Water heaters have two functions
1. heat water
2. keep heated water warm
The heater tank is lined with a layer of insulation and designed to retain heat like a thermos flask.
This depends on many factors. Your heater location, usage and climate play a part. Generally, owners in Singapore expect a 4ºC drop over 8 hours.
Well insulated water heaters keep water in the tank hot for a long time.
When water does not constantly have to be reheated, your electric costs will be lower.
A. O. Smith insulation is much thicker than that of other water heaters. In contrast, this is the actual tank cut-out of another brand in Singapore.
Poor insulation reduces efficiency of the heater, and this means higher running costs.