It does not matter that it is a center speaker, however you can only listen to one channel of sound with only one speaker. Connecting two channels to one speaker will cause issues. Ideally, you want at least 2 speakers at minimum, or you need to listen in Mono if available (as mentioned above).
This speaker is a centered MTM design, so it can be used horizontal or vertical.
Passive speakers do not have a "wattage". You need an amplifier for the speaker to produce sound.