Recently there has been a lot of coverage of domestic energy prices, and the energy price cap. Electricity tariffs can be fixed, where you pay a constant price for each unit of electricity, or variable where the price changes over time monthly, daily or hourly. The venerable economy 7 tariff makes a cheaper price available overnight for seven hours. With increasing use of smart meters, more sophisticated tariffs have been introduced to encourage electricity use outside of peak times. This month we look examples where locals are using time of use electricity tariffs to reduce their overall energy cost. Both are with Octopus that has pioneered these tariffs, but others are available from other supply companies.
Octopus Cosy Tariff (Chris Dodge)
The Octopus Cosy Tariff is designed for owners of heat pumps and provides three blocks of cheap electricity (12.65p/kWh) throughout a day, totalling 8 hours. Outside these cheap blocks, the normal day rate is quite standard (25.81p/kWh) apart from the peak rate for 3 hours between 16:00 and 19:00 (37.42p/kWh).
The aim is to run the heat-pump and ideally other electrical equipment during the three blocks of cheap power, which could lead to halving of one’s electricity bill.
We have been trying this tariff for several weeks now, so has it been worth it?
Setting Up
A heat pump is designed to provide constant low-level heat into the home, so will the home remain warm if used primarily in these three blocks? After some experimentation, we’ve set the heat pump to be OFF during the peak time, to keep the house at a slightly lower temperature during the normal rate blocks and warm the home at the normal temperature during the cheap blocks.
We also had to adjust when the hot water tank is heated, and reset other timed thermostats for various rooms so that they don’t demand heat when the heat pump should be idle.
Is the House Warm Enough?
With a bit of experimentation, the house remains warm. The heat pump does have to work outside the main three cheap blocks, which is where a battery helps.
Using a Domestic Battery
We also charge our 5kWh battery during the periods of cheap power. This generally provides enough power for the home to cover more expensive blocks, particularly the peak block. We do need to be careful, for example when cooking to try and keep electricity demand within what the battery can provide (2.5kW max).
Disadvantages
- Without a domestic battery, we would have struggled to make it effective.
- We don’t have an EV but this tariff probably isn’t suitable for EV charging. Other Octopus tariffs provide cheap electricity at night for that.
- Setup and tuning has been quite challenging, requiring adjustment of space heating schedules, hot water heating schedules, battery charging periods and several room thermostats.
- We will need to adjust in summer to optimise use of our solar electricity.
The Overall Result
We’ve been quite successful in keeping much of our electricity demand within these cheap blocks. The diagram below is typical hourly electricity usage over one day.
For this day, our estimated electricity bill was £2.69 for a total of 17kWh. On our previous tariff of the more standard Octopus Flexible, the would have cost £4.74. For the most recent three week billed period, this has reduced our electricity bill from £111 (Flexible Octopus tariff) to £68, a saving of £43 for the period.
The chart below shows the battery charging phases (red) which take place during the cheap power blocks. Heat pump operation (black) also takes place mainly within the cheap power times, but not exclusively so are powered from the battery.
So far this winter we have not had any very cold days, we’ve yet to see how effective it will be when the heat-pump needs to work harder as the 5kWh battery will not be large enough to run the heat pump for long outside the cheap power times.