strange question in several ways
if you want to just use the boost off peak , why not try switching it on during this period and look at the meter to see which one it shows up on [ it will , of course , could up as off peak ] ; I presume you have a 2 element tank , but using both for a few minutes at the beginning of the off peak period [ so the water needs heating ] , won't be a problem
why do you want to increase your costs by doing this ? it is so much cheaper to heat off peak that , provided your tank is properly insulated , the small on -cost for maintaining a tank of water hot is negligible compared to the cost to heat water at peak prices ; I have installed one of the modern factory insulated tanks , can heat the water , and it is still hot enough to have a nice hot shower after 2 days ....if I am going away have to remember to turn off the water heating a couple of days in advance