It is always advisable to have an electric hot water system connected to either off peak tariff 31 or 33 otherwise your electricity bill could be very high as these systems heat up the whole tank when they drop below 60 degrees.
Tariff 31 is the cheapest tariff but your tank will only heat up during the night. If you use all the water during the day, you may run out until the next morning.
Tariff 33 will give you a guaranteed 18 hours, but usually not during peak hours.
You may want to concider Solar Hot Water to reduce your energy bills or Gas Hot Water for hot water on demand.