The simple answer is than an 'Electrikit' converted cooker will not contribute to your hot water at all.
You may think this is bad news, it's not. Using your Aga range cooker to heat water is almost certainly the most expensive way imaginable short of a cauldron over an open fire! Aga range cooker 'back boilers' are simple gravity systems that have no 'off' switch and suck huge amounts of energy/heat from your cooker that the burners then have to work hard to replace. The simple fact is that these cookers are not primarily designed to heat water.
The water is not heated by an Aga range cooker 'that is already hot' but rather by a cooker that has to work much harder to stay hot!
You are much better off leaving the hot water to your dedicated hot water system and letting your cooker do the cooking - this applies even if you aren't converting to electric.
The pic below shows a boiler from an Aga 'Standard' range cooker, it's like a jacket full of water that hugs the main barrel and absorbs heat from it.
>>Find out more on our 'Aga range cooker electric conversions homepage'
>>Find out about our reconditioned electric Aga range cookers - meticulously made to order.