Thinus, remember that in SA water heaters exclusively provide hot water supply, with water only flowing when a tap is opened. In some colder countries, like Netherlands and Canada to mention two, the hot water from the geyser ("boiler") is constantly circulated through the pipes to heat indoor living spaces and perhaps also to keep it from freezing. In SA we have many homes with no geysers; only cold water supply. In Canada that would not be possible. You could probably safely assume that all homes in relatively cold countries have geysers, so just count the houses.