They way I figure it should be done is as follows. It should be the margin rate divided by days in a year. That number times debit amount times days held. .06 (Annual rate)/ 364 (Days a year)= .0001648 (Daily Rate) .0001648 * 10,000 Margin Debit)= 1.648 per day Isn't that right?
winter: How much margin did you use on that one day, though? $1147 is the average balance if you used $30969 one night, and $0 on the other 26 nights.
In my case it was for sure $20k margin balance for one day resulting in $100 interest charge. The failed money market sweep can explain the problem because when it fails to return cash to my account at the beginning of a day, whatever trades I make that day will be on margin and trigger interest accumulation.
You shouldn't be charged margin on intra-day trades, and if it didn't sweep your cash back in, I'd think you'd have gotten a margin call. In any case, I guess they're on it. Hard to believe a big firm like Penson doesn't have checks and balances to catch something like that (i.e. their margin income suddenly skyrocketing), and that nobody has noticed it until now.