Economy 7 and Immersion Heaters in London: Using Off-Peak Electricity for Hot Water
Economy 7 electricity tariffs offer cheaper overnight electricity rates. For London properties with a hot water cylinder, using an immersion heater on Economy 7 overnight heating can significantly reduce hot water costs. This guide explains how to set it up.
What Is Economy 7
Economy 7 is an electricity tariff that provides a lower per-unit rate for electricity used during a seven-hour overnight window — typically between midnight and 7am, though the exact hours vary by supplier and region. During the off-peak hours, the unit rate for electricity is significantly lower than the daytime rate — typically 8 to 12 pence per kWh versus 25 to 35 pence per kWh at peak rates. The tariff was introduced to encourage domestic electricity consumption during low-demand overnight hours, benefiting both consumers and the electricity grid.
Economy 7 meters have two registers — one for daytime units and one for night units — that the supplier uses to calculate the bill. The meter can be identified by the presence of two sets of figures on the digital display, or by a label confirming Economy 7. Your electricity bill will show separate day and night unit rates if you are on this tariff.
Dual Immersion Cylinders and Economy 7
A standard vented or unvented hot water cylinder holds 100 to 200 litres of water. Heating 150 litres of water from cold (approximately 10 degrees Celsius) to 65 degrees Celsius using a 3kW immersion element takes approximately three hours. The bottom element in a dual immersion cylinder heats the full tank volume. If this heating cycle is programmed to occur entirely within the Economy 7 off-peak window, the full heating cost is charged at the cheaper night rate.
A correctly set Economy 7 timer ensures the immersion starts approximately three hours before the off-peak window ends — for example, starting at 4am if the off-peak window runs from midnight to 7am. This allows the full heating cycle to complete before the daytime rate resumes. If the timer starts too late, the last portion of the heating cycle runs on the more expensive daytime rate.
Installing or Configuring an Economy 7 Immersion Timer
Most immersion heater installations include a dedicated timer switch or a programmer adjacent to the immersion switch panel. To configure Economy 7 heating, the timer is programmed to turn on the immersion element during the overnight window and turn it off before the daytime rate period begins. Mechanical timers use a rotating dial with pins that are manually set to the on and off times. Digital timers allow more precise programming and can be set to different schedules for weekdays and weekends.
Where no timer is fitted, or where the existing timer is faulty, we supply and fit a new digital programmer on the same visit as an immersion heater replacement or service. We confirm the Economy 7 window with you, set the timer to the correct on and off times, and test the heating cycle before leaving. The cost of a timer installation is from £80 including parts and programming.
Top Element Boost During the Day
If the overnight Economy 7 heating has been consumed and additional hot water is needed during the day, the top element boost switch heats only the upper portion of the cylinder — typically 40 to 50 litres — rather than the full tank volume. This smaller volume heats much faster than the full tank (30 to 45 minutes for the top portion versus 3 hours for the full tank) and at lower cost than reheating the entire cylinder. The boost switch should be turned off once the hot water requirement is met to avoid unnecessary heating at the daytime electricity rate.
When Economy 7 Makes Financial Sense
Economy 7 is most financially beneficial in properties that have high electricity consumption overnight — typically properties with storage heaters, an electric vehicle charger on a night rate timer, or a large immersion cylinder used heavily. For a property that uses hot water primarily during the morning and evening, Economy 7 overnight immersion heating reduces the cost of the largest regular electricity consumption event in the property. Contact Prestige Engineers for immersion heater timer installation, Economy 7 setup, and hot water cylinder servicing across all London boroughs.