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COP or Cost Per kWh

bontwoody · 2024-09-10T13:29:27Z

There has been much discussion of how important COP is with respect to running a heat pump, but for many homeowners the important metric is how much it costs to run a heat pump and whether it is cheaper than other forms of heating. Having made some changes to my heat pump set up, I have been very satisfied to see my position in the OpenEnergyMonitor league table improve until I am now close to the top. However, as much as I would love to think I have done a great job with my installation, I know that this current position is misleading. One glaring advantage that I have is that my Domestic Hot Water (DHW) is not heated by my heat pump, instead I chose to go with a Mixergy cylinder heated by an immersion coil. I did this in the knowledge that I can fit a heat exchanger to rectify this and have installed all the pipework and valve to enable it. The reason I have not done so, is that I am on the Intelligent Octopus Go tariff, which currently gives me overnight electricity at about 7p per kWh. This means that the COP leverage I would get from the heat pump is not that substantial financially and I would be looking at a payback period of perhaps 10 years to recoup the cost of the heat exchanger. Another factor is that using the immersion heater, utilises the design of the Mixergy cylinder to its full potential, meaning I don’t have to heat the whole tank as I would with the heat pump, so there is another potential saving there. So to get to my point, is a metric of Cost/kWh of heat produced more important than COP? To illustrate this I took a short period in August and calculated a combined cost/kWh for heating and DHW. Over the period this gave me an average of 7p per kWh of heat, just about on par with gas taking into account a gas boilers efficiency. However this isn’t the whole picture as I have solar panels and a small battery so I wont be paying 22p per kWh in the daytime and this would lower the average cost per kWh. The ratio of house heating to DHW also affects the price, as in the winter, the vast majority of heat pump use is for the former and this will again lower the average. It will be interesting to do the same calculation in 3 months time.

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bontwoody

(@bontwoody)

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Topic starter 17/09/2024 2:33 pm

@majordennisbloodnok Hi, as you say carbon intensity is the best measure if you can use that. What really interests me is why the variation in correlation is so large.

Some days price is a very good indicator of carbon intensity and others not so good. On the 8th Sept there was actually no correlation between the two which is mystifying to me, especially as it had a similar intensity to the 29th Aug which had a good correlation.

I only took one month of data in autumn so its possible winter and summer might yield different results.

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60

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Derek M

(@derek-m)

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17/09/2024 2:52 pm

@bontwoody

The price will be dependent upon which generators are operating, and also how many 'middle men' are taking their cut.

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bontwoody

(@bontwoody)

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Topic starter 17/09/2024 2:59 pm

@derek-m I suppose it was unrealistic of me to expection production and customer prices to track each other. Just look at petrol prices 🙂

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Ringi

(@ringi)

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09/12/2024 7:50 pm

Carbon intensity can be high for some grid zones while its zero for others due to limited connections between grid zones. Marginal price is set nationally, so for example can be high while wind turbines in a zone can't generate due to low local demand.

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Ringi

(@ringi)

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09/12/2024 8:12 pm

If all we cared about was cost per kwh of heat, we can get £0.07 using oversized traditional storage heaters, but on many days we would open windows due to limit control over discharge rate. So clearly we care more about comfort than kwh of heat, for example a drafty home needs to be heated to a higher temperature to feel comfortable.

But can add 30kwh of basic Chinese batteries to an exisiting inverter for a little over £3k, the £3k would not pay to upgrade many radiators and pipework when chasing higher COPs. If the choose is between a replacing a functional DHW tank with a heappump optimized DHW tank or 15kwh of batteries, (assume EV driver) then the batteries are likely to lower per kwh costs more.

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