Peercoin

Peercoin, also known as PPCoin or PPC, is a peer-to-peer cryptocurrency utilizing both the proof-of-stake and proof-of-work systems.

Unlike Bitcoin, Namecoin, and Litecoin, Peercoin does not have a hard limit on the number of possible coins, but is designed to eventually attain an annual inflation rate of 1%. There is a deflationary aspect to Peercoin as the transaction fee of 0.01 PPC/kb paid to the network is destroyed. This feature, along with increased energy efficiency, aim to allow for greater long-term scalability.

Proof-of-stake
Peercoin is the second notable cryptocurrency address the ASICs. It uses proof-of-stake on the client-side, integrating a miner into a full node wallet in the form mining existing coins, known as minting. The system was designed to address vulnerabilities that could occur in a pure proof-of-work system.

Proof-of-work
However, the network as a whole relies on the SHA-256 proof-of-work algorithm. For each 16 times increase in the network, the proof-of-work block reward is halved. In July 2016 the Bitcoin mining reward halved causing a notable minority of miners to switch to mining Peercoin for better profitability. Researcher Adam Hayes explained that the Peercoin network hashrate surged from roughly 500 TH/s to 6,500 TH/s following the halving.

Peercoin's proof-of-stake system was developed to address the high energy consumption of bitcoin. For example, as of April 2013 the generation of bitcoins was using approximately $150,000 USD per day in power consumption costs. The proof-of-stake method of generating coins requires very minimal energy consumption; it only requires the energy to run the client software on a computer, as opposed to running resource-intensive cryptographic hashing functions. During its early stages of growth, most Peercoins will be generated by proof-of-work like bitcoin, however over time proof-of-work will be phased out as proof-of-work difficulty increases and block rewards decrease. As proof-of-stake becomes the primary source of coin generation, energy consumption (relative to market cap) decreases over time. As of January 2014, roughly 90% of new coins being generated are still from proof-of-work and the energy consumption of Peercoin uses roughly 30% of the energy consumption of bitcoin (scaling for market cap - in terms of value secured per GH/s).