Normally, the voltage would be specified under load so without one you would see a slightly higher voltage. If you need a 20KW load, the trick I have used in the past is to wire up lots of domestic heating elements in series/parallel arrangement to make up the voltage and power you need. Use the elements power rating rather than measure it's resistance because element power is the rating when hot and a cold resistance will be lower than Ohms law would suggest.
For example, if you have 220V 1KW elements, wire 3 strings of 8 elements in parallel to make a 1.76KV 24KW load and work from there. The slightly higher voltage requirement will result in a slightly lower powered load. You may need to add or remove elements to get an exact load if necessary. It will get VERY VERY-VERY hot - take great care.
Brian.