An electric power supply apparatus includes an electric energy storage, an external power supply circuit, an internal power supply circuit, and circuitry. While the transport apparatus stops, the circuitry is configured to monitor the electric energy storage, the external power supply circuit, and the internal power supply circuit. While the transport apparatus stops, the circuitry is configured to calculate a remaining time during which the electric energy storage is to supply electric power to an external electrical load based on a total power amount, an external electric power, and an internal electric power.