how much power does a transformer use when idle?

When idle, a transformer consumes a small amount of power known as “no-load loss” or “core loss.” This power is primarily due to the magnetization of the core and is typically expressed in watts. The no-load loss varies depending on the transformer’s design, size, and materials, but it generally ranges from 1% to 5% of the transformer’s rated capacity. This idle power consumption is crucial for understanding overall energy efficiency.