Servos only take power when they moving. Small servo have less power so take less mA, typically less than 500mA. Do ensure the servo linkage and the control surface itself are free running and mechanically efficient so
all the servo power available is used to move the surface against the airflow. Done well you can safely use a small low power servo.
You might get away with as little as 22W/lb. It will depend on how aerodynamically efficient the plane is, particularly its wings, but that power level will only maintain height. Climbing requires virtually double the 'maintain height' power and even then that only allow it to climb as fast as it sinks with the power off. For an efficient glider that is quite slow.
Good luck hunting thermals.
It is not that easy even for pilot in a full size glider when he has instruments, much harder when piloting remotely from the ground.
I've done both.