Here's the thing about fencing - if your horse is happy and content, with adequate forage, water and company, you can keep him in with baling twine. If your horse is hungry, thirsty, lonely or bored, you can't keep him in with a 6' brick wall.
This is another dimension of the horsekeeping on small acreage problem, as a general rule, the smaller the area, the better fence you need, because the horses have more incentive to break out.
This is just my opinion and my experience, others reasonably and legimately have different opinions and experiences, but I do not trust electric and T-posts alone to keep horses in. Electric braid or electric rope on properly sunk posts, with corners braced, sure. Electric wire on stepped in or hand sunk posts? Nope - too much can go wrong. The charger can fail, power outage, branch on the fence grounding it out, deer charging through it, wet ground causing the posts to shift and lean, smart horse listening to the charger and figuring out when the current pulses, and on and on and on. Since my horses are out, unsupervised, most of the time, I needed something I could trust to keep them out of the road. I have electric on the inside perimeter of all my fences to keep the horses from leaning on the fence, and that's great. As temporary cross fencing? Yup, it's great for that too. I just don't trust it as the ONLY barrier keeping the horses out of the road.
Just IME and IMO, I know plently of people will post that they keep their horses in electric only fence on T-posts and have successfully for years, and I'm sure that's true. I've just had way too many bad experiences to trust it.