This study presents a comparative evaluation of five widely adopted blockchain consensus algorithms—Proof of Work (PoW), Proof of Stake (PoS), Delegated Proof of Stake (DPoS), Practical Byzantine Fault Tolerance (PBFT), and Proof of Authority (PoA)—based on energy efficiency and performance metrics. As blockchain systems scale into broader applications, key indicators such as transaction throughput (TPS), latency, security, centralization risk, and scalability become increasingly critical. Each algorithm is assessed through theoretical analysis, empirical data, and simulation-based studies found in the literature. The findings indicate that Proof of Work is the most secure, but it does not environmentally sustainable due to its energy needs. Proof of Stake and Delegated Proof of Stake are more energy-efficient and have a high throughput, but it can lead to centralization. A Practical Byzantine Fault Tolerant consensus algorithm is great in permissioned networks, but it doesn't scale well. Furthermore, those networks aren't really decentralized. Proof of Authority is the most environmentally sustainable option, but that energy efficiency comes at the cost of decentralization. This research highlights the lack of sustainable consensus mechanisms, that is, consensus protocols that are both secure and scalable and environmentally sustainable—as a gap that future research can address.

Keywords - Blockchain, Consensus Algorithms, PoW, PoS, DPoS, PBFT, PoA, Scalability, Sustainability

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