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Discussed this with some friends and the view we came to is that your momentum relative to both the portal and your surroundings is preserved (which explains how you could portal to the moon and not get liquefied by the difference in rotational momentum between earth and the moon). The portal speeds you up or slows you down depending on local conditions on the other side to preserve your relative momentum. This would, logically, indicate that energy is created or destroyed depending on the difference, which (to me) means that 'portals' technically exist outside our universe as a concept and are therefore not subject to conservation of energy.
not subject to conservation of energy
That's already the case simply because you can go through a portal from the bottom of a cliff to the top of a cliff without doing work; this obviously lets you build a perpetual motion machine.
(Do gravitational fields pass through portals? If you try to go through a portal from earth to deep space, will Earth's gravity pull you back?)