The difference between and electric based geyser, and a gas based geyser is that the gas based system, is a heat on demand, where as the electric geyser is that the water is maintained at a value on a continuous basis..
In other words, with the gas based water heater, only when you start using the water, does the gas through the flow/pressure switch, trigger the gas burner to heat the water as it travels through the heating coil.
When you close the tap, the gas trigger switches the burner off.
Advantages of this is that you only use the gas when you use the water.
Disadvantages, depending on the coil size and burner, if the incoming water is very low temperature, like in winter, then the amount of energy transferred from the burner to the water may not be sufficient to have a comfortable shower. Also if the flow rate is too high, such as two people showering, or more than one source of use, then the flow will reduce the temperature that the gas burner can transfer to the water. This is more noticeable in winter where users want a higher temperature to overcome the lower ambient temperature during showering.

With respect to an electric geyser, the element basically heats up the 150L of water during a heating cycle, and depending on the size of the element, and the start temperature, it may take between 40 minutes to an hour to heat up the water to approximately 60 degrees centigrade, or whatever the thermostat has been set on. Irrespective of the water demand, the temperature of consumption will remain constant until almost all of the heated water is consumed in one go. Electric Geysers have been designed to force all the hot water to the outlet. However if you stop at any time and wait too long, then the incoming cold water will mix with the hot water and drop the temperature, which will then require the element to reheat the water before consumption again. The thermostat maintains the water at the maximum setting 24/7.
Disadvantages is the consumption of electricity during periods that the hot water is not used and the thermostat maintains the water temperature. To reduce this consumption, geyser timers are used to synchronise the water heating to your hot water usage times.

I suspect that your lifestyle must be adapted to suite the type of water heating system that you have.