Answer: In a perfectly isolated system, the total energy is the same.
Explanation: The conservation laws for mass and energy hold in a system that is isolated from its surroundings. The enegy released by the dissolution of solid NaOH in water is absorbed by the water. So the total energy of the syetm remains the same. It is simply converted from lattice energy of the NaOH into heat energy as measured by the temperature of the water. As long as it is operfectly isolated, the total energy remains the same.