I only have one 4ft tank in the lounge and it is enclosed on 3 sides by a wall unit. The lounge is kept warm although it can drop to 15 overnight sometimes (less often now we are double glazed). Our power bills don't vary much between winter and summer as we use the air cons year round to both heat and cool. I suspect water heating may go up a bit as we have longer, hotter, showers in winter 8)
Tanks in a well heated, insulated, home shouldn't make any difference to your power bill in winter.