This paper compares a clean energy standard (CES) and a carbon tax (CT), using theory and quantitative experiments. A two-stage duopolistic competition in the electricity sector between a polluting plant and its non-polluting rival anchors the model underlying these experiments. The CT induces both plants to contribute to clean electricity, whereas the CES only incentivizes the non-polluting plant. Ultimately, what matters for the ranking of these instruments is the size of the pre-existing competitive gap between the two rival plants. When this gap is sufficiently small, the CES becomes the more cost-effective instrument, irrespective of the pre-specified emissions reduction target.