Quantization is critical for realizing efficient inference of LLMs. Traditional quantization methods are hardware-agnostic, limited to bit-width constraints, and lacking circuit-level insights, such as timing and energy characteristics of Multiply-Accumulate (MAC) units. We introduce HALO, a versatile framework that adapts to various hardware through a Hardware-Aware Post-Training Quantization (PTQ) approach. By leveraging MAC unit properties, HALO minimizes critical-path delays and enables dynamic frequency scaling. Deployed on LLM accelerators like TPUs and GPUs, HALO achieves on average 270% performance gains and 51% energy savings, all with minimal accuracy drop.