Summary: Supply function equilibrium models are used to study electricity market auctions with uncertain demand. We study the effects on the supply function equilibrium of a tax, levied by the system operator, on the observed surplus of producers. Such a tax provides an incentive for producers to alter their offers to avoid the tax. We consider these incentives under both strategic and price-taking assumptions. The model is extended to a setting in which producers are taxed on the benefits accruing to them from a transmission line expansion (a beneficiaries-pay transmission charge). In this setting, we show how this tax may lead to lower prices in equilibrium.