The available generation capacity can be divided into three sections， i.e. static inadequacy， dynamic risk adequacy and dynamic adequacy. Margins of static adequacy and dynamic adequacy are defined respectively， the effects of static and dynamic disturbances could be quantitatively analyzed. Risks of generation capacity are dealt with in a 3-D space consisting of static adequacy margin， dynamic adequacy margin and cost. The risk-optimized uninterruptible load can be estimated by treating interruptible loads as one kind of emergency generation reserve. Simulations show the validity of the presented method.