Hi there Telford and welcome to the forum
I'm not an electric brewer so I can't help you out with the wattage. This could well be an issue though as many urns are often a bit under-powered so take longer to reach temperatures and then may only maintain a poor boil.
If you want to get 6 gallons or 23 litres into your fermenter, then this is also too small - you really need a 40 litre urn.
There are ways to get larger volumes of beer from smaller pots (see
here) but if you want to do this then you need at least another pot and these smaller pots can often be heated just on your stovetop.
The urn you linked, assuming it will be able to give you a good boil, should give you about 13 litres of beer to drink.
Hope that helps a bit,
PP
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