Without a time switch installed, electricity (*not* energy from the sun) will heat up the cold water faster than the sun can.
With a time switch installed, *only* solar energy will be used to heat up the cold water - electricity will kick in if the time switch is set to use electricity for critical time periods (early in the morning and late at night).
See here for more details: Eskom info on using a time switch with solar panels?
*) Installing a time switch in a solar installation will:
1) ensure that electricity is not used on a sunny day (i.e. on a sunny day you would rather have a slower heating up of the water by the sun, rather than use electricity and solar to heat up the water faster) and
2) ensure that electricity is used so as to guarantee that there is hot water early in the early morning and, in late afternoon/night on cold/cloudy days.
Remember to also turn the temperature on the geyser's thermostat down! (mine is set to 35 degrees C, and it's fine i.e. this is the temperature of the water on cold/cloudy days, but on sunny days the temperature is much hotter due to the solar panel doing its job)
Update on 16June2010:
Watch out or, at least, be aware: #WTF Geyser's element has just blown <3 years after installation... 5 year geyser warranty, but only 3 months on the element! #FFS #RipOff