A battery having 1.5 volt means that it has electrical potential energy difference of 1.5 volt right?
There is a potential difference of 1.5 V.
This means that if you moved a +1 C test charge from one terminal to the other, you would need to do 1.5 J of work on the charge to move it from the negative terminal to the positive terminal. And this would add 1.5 J to the electrical potential energy of the test charge.
But the potential difference is defined even if there is no charge there to be moved or to have potential energy. Just like the electric field is there ready to apply a force to any charge that comes along, even when there's no charge actually there to have a force applied to it.
That means this potential to do work or energy is supplied when the battery is connected to the circuit and electrons flow as they recieve the energy?
When you connect the circuit, then the battery actually does work on charges to push them through the circuit. The "potential" or possibility to do work is there even before the circuit is connected.