |
| Energy use | High by design | Minimal |
| Attack cost | Out-compute the network | Acquire and risk a large share of stake |
| Penalty for cheating | Wasted electricity | Stake destroyed, called slashing |
| Used by | Bitcoin, Litecoin, Dogecoin, Monero | Ethereum since September 2022, Solana, Cardano, Avalanche |
The core argument between them is about what should back security. Proof of Work anchors it in physical resources outside the system. Proof of Stake anchors it in the value of the token itself, which critics argue is circular, and defenders argue is sufficient because attackers destroy their own stake.
The energy question, honestly
Widely varying estimates are themselves worth knowing about.
Cambridge's CBECI publishes a lower bound, an upper bound, and a best guess, because the inputs cannot be observed directly. Its early 2025 best guess was around 175 TWh annualised. A separate Cambridge survey of the mining industry, published April 2025, estimated approximately 138 TWh based on reported activity. Cambridge also revised its own methodology in 2023, cutting its 2021 estimate from 104 TWh to 89 TWh after finding older hardware assumptions had overstated consumption.
Two points of nuance that get lost in most coverage. Energy use is driven by hashrate and reward value, not by transaction count, so the widely quoted "energy per transaction" figure is not a meaningful measure. And miners chase the cheapest electricity available, which frequently means stranded or surplus generation that would otherwise go unused.
None of that makes the consumption small. It does make simple comparisons misleading.
Where mb.io fits
Proof of Work secures the asset. Getting regulated access to it is a separate question.
mb.io is a regulated crypto spot exchange backed by MultiBank Group, a financial institution founded in 2005 that serves more than 2 million clients across 100+ countries.
- Regulated by VARA in the UAE and AUSTRAC in Australia
- 10/10 security score from Hacken, an independent blockchain security auditor
- Institutional-grade MPC custody powered by Fireblocks, with segregated client funds
- A curated list of assets, so you're not sorting through thousands of tokens to find the ones worth trading
- Buy, sell, and swap in three steps, from sign-up to purchase
- 24/7 customer support, on web and on the iOS and Android apps
Open your account and start trading on mb.io.
Frequently asked questions
How does Proof of Work secure a blockchain?
By making block production expensive. Rewriting history would require redoing the computational work for every subsequent block faster than the rest of the network produces new ones, which costs more than the attack could realistically return.
Why does Proof of Work use so much electricity?
Because the expense is the security mechanism rather than a side effect. Difficulty adjusts so that miners collectively spend up to the point where mining stops being profitable, which ties consumption to the value of the reward.
What is a 51% attack?
Controlling more than half a network's computing power, which would allow an attacker to reverse their own recent transactions and exclude others. It would not let them steal from other addresses or create coins outside the schedule.
Is Proof of Stake better than Proof of Work?
They make different trade-offs. Proof of Stake uses a fraction of the energy and ties security to the token's own value. Physical resources external to the system are what Proof of Work anchors security in. Which is preferable depends on what you think security should rest on.
How much energy does Bitcoin actually use?
Estimates disagree. Cambridge's CBECI best guess was around 175 TWh annualised in early 2025, while a separate Cambridge industry survey put it near 138 TWh. The inputs cannot be observed directly, which is why credible sources publish ranges.
Which cryptocurrencies use Proof of Work?
Bitcoin, Litecoin, Dogecoin, and Monero among the larger ones. Ethereum used it until September 2022 and now runs on Proof of Stake.
What is difficulty adjustment?
The mechanism that keeps block times stable. Bitcoin recalculates the puzzle target every 2,016 blocks, roughly two weeks, making it harder when more computing power joins and easier when it leaves.
