this is difficult to answer. How much is your bill? How much are the lights on in the basement? How often does someone use the outdoor outlets? If you think $15 is fair, great. If you think it should be $50 tell your landlord that. This is something you have to negotiate between you no one can tell you what is appropriate.
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You may also want to contact the local power company and see if this has been going on a long time or only recently. If it is fairly recent, a fair way to determine how much is owed would be asking the power compnay for a couple of years history and seeing what the unit individually was charged and what the common areas were charged, nad determine the percentage of usage for each. If it has alsways been this way, get the HOA /power company to install the correct meters and then determine what your usage is based on the next 4 to 6 months (or if you can be patient, 12 months) and base it on percentage each is using power now. But if you delay determining a number amount, make sure you get something in writing from the landlord or HOA that they agree to reimburse you based on the percentage of use (and how you decide it should be determined)
This is not intended to be legal advice or create an attorney-client relationship. If more information is needed, you should consult with an attorney in your state regarding the specifics of your situation and the options available to you.