why inverter rating in KVA?
Your this question is related to drive or with power source? If it is related to drive then inverters are rated in Kw. If this is related to power source then the rating of inverter is in KVA. Generally in case of power source, the connected load equipements rating is provided in watts but their actual current varies as those equippements power factor are differents.
e.g. tube lights p.f. 0.4, CFLs p.f. 0.6 ,fans p.f. 0.8, T.V. p.f. 0.6 set etc. Hence actual total load current needs to be calculated & accordingly inverter is to be selected.If it is not calculated then inverter rating may be undersized & this may lead to overloading of inverter. e.g. 700 VA inverter at 230 Volt is supposed to supply 3 A load. (500 W max load you can connect on 700 VA & not 600 or 650 watt
Hope this helps to solve your query.