Auctions are a promising tool to accomplish renewable energy support (RES). However, they involve diverse risks for the auctioneer and the bidders. The major risk for the auctioneer with respect to the expansion target and the acceptance of auctions for RES in general is the risk of non-realization: awarded bidders do not realize their projects. The main reasons are bidders' uncertainties concerning their project costs. The auctioneer can reduce the non-realization risk by taking various measures. The most discussed and implemented measures are financial and physical prequalifications and penalties. An evaluation of these measures has relevant implications for an appropriate auctions design for RES. The non-realization option induces lower bids. A suitable combination of financial and physical prequalifications can reduce the non-realization risk significantly, however, involves sunk costs. Counterintuitively, first-price auction and second-price auction lead to different expected award prices and different realization probabilities. Penalties bear a high risk of inefficient auction outcomes. The appropriate parametrization and combination of the measures is challenging. Under consideration of all implications, we recommend high financial and adjusted physical prequalifications to achieve a sufficiently high realization rate while maintaining efficiency.