There is a difference between sunny and hot. Cupertino has plenty of sunshine, but is not hot so doesn't require a ton of energy for cooling (the building doesn't even have AC!). If they were able to pull this off in say, Phoenix, it would be a lot more of a cost savings.
> Jobs hated air-conditioning and especially loathed fans. (He vigilantly tried to keep them out of his computers.) But he also didn’t want people opening windows, so he insisted on natural ventilation, a building that breathes just like the people who work inside it.
Energy costs are typically, and by far, the lion's share of the operating costs of a major building like this.