My heat pump has been installed just over two weeks now.
I have also, *finally*, got my own monitoring setup. Similar to OpenEnergyMonitor, it uses an electricity meter and a heat meter to monitor what the heat pump is doing.
If you want to read about my heat pump and its installation, I’ve created a dedicated page
The old way
Before my heat pump was installed, I used the immersion heater in my Mixergy hot water tank. Why the immersion? Well, given the cost of gas and my boiler’s 80% efficiency, the immersion worked out to be cheaper when run at off-peak prices. Sure, it was slower, but I figured that sometimes it would be less carbon intensive.
A typical day saw the immersion turn on at 3am and run for between 1 and 2 hours, sometimes longer!
Here is a graph from the 18th of September 2025.
It shows the tank running for one hour and forty-five minutes, heating the tank up to 75% of its charge. That’s about 158L of water. The immersion consumed 5.4kWh of electricity. My boiler would have used about 6.5kWh of gas for the same job, but it would have been much quicker.
The new way
Here is another graph, from Mixergy, showing the 18th of September 2026.
This run took an hour, and the tank was fully charged. That’s about 210L of hot water.
You’ll also notice the tank reports 0kWh of Hot Water Energy Usage. That’s because it doesn’t know what the heat pump is doing.
If we look at my monitoring, it shows the heat pump delivered 8kW, but only 2.5kWh of electricity used!
Making direct day-to-day comparisons is difficult, but the starting conditions aren’t always the same. The immersion started its run with some hot water already in the tank. The temperature at the top of the tank was ~35°C, compared to the ~21°C for the heat pump.
But the numbers roughly hold. If the immersion heated the tank fully, it would have needed ~7kWh of electricity. The maths also shows that. To raise 210L of water from 19° to 50°C requires 27,250kJ. That’s roughly 7.5kWh of energy.
For gas, with an 80% efficiency, I’d have needed 9kWh to heat the tank.
The cost
My off-peak electricity rate on Octopus Intelligence Go is 6.57p for a kWh of electricity and 6.93p for a kWh of gas.
For 210L of water, at 50°C, it costs
| Immersion | 49p |
| Boiler | 62p |
| Heat Pump | 17p |
The figures pretty much speak for themselves.
The heat pump was 66% cheaper than the immersion and 73% cheaper than gas!
Comparing apples to oranges
I knew my heat pump was going to be cheaper than both my immersion and boiler, but I do want to caveat those numbers. I don’t want you to finish reading this article thinking a heat pump is automatically cheaper to run!
I’m running my heat pump during the off-peak window of my Octopus Tariff. That’s 6.5p vs 27.5p for the peak rate!
If we ran the numbers again, using the peak rate, it’s a different story.
| Immersion | £2.06 |
| Boiler | 62p |
| Heat Pump | 68p |
The immersion, as expected, would cost a fortune. The boiler remains unchanged. The heat pump jumps quite a bit.
During the day, at peak rates, my boiler would be the cheapest way. In fact, we did use the boiler to top up the water during the day as required.
Another factor to consider is the weather. As the outside temperature drops, the heat pump will require more power to heat the hot water. The opposite is true during the summer. Both the immersion and gas boiler are immune to that variable.
I will need more data before I can make a more definitive comparison.
Summary
Overall, I’m pleased to see that my heat pump has started saving me money. It’s also using less power.
It’s 10°C outside this morning, so I think the heating will be going on very soon. Let’s see how that stacks up against my boiler too!
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Thanks,
Tom!
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