Minimum Energy Performance Calculator – Performance Outputs tab

The rated capacity of a PV arrary for example, is the total peak value of the rated panel production at optimal conditions. So yes maximum peak value in kW.

"rated" capacity is the capacity measured under a set of laboritory conditions, i.e. at a specific operating point usually specified by some standardized testing proceedure. It's often even more complicated in that the testing proceedure involves reporting the achieved outputs at a range of different operating points and then uses a weighting formula to deturmine the "rated" capacity. So it ties very closely to which standard is put to ground to "rate" the equipment. The idea is that with a standardised rating test, different equipments can be compared to one another for the consumer to know how well they compete.   Your data sheet will usually always show one capacity value that ties in with a set of opperational points/conditions which will also be listed. So for those conditions, the capacity will be valid. When designing, the design capacity is usually the rated capacity, which means the theoretical "worst-case" you would like the equipment to cover. Because it's not practical to always cover every spike, you could consider for example covering the hourly averaged daily "peak". I would try, however, sticking to the datasheet's rated values when entering the date into the table, because they may make more sense for that type of equipment.