Cooling 1g of water from 25 C to 0 C takes 25 calories. Freezing 1g of water at 0 C takes 80 calories, a total of 105 calories.
1 Watt is 859 calories/hour, so at 100% efficiency, 1 Watt could freeze 8.1 grams of water per hour. Figure on a refrigerator being around 3040% efficient, so say 23 grams.
Note that Watts are a unit of power  what you are looking for is wattshours, which is a unit of energy.
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