I know this is an old thread, but I am doing some work on my system once more, and thought I might say something which I feel is important.

Just because Vaillant calculate their 'COP' that way doesn't make it right. It produces a figure that looks far better than simple kWh input-to-kWh output, a simple ratio and is the only sensible way to compare S/COPs, especially across multiple manufacturers. Vaillant's 'calculation' is dissembling or plain distraction from the true COP, or ratio between power in to power out. Otherwise it is perfectly meaningless to the average punter, who is perhaps not so arithmetically inclined as some.  

I don't have an easy answer to your oil/ASHP comparison.  However, the average house using gas, uses about 12000kWh per year.  From what you say your house is older and leakier than average. Is it really feasible that your older, leakier than average house used less than the average amount of energy to heat all of it to the average indoor temperature? What did your EPC say the heating requirement was?

I think it's possible that your COP calculation (or rather, Midea's numbers) is out by quite a lot.  You have an ASHP that's struggling in cold weather, running flat out at (I think) quite a high flow temp.  You also (I think) have a LLH or PHE. Your COP could easily be in the low 2s rather than the mid 3s you report. You say somewhere up-thread that your rooms may be a few degrees warmer with the ASHP than with the oil CH. If this is true and you're heating more of the house for longer (IIRC your heating is now on all night) then that's going to use a lot more energy than before.   

I might be barking up the wrong tree but I think you need to (1) compare how you used your oil system - temperatures, rooms, timings - with what you do now with the ASHP and work out the difference in energy required and (2) Try and find a way to calculate/estimate/measure a more realistic COP. 

A new thread sounds like a good idea.       

I just know I’m toasting now for less money.

I think you are probably using 1kWh per hour for 24 hours, making a total of 24kWh over 24 hours. I too find all this confusing, because the names overlap, and distinguishing between power (kW) and energy (kWh) is not intuitive. The way I try to remember the difference is power is about how powerful something is, how much work it can do in a given period of time, while energy is what is burnt, literally or figuratively, to do the work (heat the house, move the car, whatever the energy is being used for).  

Thus a 2kW convector heater has a power of 2kW, and so is twice as powerful as a 1kW heater. If we run the 2kW heater for an hour, it will 'burn' at 2kW for an hour, and so 'burn' 2kW x 1h = 2kWh of energy. If we ran it for half an hour, it would 'burn' 2kW x 0.5h = 1kWh. Likewise, a 1kW heater running for two hours would 'burn' 1kW x 2h = 2kWh of energy.

The maths isn't complicated. 1 litre of kerosene contains just over 10kWh of energy (10.35 to be exact), meaning you can easily convert litres of kerosene into the amount of energy in kWh it will produce. At 100% efficiency, an oil burner using 3000L of kerosene will produce 31,050 kWh of energy (heat), I knocked a bit off for efficiency losses, but kept it at a round figure, 30,000 to keep things simple. Likewise, the heat pump energy output is a simple calculation, energy input multiplied by the COP, For each kWh in, you get 1 x COP kWH back. I then took your current daily use, 24kWh, multiplied it by 30 days and then by nine months, and then by the COP (I used 3.5 as being a more realistic overall COP) to get what your heat pump will put out per year, 22,680kWh. It is a very rough and ready calculation, but the results, 30,000 vs 22,680 are a long way apart.

Which brings us back to my original point, there is something seriously wrong with the calculations, especially in my case, but also in yours, and what's more, yours goes the other way (your case: oil needs more kWh to achieve the same heating, my case: heat pump needs more kWh to achieve the same heating). They should be the same, a kWh doesn't know where it came from or where it is going, if your house remains unchanged, the heat demand will remain unchanged, and the kWh needed to meet that demand will remain the same (with adjustments for different weather etc but over long periods of time, like a year, it will remain broadly the same).

The key point is our houses should need very similar amounts of heat energy delivered to heat our homes to our preferred level of comfort, wherever the kWh came from. The problem is the calculations don't confirm this. Ergo, there is either something wrong with the calculations, or I have gone mad.

There is in your case one remote possibility: your old oil boiler was extremely inefficient. It had to burn 30,000 kWh of energy to produce 22,680 kWh of delivered heat (the rest literally went up the chimney). I wonder, might that have been the case?

There is another possible thing to consider – with fossil fuel boilers of any type, the flow temperature is naturally going to be, probably, at least 60°C, and at lower flow temperatures, with average radiators for such systems, you will never be warm enough. But if you open the door to the outside with such a system, because the heat source is pretty hot, room temperature recovers quite quickly. With the low flow temperatures of heat pumps, and with large rooms, especially if the calculations for radiator capacity hasn’t been done. Well, you might never feel warm enough. Then you bump the temperature settings up on the controller, then Bob’s your uncle, massive bills. However, even with the maximum flow temperature of my system set to 45°, I have never seen the flow temperature exceed 40°, as the weather compensation/heat curve calculates the right flow for the system. 

With all of the TRVs fully open, it should be possible to regulate the temperature in the house without the system ever shutting off. This is what you should be looking for, absolute minimal cycling, whilst keeping the temperature where you want it. I am getting on in Age now, have a couple of health issues, and I have the house at 23° all day, set back to 21 at night. Even at these values, I think my power consumption is really very reasonable. Although compared to the cost of heating my old house down in Bristol, 600 miles south by road, I was gobsmacked at what I was spending on heating when I was first in Wester Ross. I got used to the cost of staying warm here very quickly, and stopped fretting about it when I realised how many of my friends and neighbours were spending similar amounts of money. It doesn’t necessarily get that much colder here than down south, but it is cooler for more of the year, and it is not unusual for the heating to come on at any point during the summer. It is not unusual for the heating to come on at any point during the summer. Days above 20 C are relatively rare, and often only come in single days. Our sunniest days are in spring, when it can often be very cold at night. And because houses here don’t benefit from much solar gain, it often remains cooler in the house than it is outside, so a day when it is 20 or 22° outside it might still only be 17 or 18° in the house.

Apologies again for my wordiness… 

Currently using 1 kWh for 24 hours = 24 kw (currently February)

I put in 3000 litres of heating oil every year.

I’m not a mathematician or whiz with formulas

I’m only saying that’s what I spent and that’s the consumption I have.