AB is part of a circuit as shown that absorbs energy at a rate of 50 W. E is an emf device that has no internal resistance 

Resistance absorbs energy at the rate of 2 W

Potential difference across AB is VAB. I = 50 W

$V_{AB}=50V$

Drop across resistor is 2 V, therefore emf of E is 48 V.

As AB is absorbing energy at the rate of 50 W, 48 W is being absorbed by E. Thus, E is on charging i.e., current is entering from positive terminal of E.