Data centre services hold promise for reducing societal carbon emissions, but an imperfect and evolving portfolio of performance metrics obscures which data centre characteristics correspond to low-carbon operations. Meanwhile, policymakers face a pressing question: can we identify and promote tangible characteristics that reliably represent low-carbon data centres today while the world awaits better metrics? Fortunately, data centre energy models can provide actionable guidance. Here, we present results that identify such characteristics and illuminate the factors that govern a data centre's actual carbon performance. These results can help public and private sector policymakers accelerate the transition to a low-carbon Internet by aligning data centre incentives with factors that truly matter.