PJM Manual 21 requires temperature adjustments of test results for combustion turbine and combined cycle units but does not explicitly require temperature adjustments for other types of units. A significant number of generation owners/operators choose not to temperature adjust their test results. In addition, Manual 21 permits hydroelectric generation owners to perform their unit verification tests at any time during the Delivery Year, rather than requiring them to perform the test over the summer period. These two rules in Manual 21 may result in generator ratings that over-state the actual output of some PJM units under peak summer conditions. The use of inaccurate generator ratings in planning studies will result in inaccurate study results.