Why does every page about a generator but the page of a nuclear reactor have information like: power output?
The other information is useful, but I would like to know what sort of 
I should use.
Nuclear reactor page has no energy information
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Output depends entirely on the setup you go for. Anywhere between 2 and 10 million EU total.
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And sometimes, (while I was toying with a critical system with no cooling), I nailed just a little over 10 million EU total. Keep in mind that it really depends on how you set the EU values in the config.
But, as a general rule of thumb, if you plan on using something like a MK. II or III, I'd personally recommend 4x Insulated HV Cables.