Economy 7 is a differential tariff in the United Kingdom for electricity usage. The day time electric rate remains the same, however, at a set time at night the electricity rate drastically drops to promote more off-peak hours for emersion water heaters. You control the electricity to an emersion hot water heater and take advantage of the cheaper electricity rates through a keypad wired to the heater.
Turn the clock dial clockwise to align the hour you want the heater to turn on with either the "Summer Time" or the "Winter Time" mark at the top of the timer. The clock dial is in a 24-hour format and will turn the water heater on at this time to provide you a tank of hot water for the morning.
Press the toggle switch on the right side down to "Timed." The "Off-Peak" light will illuminate when the immersion heater is on.
Turn the boost timer dial on the left bottom side counterclockwise up to the 1 hour setting. The "Boost" indicator light will illuminate to show that the boost mode is selected. This will turn on the hot water heater during the day when you need warm water for washing dishes, cleaning or other tasks.
Your local installer sets the off-peak time according to the local electric company's rate change time. The less expensive off-peak time lasts for 7 hours during the night.