The smart electricity grid introduces new opportunities for fine-grained consumption monitoring. Such functionality, however, requires the constant collection of electricity data that can be used to undermine consumer privacy. In this work, we address this problem by proposing two decentralized protocols to securely aggregate the measurements of n smart meters. The first protocol is a very lightweight one, it uses only symmetric cryptographic primitives and provides security against honest-but-curious adversaries. The second protocol is public-key based and considers the malicious adversarial model; malicious entities not only try to learn the private measurements of smart meters but also disrupt protocol execution. Both protocols do not rely on centralized entities or trusted third parties to operate. Additionally, we show that they are highly scalable owning to the fact that every smart meter has to interact with only a few others, thus requiring only O(1) work and memory overhead. Finally, we implement a prototype based on our proposals and we evaluate its performance in realistic deployment settings.