A set is how many times you are going to lift and then take a break. So if you lift the weight five times, take a break, and lift again you will have done two sets. The five times would be your reps. My coaches like to write the workout like so: Squat 3x2, Bench 2x5, and so on.
Evaporative coolers use both electricity and water. How much water is used depends on the humidity of the day and the fan speed that the unit is set to operate on. A portable unit might use up to 4 litres per hour while a central system could use as much as 25 litres per hour. Ducted evaporative coolers must refresh their water supply otherwise it can become too salty, and a range of water management systems are used by different brands/models.
Since most retailers and manufacturers don't have specific estimates, consider using 7 to 9 gallons / 26 to 34 litres per hour as an average estimate, realizing your actual numbers will vary. Multiply average water use by the number of hours you plan to use your swamp cooler. Using it for the hot hours of 11 a.m. to 3 p.m. would mean 4 hours of use. 7 to 9 gallons / 26 to 34 litres per hour for 4 hours would be 28 to 36 gallons / 104 to 136 litres per day. Then simply multiply that number by what your water company charges per gallon / litre of water.