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Working out Air Source Heat Pump running cost?

Posts: 19 Forumite
Hi all,

Does anyone have any experience working out the running cost of an Air Source Heat Pump?

I've been very bad at recording readings from it so only have a few readings plus the start date to calculate from... in principle though, is this equation right?

kW used / (Days elapsed x 24hours) x tariff price per kW = cost per day

So, last winter would be:
2682kW / (87 days x 24 hours) x £0.21 = £0.27 per day

And, this summer would be:
719kW / (192 days x 24 hours) x £0.21 = £0.03 per day

Does that look right? Many thanks in advance for any help!:)
«13

• Posts: 29,048 Forumite
If your ASHP used 2,682kWh in 87 days, that is an average of 30.83kWh a day and an average of 1.28kWh per hour over the whole 87 days. If you pay 21 pence for a kWh(extremely high cost) then it costs £6.47 a day and 27p an hour.
• Posts: 8,688 Forumite
edited 29 November 2017 at 12:03AM
I think we need a bit more info. Are the consumption figures you are giving for the ASHP alone or for the whole house.

Is the ASHP on a seperate meter or have you got in on the day rate of an E7 tariff or some strange complex metering system. As Cardew says 21p a kwh is insanely high what tariff are you on. I'm moaning about having to pay 11.55p/kwh to run mine.

My ASHP cost very little to run in the summer but iit can look quite expensive in the winter but I reckon mine uses around 3000-3500kw a year which equates to between £350 and £400 a year.

As it responds to the house heating requirements and it has weather compensation it's consumption can vary between 10-50kwh in a day depending on how cold it is outside. That's anywhere between £1-£6

You can only determine what it's using by continuously monitoring it's consumption.
Never under estimate the power of stupid people in large numbers
• Posts: 19 Forumite
Hi there,

The 2682 figure is just a kW figure.... not a kWh figure!

Stan
• Posts: 19 Forumite
edited 29 November 2017 at 12:08AM
The figures given are for the heat pump only.
I'm on a Superdeal/Flexiplus setup where there are three tariff prices:
Day @ 20.77p / kWh
Night @ 10.99p / kWh
Stored heat @ 9.92p / kWh

Storage heaters heat the old part of the house and a new extension has a wet underfloor setup powered by the ASHP
• Posts: 19 Forumite
The usage figure i'm quoting is the kW used by the heat pump. Not kWh.
• Posts: 8,688 Forumite
edited 29 November 2017 at 12:15PM
Energy consumed by the heatpump will be measured in kwh, ie if it uses 1kw and it's on for one hour then it's used 1kwh. Your leccy meter measures in kwh

Heat energy produced by the heatpump is also measured in kwh and is done by using a heat meter which measures the temperature of the water going into the heatpump, the temperature of the water leaving the heatpump and the volume of water flowing through the pump. These are integrated together which results in the quantity of heat produced by the pump.

The COP of the heatpump is ithe difference between the energy supplied to the heatpump from the electricity supply and the heat energy output from it. So if it consumes 1kwh of leccy and produces 3.2kwh of heat it has a COP of 3.2.

The quantity of energy produced by the heatpump varies with the outside temperature and the flow temperatures so isn't a constant.

Most heatpump COPs are spec'd at 7 degrees ambient to 35 degrees flow (7/35) and a change to either of those temperrtures will result in a different COP.

If you assume that your heatpump has a COP of 3 then your 2682kwh of leccy should have produced about 8000kwh of heat which means you are paying the equivalent of around 7p/kwh during the day and about 3,6p/kwh at night assuming you get some benefit from using it from the off-peak supply.

I'm interested in how you are measuring the consumption - is it separately metered on it's own circuit, indepenedant from everything else or have you got an energy monitor connected to it?
Never under estimate the power of stupid people in large numbers
• Posts: 29,048 Forumite
edited 29 November 2017 at 10:40AM
Stan79 wrote: »
Hi there,

The 2682 figure is just a kW figure.... not a kWh figure!

Stan

Believe me the figure of 2,682 used over 87 days is kWh!

Your heat pump could be rated at anything between 9kW and 14kW

As said above, the price you are paying for electricity is unbelievably high. There are plenty of tariffs available where you pay around 12p/kWh to13pkWh all day and night i.e. 24/7, and that includes all the electricity consumed in the house - not just for the ASHP.
• Posts: 19 Forumite
Ah... i've found part of the problem... The meter i have been quoting is a meter linked to the RHI payments and measures the amount of hot water produced in kW. What a prat. Sorry to waste your time.

Still no closer to working out how much electricity the heat pump is consuming!!!

Changed tariff today and am now paying 16p day, 8.5p night, 7p stored/heat. Works out to £200 saving based on projection and cheaper than their lowest single rate tariff. Projected bill is £726 for a family of 4 in a 4 bed house.

As i say though, I'm still no closer to working out how much electricity the heat pump is consuming!!!

It's a 10kW model...
• Posts: 8,688 Forumite
If you are having to provide meter readings to get RHI then you should have two meters, one is the heat meter which works out how much energy your unit is producing and you should also have a separate electricity meter to record how much electricity you are inputting. so your RHI should be based on the difference between the two
Never under estimate the power of stupid people in large numbers
• Posts: 19 Forumite
I don't have to provide meter readings. I think the fitters installed it "just in case"...

Unless the control box for the heat pump logs kWh usage? I will have to have a look!
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