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But now that he has all that electricity, why not just sell it to the grid? I know that those that have solar panels can do that if the panels generated more electricity than the owner used.
At least in the US each state has a Public Utilities Commissions that put limits on your self-generation capacity and what you can do with it. They work with the actual Public Utility (PU) providers who own and run "The Grid(s)" in that state to properly govern the grid(s) in the state. That's why. Most PU's will require that you have a proven x amount of power usage over several months and then limit your solar/wind/water/whatnot generation capacity to that value + some reserve margin over it.
The reason for that is simple: The PU's are required by law to do their practical best at providing reliable power to the areas they serve. Amongst other things that means having a good idea of how much
reliable power is available on 'the Grid' and how much is being used. They cannot have a huge number of folks acting as an independent outside power source because said folks have little to no incentive to guarantee the quality and amount of power they are feeding into the grid.
That said -- if you are talking about generating a few MW or more of power that is different story. Anyone can setup a private (or is often the case, local community owned) power generation site, become regulated service provider, and tie it to the grid. Especially in rural areas it is fairly common. As I recall, in Michigan the generation capacity threshold to become a service provider is a minimum designed
constant capacity of 500kW with at least that amount going to feed the grid.
To do that the site has to meet many regulatory points one being that it must be built to meet all requirements of the Regional Grid Operations Authority (the US has 3 - the Eastern Grid, the Western Grid and the Texas (ERCOT) Grid, with the Eastern Grid being the largest of the three) as well as be operated per their requirements.
OK, then what stops the big utilities from using their electricity to mine as opposed to selling it to the consumer?
Really?
They are PUBLIC UTILITIES - not a business. They were created to serve the greater good by providing reliable power to the areas they serve and are bound by law for that to be their primary purpose.
But - when it comes to
excess power generation and/or normally idling reserve capacity built for handling peak loads they are free to do what they want with it. Some utilities DO use miners located at and fed by their peak-power leveling generation sites to set a heavy baseline load for easier and more efficient site operations. They then start shedding miners as the reserve capacity is called upon during time of peak loads. Once the loads drop off miners are brought back online to keep the plants operational loads fairly constant.