That's more often than not already the case, and it will likely start to 'affect' the regular power requirements needed for everything not related to mining, if nothing changes. That being said, the main power suppliers always have plenty enough extra capacity they can use in case there are extreme usage peaks. In case their extra capacity isn't able to cope with the fierce miner demand, they should consider upgrading their entire way of operating. If the long term demand is there, which is almost a guarantee, then I don't see why the power suppliers aren't willing to upgrade. If they aren't willing to upgrade, which is their loss, then miners will look for an alternative location, it's that simple. It's all about the business opportunities the power hungry miners offer, and there are enough power suppliers ready to serve them. If it isn't Iceland, another country will happily welcome them.