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Data Accuracy Problem: Daikin 8kW Heat Pump's Onecta App vs MMI Power Consumption

 
Air Source Heat Pumps (ASHPs)
Last Post by Toodles 6 months ago
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 Nursethescreens
(@nursethescreens)
New Member Member
Joined: 6 months ago
Posts: 4
Topic starter 30/12/2025 8:52 pm   [#2692]

Good Evening.

Has anyone found the following when using a Daikin 8KW ( or other type of) The power input figure on the MMI varies from the information on the ONECTA App?.

I started a log from the beginning of December 2025 ( the ASHP was installed mid September) and going by the input figure for the 1.12.2025 ( 347 KWH )and the input figure for 30.12.2025 ( 601 KWH )  I got the total usage for that period as 254 KWH .

But the ONECTA App showed 354 I understand that unless I use something like Shelly or home assistant the figures will not be Spot on but this does seem a big difference .

iam going to continue with a log / spread sheet going forward so the New year ( January at least ) may have a better outcome.

regards to all .

and a Happy New Year.

Richard.

 


This topic was modified 6 months ago by Mars

   
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Toodles
 Toodles
(@toodles)
Famed Member Contributor
Joined: 4 years ago
Posts: 2808
31/12/2025 7:17 pm  

The data accuracy of the Daikin MMI is somewhat iffy to say the least; Jonathan Tracey produced a Youtube presentation on the subject, he has a Shelley meter in circuit and he concluded the ‘rounding errors’ were one of the villains responsible for the variations! I also came across the following from Homely in the Connect app support notes:

Energy and heat estimation methodology

Close

Heat pump heat output

To ensure a consistent method for estimating heat output across various heat pump brands, we use three key measurements: flow temperature, return temperature, and flow speed. Since heat pumps are not specifically designed as metering instruments, each sensor inherently carries some degree of uncertainty, leading to moderate overall measurement accuracy.

Additionally, assumptions about the working fluid's composition are necessary. Our standard assumption is that the working fluid is water; however, if your system uses glycol or another fluid, appropriate adjustments must be made to the calculations.

Electricity consumption

Most heat pumps do not report electricity usage over Modbus. To ensure a consistent method for estimating electricity consumption across different brands, we derive this consumption from the Coefficient of Performance (CoP) tables published by heat pump manufacturers. By using these CoP tables, along with the calculated heat output, flow temperature, and outside temperature, we can accurately estimate the electricity consumption for each heat pump.

While these statistics provide reasonably accurate and useful indicators of a heat pump's performance, they should not be mistaken for actual meter readings, and Homely cannot guarantee their absolute accuracy. Therefore, they are typically not suitable for sharing publicly, such as on social media.

 

Please note that the power consumption calculations do not account for the power usage of backup heaters, tank booster heaters, circulation pumps, or external heating sources.

(Thank you Homely)

Regards, Toodles.


Toodles, heats his home with cold draughts and cooks food with magnets.


   
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 Nursethescreens
(@nursethescreens)
New Member Member
Joined: 6 months ago
Posts: 4
Topic starter 31/12/2025 8:50 pm  

@toodles 

Good evening.

Thankyou for the response I too have been following Jonathan T and do recollect his comments on this subject and May rerun his video on the subject again . And the inclusion of the homely notes in your response.

Much to learn about this System and a few tweaks here and there , for me it’s not so much chasing cop values but making sure the other occupants in the house are warm ( comfortable) and there’s a reasonable amount of Hotwater!.

And the Electric bill doesn’t exceed what I would of used in Gas and electric combined by to much or anything at all.

 

Happy New Year.

Regards .

Richard.

 



   
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Toodles
 Toodles
(@toodles)
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Joined: 4 years ago
Posts: 2808
01/01/2026 11:40 am  

@nursethescreens Yes, comfort and happiness of the family (and particularly for SWMBO!) is the secret to a contented existence - even if it is at the expense of the COP! Chasing and optimising COP and energy consumption comes second to Family Contentment!!!😉 Toodles.


Toodles, heats his home with cold draughts and cooks food with magnets.


   
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