All transformer windings and transformer cores produce copper and magnetic losses that appear as heat:

VA_in = VA_out + W_loss
where VA = volts X amperes

W_loss = W_cu + W_fe

W_cu = watts dissipated through copper
losses in the windings

W_fe = watts dissipated through magnetic
losses in the core

The toroidal transformer typically requires only 10% of the magnetizing current required by laminated transformers. Higher flux densities are permitted because the direction of the magnetic flux is the same as the steel core grain. High flux densities allow fewer turns of copper wire, reducing the DCR of the winding. The graph shows the efficiency that can be expected in terms of VA power ratio.