Hiya mate, it's so you can heat your water up overnight while it's cheap. One of the switches will be to tell it to come on between 11pm and 7am and the other will be so you can turn it on outside of these hours if you want an extra boost. Hope it says which one is which but if you don't know just try each one in turn in the daytime and whichever heats the water, you want the other one on all the time (the night one)
Dan, January 2009