December Solar Generation and Export Numbers
As expected for December, solar generation fell further as we moved into mid-winter, although performance remained ahead of last year. We generated 67.00 kWh, compared to 53.00 kWh in December last year, an increase of 14.00 kWh year-on-year. Solar accounted for just 8% of total household consumption, which is typical for December. Total household consumption for the month was 852.80 kWh, reflecting higher seasonal demand.
Despite December being a low-generation month, it capped off an exceptional year for solar. 2025 was a record year, with total generation of 4,586.30 kWh, beating 2024 by 104 kWh. The Met Office reported 2025 as the UK’s sunniest year on record, and this was reflected in our system performance. December itself was unusually mild during the first half of the month, helping maintain generation levels. Each year so far, the system has continued to outperform the original MCS prediction, with 2025 exceeding it by over 1,000 kWh.
Battery usage remained heavy as solar contribution reduced. The batteries were charged with 517.80 kWh and discharged 501.50 kWh, showing sustained cycling to minimise higher-cost grid imports. We used 313 charging slots during the month, while the battery was suspended for 171 slots, largely to avoid unnecessary cycling when the grid price matched the Cosy charging rate.
The average charging slot cost this month was 15.04p per kWh, with the cheapest slot at 14.62p and the most expensive slot used at 29.82p. As with November, this strategy ensured the battery was preserved for use during higher-priced periods later in the day.
There were no days where solar generation exceeded household consumption or grid imports, resulting in no self-sustained days by either measure this month.
Grid imports totalled 885.40 kWh. The cost of electricity imported on the Cosy tariff was £104.51. For comparison, importing the same energy on the standard tariff would have cost £234.90, while supplying the household entirely on the standard tariff would have resulted in a total cost of £226.25. This represents a saving of £131.42 compared to standard tariff imports, and £122.77 compared to standard tariff house consumption.
Exports were minimal, as expected for December. We exported 6.90 kWh during the month, delivering £1.04 in export savings. With limited solar availability, most generated energy was consumed directly or used to support battery charging.
Looking at the wider picture, the average unit price paid across 2025 was 16.13p per kWh. For much of 2024 we were on Agile Octopus, but towards the end of that year Agile became less attractive. In February 2025, we moved to Cosy Octopus, which has proven to be a better fit for winter operation and battery-led optimisation. The average price cap cost was 26.29p in 2025.
Overall, December shows the system fully embedded in winter mode. Solar contribution is minimal, battery usage is critical, and savings are driven almost entirely by intelligent tariff selection and charging strategy. While generation is low, the year as a whole highlights just how effective the system has become, delivering record solar output and consistently low average energy costs compared to the standard tariff.
According to the Octo-Aid app this was the breakdown of our usage per rate.
Grid Usage by Rate (High, Mid and Low)
As solar panels output declines over the years I have added the previous years generation to the table at the bottom. Of course weather comes into it as well, but I thought it would be interesting to track. I have added this post that shows each year as a graph.
Octopus Energy bill
Octopus Watch Tariff Comparison Report. This does not include exporting
Best Solar Generation
| Date | kWh | |
|---|---|---|
| Generation | 26th May 2023 | 32.90 |
| Front generation | 14th June 2023 | 19.80 |
| Back generation | 7th June 2023 | 14.70 |
| Worst generation | 12th December 2022 | 0.20 |
Solar Generation December 2025
| Date | kWh | |
|---|---|---|
| Best generation | 2nd | 4.00 |
| Best front generation | 2nd | 3.00 |
| Best back generation | 14th | 1.80 |
| Worst generation | 23rd | 0.30 |
| Worst front generation | 23rd | 0.20 |
| Worst back generation | 23rd | 0.10 |
Best Solar Export and Profit
| Date | kWh | Profit | |
|---|---|---|---|
| Exported kWh | 15th June 2024 | 22.20 | |
| Exported profit | 15th June 2024 | 22.20 | £3.15 (15p kWh) |
| Profit day | 14th April 2024 | 18.20 | £2.67 |
Solar/Battery Export and Profit December 2025
| Date | kWh | Income/Cost | |
|---|---|---|---|
| Best kWh export - Battery & Solar | 4th | 0.30 | |
| Best kWh export - Solar Only | 4th | 0.30 | |
| Best export profit | 4th | 0.30 | £0.05 (15p kWh) |
| Cheapest Day | 22nd | £3.71 |
The graph below shows the generation for both front and back panels
Total Solar Generation
The graph below shows generation, battery discharge, battery charging, export, import to use and consumption for 2025
Showing generation, battery discharge, battery charging, export, import to use and consumption for 2025
Overview for December
The graph below shows the generation for each day
Graph showing production for the month
The graphs below show the generation split between the front and back panels
Graph showing production for the front facing panels
Graph showing production for the back facing panels
The graphs below show the yearly generation split across both the back and front panels
Total Front Solar Generation
Total Back panel generation
Octopus Cosy
The costs shown here do include the standing charge. Any comparisons are compared to the unit cost if we were not on Cosy Octopus. There are two comparisons. One is the cost of what we imported from the grid for battery charging. The other is, the cost of the energy used by the house, house consumption.
December Grid Information
| Standard Tariff | 26.53p | ||
| Standing Charge | 0.48p | ||
| Cost from the grid | £ 104.51 (Cosy) | £ 226.25 (Standard) | |
| Savings including export | £131.42 | ||
| Exported | 6.90 kWh | £1.04 @ 15p kWh | |
| Charging slots used | 313 | ||
| Battery Suspended Slots | 171 | ||
| Average unit slot price | 15.04 | ||
| Cheapest slot | 14.62p | ||
| Most expensive slot | 29.82p | ||
| House consumed | 852.80 kWh | ||
| Solar generated 2025/2024 | 67.00 kWh | 53.00 kWh | |
| Battery Charged/Discharged | 517.80 kWh | 501.50 kWh | |
| Percentage generated by solar | 8% | ||
| Self Sustained Days 2025 (More solar than consumed) | 182 | ||
| Self Sustained Days 2025 (More solar than we imported) | 96 | ||
Here is the Octopus Watch report summary
Octopus Watch summary
Featured posts
Solar panel performance declines over the years. I thought it might be interesting to see how mine compares. Of course weather comes into it, but hopefully this will be a useful guide.