After suffering through cold evenings for some three weeks and nightly temperatures only getting colder, I gave in and bought this electricity vampire:

Considering the cottage in which I live, the roof is made from corrugated iron and a heat leak, windows are single pane, there is zero insulation, and the metal door to the outside has no doorstep (hence draughty). The floor is covered with stone tiles.

It took the heater full capacity to actually keep the room temperature stable let alone warm it.

Johannesburg has a long way to go.

BTW, if your insulation is all right, it may actually be more energy friendly for you to keep your daily temperatures  a bit higher - maintaining a temperature at 15 degrees C may cost less than heating up seven degrees to 20 degrees C. Or that was the advice my family got in the Netherlands... would love some verification on that.