Proof of Capacity (PoC): The Storage-Based Mining Algorithm
Proof of Capacity (PoC) is a blockchain consensus algorithm in which miners pre-compute and store cryptographic solutions (called plots) on hard-drive space rather than solving hashes in real time. The more storage a miner dedicates, the more pre-calculated solutions they hold and the higher their probability of producing the next block. Because plotting happens once and ongoing mining mostly reads from disk, PoC consumes a fraction of the electricity that Proof of Work demands, while keeping a lower hardware barrier than ASIC mining. It is sometimes called Proof of Space, and a refined version, Proof of Space-Time, underpins networks such as Chia.
What Is Proof of Capacity?
Proof of Capacity (PoC) is a blockchain consensus mechanism in which miners commit unused hard-drive space, not processor power, to secure the network. Instead of racing to solve a hash in real time, a PoC miner pre-computes a large set of cryptographic solutions ahead of time and writes them to disk in a one-off step called "plotting." When a new block is needed, the protocol picks the miner whose stored solution is closest to the target. More allocated storage means more stored answers, which means a higher chance of winning the block reward. Because the heavy computation happens once and ordinary mining is mostly a disk read, PoC is dramatically more energy efficient than Proof of Work.
Why a Third Consensus Model Was Needed
The two best-known ways to secure a blockchain each carry a structural cost. Proof of Work, used by Bitcoin, asks miners to brute-force a SHA-256 hash by iterating a nonce until the output meets the difficulty target. As difficulty rose, mining migrated to specialised ASIC hardware that burns large amounts of electricity, and every miner who fails to find the nonce in time has simply wasted that energy.
Proof of Stake, the model Ethereum adopted, cuts that energy bill by selecting block producers based on how many coins they lock up. The trade-off is a different one: capital concentration. Participants holding the most coins gain the most influence, which can push validation toward a small set of wealthy nodes. PoC was conceived to sit between the two — far cheaper to run than PoW, and cheaper to enter than buying a large coin stake.
How Proof of Capacity Works
PoC has two distinct phases: a one-time plotting phase and a continuous mining phase that reads from those plots.
Step 1 — Plotting the Hard Drive
During plotting, the miner repeatedly hashes their unique account ID through a memory-hard function (the early Burstcoin implementation used the Shabal algorithm, which is deliberately costlier to compute than SHA-256). The output is a set of nonces written to disk. In the classic Burst design, each nonce contains 8,192 hashes, organised into 4,096 pairs called scoops, so the protocol can address any single scoop by number from 0 to 4,095.
Step 2 — Mining From Stored Plots
When a new block opens, the network derives a scoop number from the previous block. Every miner reads that one scoop from each of their stored nonces and uses it to calculate a value called a deadline — the number of seconds the miner would have to wait before being allowed to forge the next block. Each miner keeps their single shortest deadline. The miner with the lowest deadline across the whole network wins, creates the block, and collects the reward. Because Burst targeted roughly one block every four minutes (versus Bitcoin's ten), reading pre-stored answers is essential — solving them live would be far too slow.
A Worked Example
Imagine three miners competing for the next block:
- Miner A has plotted 1 TB of storage.
- Miner B has plotted 4 TB.
- Miner C has plotted 8 TB.
Each terabyte holds a roughly proportional number of pre-computed nonces, so each miner's chance of holding the shortest deadline scales with their share of total storage. Total dedicated space is 1 + 4 + 8 = 13 TB.
| Miner | Plotted storage | Approx. win probability |
|---|---|---|
| A | 1 TB | 1 / 13 ≈ 7.7% |
| B | 4 TB | 4 / 13 ≈ 30.8% |
| C | 8 TB | 8 / 13 ≈ 61.5% |
The takeaway: in PoC, your expected block share is governed by how much capacity you contribute relative to the whole network — the storage equivalent of hashrate in PoW.
PoC vs PoW vs PoS at a Glance
| Property | Proof of Capacity | Proof of Work | Proof of Stake |
|---|---|---|---|
| Scarce resource | Disk storage | Compute / electricity | Locked capital |
| Energy use | Low | Very high | Low |
| Main hardware | Consumer hard drives | ASICs / GPUs | Standard server |
| Entry barrier | Low (cheap disks) | High (ASIC cost) | High (coin stake) |
| Reusable hardware | Yes — wipe and reuse | No (single-purpose ASICs) | Yes |
| Centralisation risk | Storage-rich actors | Hardware-rich actors | Capital-rich actors |
Benefits of Proof of Capacity
- Energy efficiency. After the one-off plotting step, ongoing mining is mostly disk reads, so power draw is a fraction of PoW.
- Low hardware barrier. Commodity hard drives are cheap and widely available, lowering the cost of entry compared with ASIC mining.
- No specialist arms race. Newer drives offer little advantage beyond capacity, so miners do not have to chase the latest hardware to stay competitive.
- Reusable equipment. When you stop mining, you simply delete the plot files and the drive is a normal storage device again — unlike a single-purpose ASIC.
Risks and Pitfalls
PoC is not a free lunch. Several trade-offs deserve attention:
- Storage centralisation. Low entry cost cuts both ways. Nothing stops a well-funded actor from buying enormous arrays of drives and dominating the deadline lottery, recreating the concentration problem PoC set out to solve.
- Wasted, single-use data. Plot files have no value outside mining. Large volumes of disk space sit occupied by data that is meaningless for any other purpose.
- Stealth abuse. PoW malware often reveals itself by slowing a CPU. PoC plots mostly idle until a block opens, so unauthorised use of spare drive space — a storage-flavoured cryptojacking — can be harder to detect.
- Drive wear. Heavy plotting and constant reads can shorten the lifespan of consumer SSDs in particular.
Where PoC Stands Today
Proof of Capacity began with Burstcoin in 2014 as a niche, single-chain experiment. The idea has since matured into the broader Proof of Space and Proof of Space-Time families. The most prominent modern example is the Chia network, which added a time element to discourage "grinding" and brought storage-based consensus to a much larger audience. The core promise — securing a chain with disk capacity instead of electricity — remains an active line of research as the industry hunts for greener alternatives to Proof of Work.
For deeper context, see our explainers on [how Bitcoin mining works](https://en.coinotag.com/guide/what-is-bitcoin-mining), [Proof of Stake mining](https://en.coinotag.com/guide/comprehensive-guide-pos-mining), and [how mining pools operate](https://en.coinotag.com/guide/how-mining-pools-work).
COINOTAG Perspective
PoC is best understood not as a winner that displaced PoW or PoS, but as a proof of concept that decoupled blockchain security from raw energy consumption. Its lasting contribution is the insight that any scarce, verifiable resource — storage, in this case — can anchor a consensus game. Whether storage-based chains scale to compete with the largest networks is still unproven, but the design pushed the entire sector to take energy and hardware accessibility seriously, and that influence outlives any single coin.