Alaric

When it comes to the greens you really have to bear in mind the circumstances in which they exist. A green party majority government is simply not on the table and not what you should be voting for. Instead, bear in mind that the green party does not enforce any kind of "party whip" -- it's in essence a party of ideologically-aligned independent candidates, and should very much be approached on a candidate-by-candidate/riding-by-riding basis, rather than something like a low profile backbencher CPC/LPC MP where you're essentially just voting for the party itself. You vote for them if you think that they can be a vocal advocate for your community and your causes in parliament, not because you think they'll perform well or poorly if they hypothetically formed government -- they'd have a long road to get there, and if they were approaching that they would pick up more funding, consultants, etc. to help that transition.

One thing that has always stood out to me when hearing from or speaking to green representatives (including my own green MPP) is the level of passion, compassion, and candidness which I have found lacking from many politicians, which resonates greatly with me. But it's something that really should be approached at the individual riding level for the reasons I outlined above.

The other responder answered how many votes are considered extras, but I'm assuming you may be wondering about how those extra votes get allocated. If so, they would be divided up proportionally based on the 2nd choices of all the voters for that candidate or party. E.g. if Armchair won with extra votes, and all of Armchair's voters 2nd choices were: 50% for Couch, 30% for Loveseat, and 20% for Stool, then the extra votes would be divided up to match those percentages.

This is a tangent, but when talking about STV I have seen at least one person comment that it does not solve for strategic voting, noting that if you liked a candidate but also knew that:

1. they were very popular and likely to win,

2. you had a rough idea of how the surplus votes would be split (possibly from prior polling),

you could make your vote count for more by voting for your 2nd choice (e.g. Stool) as your 1st choice, making your vote for Stool 5 times more potent than if you were just another extra voter for Armchair. I.e. it doesn't eliminate strategic voting and so could cause distorted/degenerate outcomes. However, that relies on a lot of assumptions and is much less predictable than under First-Past-The-Post. There's also at least one research article saying that strategic voting under STV is computationally difficult https://sci-hub.st/https://www.jstor.org/stable/41105995

An Alternative Vote system does not solve the major issues of First-Past-The-Post, and can still have wildly distorted outcomes relative to vote preferences. Unless combined with multi-member districts (aka Single Transferable Vote) it's barely/not an improvement for Canada -- it's a convenient option for the largest parties because it rarely makes a difference to the outcome and the votes all trickle back to them anyway, so they can make a change without making a change.

Here's an article worth reading. https://www.fairvote.ca/expert-dennis-pilon-sets-the-record-straight-about-the-alternative-vote/

The only desirable outcome of electoral reform is one which introduces at least a degree of proportionality -- Single Transferable Vote if you're really itching for a ranked ballot, or Mixed-Member Proportional Representation otherwise (my preference - but either would be a great improvement).