On diagrams you'd use + as the "source" of elecricity, i.e. you assume electricity flows from + to - (poaitive to negative). Electricity as far as physics goes is an effect created by electrons, which are defined as negative in charge.

DC is electricity where the literal flow of electrons from point A to point B make the current (so it flows from negative to positive, since it's the flow of "negative" electrons that carries electricity). Benjamin Franklin assumed logically that electricity obviously must flow from positive to negative (since it's the logical choice), but alas, he was wrong as far as history sees it. So today, whenever you're dealing with electrical diagrams current/electricity is assumed to flow from + to - while in the physical domain it's the negatively charged electrons that create what we call electricity.

AC is a bit different - here electrons aren't flowing directly from point A to point B, but rather wiggling about or "alternating" in place and it's this alternating movement that carries the (still negative) charge. But even for AC it still holds true that electrical charge is the "negative" charge of electrons and that this movement of electrons alternating in place enables them to move this "negative" charge of theirs from one place to another.

I assume you know about the saying "opposites attract" - for electricity and charge it's literally true, so you can view power consumption as the "positive" charge of protons (which is immovable because protons are bound to the cores of their atom), while it's the "negative" charge of electrons which are located in the outer shells of metal atoms that can leave their atoms and move their charge that are viewed as the source/carrier of electricsl energy.

I put negative and positive in quotes because to get back to your question about defining why Franklin was wrong:

As it stands, there are two conventions on electricity. One is used in diagrams and often attributed to Franklin, the one that says that electricity flows from the positive (+) to the negative (-) pole. The other is the physics convention that protons hold positive charge while electrons hold negative charge, and this is where the disparity comes from. I don't know which convention was chronologically earlier, but I assume it's the physics one since Franklin is the one cited as "wrong".

Obligatory I'm not an electrical engineer - this is only what I remember from my physics classes. Please assume it mostly correct but maybe not technically for every minute detail (the only use of "power" is technically very wrong among other things, but that's the gist of it).