We consider a massive MIMO system with frequency-division duplex, where a base station (BS) transfers energy to multiple users in the downlink and the users simultaneously transmit information to the BS in the uplink using the harvested energy. The objective of this letter is to optimize the round-trip energy efficiency (EE) by transmit power allocation at the BS while guaranteeing the delay outage requirements of different users with Poisson traffic arrivals. The service rate and the delay outage probability are first derived. Then, the power allocation problem is formulated as a non-convex problem, which is then transformed into a convex one. The results indicate that the proposed power allocation can significantly improve the round-trip EE of wireless energy powered massive MIMO system.