Why transformers are rated in kVA and not in kW?

For understanding of losses (Eddy Current and Magnetic Hysteresis) with Mathematical equations the link to this video lecture: https://youtu.be/PIsjT56c5Dk

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(Electrical Engineering & PLC Automation 85)

Why Transformer is rated in KVA and not in KW?

Course Outline of today video lecture (CLO)
Losses in a Transformer
Reason behind this electrical statement?
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Видео Why transformers are rated in kVA and not in kW? канала 𝑺𝒐𝒍𝒂𝒓 𝑺𝒚𝒔𝒕𝒆𝒎𝒔 𝑫𝒆𝒔𝒊𝒈𝒏 𝒂𝒏𝒅 𝑰𝒎𝒑𝒍𝒆𝒎𝒆𝒏𝒕𝒂𝒕𝒊𝒐𝒏 85