Lousen, most of the facilities we maintain or assist in maintaining set a time in the wee hours of the morning.
We do help one hospital that does their tests on Friday afternoon. In the test function of the transfer switch, there should only be a quick blip in the transfer from normal to emergency power and vice versa. This will not be enough to effect normal operation of the facility.
Computers could see some problems if they are not on UPS's
The 45kva load would not meet the 30% as described here http://www.mgi-hcn.com/archive/search/testing/Differences022701.htm
However you might still meet operating temperature. You would need a temperature sensor of some type to be able to record your test. The manufacturer should be able to help you with this.
Hope this is some help.