Let I be the current in the circuit at the steady state. There will be no current in capacitor arm.
Applying Kirchhoff’s second law in the closed loop $PQSTP$, we get
$2.5 - 10I - 20I - 1.0 = 0$
$I = \frac{2.5 - 1.0}{(10 +20)} = 0.05A$
$\therefore $ $- 1.0 - 0.05 \times 20 = - 0.5 + V_{AB}$
or $V_{AB} = - 1.5 \, V$