Like many people switching from storage heaters to a heat pump, we were faced with the decision on whether to change our electricity tariff or not. Heat pumps run continuously, so it is the percentage of night use that matters, rather than the ability to use only night time electricity for heat.
The first thing to consider is the economy 7 tariff itself. In our case the break even night percentage is very close to the 7 hours in 24 which is 29%. That is, if electricity use is evenly spread throughout the 24 hour period, we would only pay as much as we would on a standard tariff. Any bias towards night use would produce an overall saving.
The second thing to consider is whether or not there is any night bias to electricity usage. This can be split between heat pump for heating, heat pump for hot water and all other electricity use.
For heating it was immediately apparent that nights tend to be colder than days, which means that the heat pump would be working much harder during this period. On investigation I found that the Daikin software used to select the heat pump system had an input to allow for a period with a lower electricity tariff. Entering the Economy 7 start and finish times in the model gave me an Economy 7 night time percentage of 44%. This is well above the break even night percentage for our Economy 7 tariff, so for heating, Economy 7 was the right tariff.
Hot water offers a lot more flexibility, and how this works really depends on your hot water requirements. In theory you could heat 100% of your hot water at night and focus your hot water consumption in the morning. For us, we decided to continue with our 50% night, 50% day usage, so for hot water we have a 50% night bias.
Heating and hot water are the biggest users of electricity, but all the other lights and appliances in the house will contribute to the night time percentage. Our habit of being early risers and using cheap rate electricity in the morning helps, but the electric oven and hob, vacuum cleaner, lawn mower, iron etc tend to be used during the peak tariff period. Other than using timers to switch the washing machine and dish washer on at night, or ironing in the early hours, there is not much we can do to reduce our consumption during this period. However, there is one thing that does happen when the Economy 7 tariff finishes in the morning. The sun rises.
We therefore decided to introduce more night tariff bias to our Economy 7 usage by having PV panels on the roof. These are very cost effective for houses that qualify for the maximum FITS payment, but even more so when everything in the house is powered by electricity. With the heat pump heating on all the time during the winter, even a cloudy day must make some contribution to reducing the electricity bill.The contribution from PV can only get better as the year progresses to Spring and Summer.PV therefore appears to be an ideal complement to a heat pump, with the added bonus of reducing our electricity consumption during the peak day period of the Economy 7 tariff.
It is early days, but we are getting night time usage above 40%, which reduces our electricity costs by between 15% and 20%. Nights are not always colder than days, so there are distortions when looking at a short period of a few days, but over time the average should settle down as the seasonal average pushes the night time percentage upwards. I will provide an update after a year.
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