Elson Opal tank: how do I actually use it?

[Astrolabe]

I am renting a flat with an Elson Opal tank, same as this one here:
https://www.diynot.com/diy/attachments/img_2113-jpg.168795/
The electricity here is on a three-price system using two meters, will I be right in thinking the lower switch is for the cheaper stored heat energy? If so, is the idea to leave it on all the time?

I ask because the hot water is only used for the taps and I live alone, so if I turn the top switch on for an hour every day or so that has given me enough hot water so far. It seems unlikely to me that it would be cheaper to have it heating every night, even at a lower tariff rate?

 

[ianmcd]

If you have an electric shower and just use HW for hand washing and washing up either just use the top booster for an hour or switch it off and just boil a kettle