The output from even a 12 Kw unit is quite low. And to do any more than wash your hands you really do need some storage of hot water. Reducing from 120 litre to smaller size may be option but for instant hot water 25 to 30Kw is required too much for electric units only gas can supply this power. I have used 7 litre undersink units which were OK even with 1Kw element. I would do internet search for "mains pressure hot water" The Dimplex do a range 7 to 15 litres and these would allow you to run full sink full of water there are many options but I would think you would need some storage as electric is slow.
THE UNITS ARE DESIGNED TO MEET MODERN HOT WATER REQUIREMENTS FOR TWO BASINS AND A BATH/ WASHING MACHINE .i WOULD STRONGLY ADVISE AGAINST TRYING TO REPLACE WITH ANYTHING SMALLER. HAVE RECENTLY CARRIED OUT SEVERAL REPLACEMENTS OF THESE HEATERS. NEW ONES ARE GUARANTEED FOR TWENTY YEARS... SO MAYBE THATS A CONSIDERATION
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