http://www.calgary.ca/UEP/Water/Page...ater-bill.aspx
You pay for the water delivered to your abode and your wastewater carried away. It's not exactly double because the wastewater is a smaller percentage of what is delivered to account for things like irrigation and evaporation and so on. So you would be charged for 20 gallons delivered, and some amount less than 20 gallons for wastewater. I forget the exact ratio they use.
But even though you pay for water to AND from your abode, realistically, I don't think you will truly feel a huge impact in your water bill for the size of tank you have. It's part of the cost of having a tank. Overall it would likely be one of your more nomimal costs.