The air may have been nippy but I knew I could always depend on sunshine. It woke me up every morning and set beautifully every evening. South Africa is far from the equator, but the sun sticks around during the winter. It is exceptionally dry, drier even than California, and my skin paid dearly for it.
With a low of freezing and a high of 70 (21 C), the winter was somewhat misleading. This is Africa, right? I thought, ignorantly. SA gets cold in the winter and central heating isn't common. In fact, sometimes it felt as if even the natives themselves were surprised at the concept of winter. The tiny radiators of my apartment (floored with tile throughout) did little to take the edge off, so I relied on space heaters, fleece blankets, and hot showers.* Like California, Southern California in particular, Johannesburg was often quite warm in the afternoon but cold after the sun went down. It is also more than a mile above sea level.
The NGO where I worked technically had central heating, but everyone had a space heater in their office. The lucky ones had offices that faced the sun. Those of us who were not so lucky wore our coats indoors, though we were rewarded with rather splendid sunsets at the end of the work day.
The cold served us well on safari. Animals hide in or under trees when it gets too hot, so winter is the best time to see them. People hide in or under blankets during the winter, so there were fewer people hogging the best viewing spots.
I'd recommend seeing South Africa during the winter. You will be assured of great weather (the summers are supposed to be quite wet) and if you're interested in seeing animals, they'll be more likely do interesting things while you're awake.
* I am forever indebted to D who lent me not only a blanket, but also a large space heater for the duration of my stay in SA. The showers were heated by solar power water heaters. Thanks to the ubiquitous sun, there was always plenty of hot water.