We recommend setting up electric sites as a separate site type. This way you have control over the allocation.
The downside of using a general site type with an optional extra for electric hook-up is that you have no way of knowing how many customers will choose the electric extra. If all customers choose the extra, you may find that you sell out of electric sites. You could also lose out on the chance to sell more non-electric sites if you set a low allocation in case all users choose electric.