An immersion heats approximately 4.5 litres of water to a high temperature every 5 minutes, so if one can decide before switching on how much water is required the pre-heating time can be calculated as follows:
- 4.5 litres for hand-washing, baby’s bath, shaving, breakfast dishes etc. = Pre-heat for 5 minutes
- 9 litres required for washing-up dinner dishes, medium-sized clothes wash = Pre-heat for 10 minutes
- 9–14 litres required for a shower = Pre-heat for 10/15 minutes
- 54 litres required for a bath = Pre-heat for 1 hour
