We based our models on typical residential, full size, pools and hot tubs. Residential pools are typically 18,000-25,000 gallons (however can be as large as 40,000).
There are a lot of factors that impact the amount of chemicals needed (and therefore the cost). Size of the pool is one, but “bather load” - how many people, how long, how much sunscreen/dirt do they have on them, weather - how much evaporation, new water (rain, or added water to make up for evaporation), and whether or not you use a cover also are big factors.
Therefore, we don't base our pricing strictly on the size of the pool, but on the chemical usage. We have made estimates based on typical residential chemical usage, so as long as your usage is in the vicinity of typical residential usage, there won't be any extra charges. If your pool needs significantly more chemicals than we expect, we'll contact you to discuss options.