echase quote:
"How do you get £0.081/kWh? Is that because in summer you fall below the usage required to get into the cheaper tariff? Not sure how they decide this tariff, is it based on a quarter by quarter usage or averaged over a year?"
E.ON calculates the higher tariff on a daily basis ie in my case 900kWh/year divided by 365 (approx 2.5kWh) charged at the higher rate for each day first plus the remaining power used that same day at the lower rate.
I had been skeptical that daily basis was used, as how do they know the daily usage when meter is read quarterly at best and probably six monthly, with rest estimated? However I just talked to a couple of utility tariff experts at Uswitch like companies and they say the companies can but vary as to whether they take a daily/quarterly view or average over the year.
Let us assume you have quarterly readings or at least quarterly estimates that are good, and that the company uses a quarterly/daily view rather than an annual average. Say in summer you use gas for water heating. Assume all the gas usage falls below the 900kWh/year (varies with plan you are on) pro rata because you have no/minimal gas cookers, etc. to take you over the 900kWh.
So all of your water heating is charged to you at the higher (about 8p plus 5% VAT) rate. So at 70% gas to hot water conversion efficiency you are paying 12p per kWh delivered to water. That is 3 times the 4p per unit often quoted on this forum. So that is the price of the gas you no longer have to buy if you have a controller. As most of the benefit of your controller comes in the summer months, because the sunshine is much greater and house electricity consumption perhaps a bit less, it means that over the whole year probably more than 50% of the cost benefit can be assumed to be at the higher rate. So that is another big cost benefit of a controller.
It's going to be a very difficult calculation to get right as so much depends on when you breach the 900kWh limit and when your quarterly bills are timed relative to summer. It is important to give the Utility readings that reflect your low usage rather than let them estimate you higher.
There is another saving you can make if you pay a standing charge rather than are on a two tier tariff as above. The standing charge is paid even if you use no gas. But if you switch to a two tier tariff, which is designed to pay a pseudo standing change in a different way, on the quarters you use next to no gas you are effectively paying less pseudo standing charge, reducing you overall bill. So that is another cost benefit of a controller.
That is all probably as clear as mud to you.
As an aside, since electricity is also about 12p it costs no more to use the immersion for top ups in summer when solar insufficient. So boiler can be completely off until you need it for home heating, saving the pilot light cost (if you have one) and arguably a boiler that is off is more reliable than one fired up a couple of times a week for short periods, which just collects corroding condensation. I don’t know what (boiler capital cost + whole life maintenance cost)/(kWr produced over lifetime) comes to but probably not insignificant and needs to be added to fuel cost.
