Most electric storage heating systems use an electric immersion to heat hot water. This may be using ‘peak’ or more commonly ‘off-peak’ electricity (usually between 11.30pm and 8.30am). If your system uses off peak to heat water then this will be controlled automatically by a timer and the whole tank will be heated for about 5 hours overnight. The water temperature can be boosted during the day if required, at peak rate, by overriding the timer. If using peak electricity only, switch the immersion on for the period required to heat the water. It is expensive to keep the immersion on for long periods during ‘peak’ times.