You will most likely need to do your and her 2024 tax returns both ways -- with and without her as a dependent -- to see what gives you the best overall result.
She is legally your dependent if:
- she is under 24 as of 12/31/24
- in school for at least part of 5 months in 2024
- she does not provide more than half of her own support for the year
- she lives with you (including the time she's away at school) for more than 183 days
This is where it gets tricky. Just because she earns money doesn't mean it counts as supporting herself. If she lives with you after she graduates and saves her earnings so she can afford to move out later, she's not supporting herself with her earnings. Also, whatever you pay towards college counts as your support. So as you can see, there are some ways to decide in advance how you'll apply funds if you do or don't want her to be your dependent next year.
If you do claim her as a dependent on your tax return, then yes, her entire income counts towards the household income. ACA subsidies are calculated annually, not monthly, so if she's your dependent, it doesn't matter whether she was on your plan for the full year or part of the year. Even if she weren't on your plan at all, you'd still have to include her income in the subsidy calculation.
If she is not your dependent for 2024, then she can still be on your ACA plan for all or part of the year. In that case, she will file her own tax return and you will have to go through the premium allocation process described in the instructions for form 8962. This is the "other" case where you just make an agreement with your daughter about what percentage of the premium belongs to you and what to her. The optimal division that maximizes the subsidy depends on each of your incomes, but you're free to give her 100% and you 0% or you 66% and her 33% or whatever division works best. Both of you have to file Form 8962 with your returns and report the other person's SSN on the form.