A geyser is the heaviest energy consumer in your household
By adding solar thermal heating you can cut you electrical water heating bill to 1/3. Solar thermal heating is done by collector plates or evacuated tubes. These preheat and re-heat your water stored in the geyser. Thus using your geyser as a battery to store energy in the form of hot water.
If you add a heat-pump to the equation, you can drive your water heating costs to under 1/4.