If you need a figure, then you may have to look for a specific example of UPS. Different designs will have differing efficiencies. The nature of the load will affect this as well.
since UPS have charged batteries, is NOT having consumption it'self. the only consumption is the load. unless you consider losses which are very little.
so the amount of power in KWH is 50% of 100KVA as you said, this is 50 kva multiply by time. this is the amount of power in KWH.
Every UPS has losses. It might be 50% or only 15%.
So a 100kVA UPS at half power might waste 25kW or waste "only" 7.5kW.
Enough heat to warm hundreds of homes in winter.