Dehumidifiers are not energy efficient, plus they give off lots of heat.[/QUOTE
Hello; I have been running a small one for about three years now. Like you I have the windows open during much of the warm weather so the unit is turned off. I turn it back on when I close the house in the fall. The energy that the unit uses helps to heat the house so I do not consider it a penalty.
I considered having the water go into a drain but decided to recycle it. I am not yet comfortable using the water in my tanks so it goes into a washing machine. As my area is now in a drought this seems the better choice. I put the setting at about half way and this will keep the humidity in a decent range.
I do not have any where near as much water in my tanks as you report so cannot give a n estimate on how often the holding tank will fill up.
Good luck