We used to be on a tariff where the cost could change every 30 minutes, in theory it meant that you could get very cheap electricity because the cost was directly linked to local demand, I've seen it where at night it was going into negative numbers(so they were paying us to use electricity), but at peak times it could be absolutely brutal too, you had to be careful and really monitor it almost real-time. We've since changed to a tariff where the cost changes daily, but the cost for the next day is announced around 1pm the day before, so I can plan things like charging my car or running appliances depending on what the tomorrow's cost is going to be.