How does alternating current transfer energy?
Whenever there is current, electrons flow (the usual case). Electrons that move carry energy in terms of voltage. Suppose that we want to transfer energy to a resistor in a circuit,
The power (which is a good way to express energy) is
For a direct current, the math is simple. For alternating current with peak current I , the effective current becomes
Hence the Power transferred is
If you are asking about transmission of energy to households, then alternating current is needed to keep changing magnetic flux through a Gaussian surface so that electricity is continuously produced via generator and altered via transformer. Direct current is useless in power generation.