September Solar Generation and Export Numbers
September brought a significant change for us as we switched our export tariff from the flat 12p Octopus Outgoing rate to Agile Outgoing on the 9th of the month. This means our export rates now fluctuate every half hour. One half hour slot reached as high as 40p per kWh. It is a notable shift that will influence how we manage our battery exports moving forward. We also enjoyed five hours of free electricity via the Octopus Weekend Happy Hours, which saved us £5.75 for the first four hours while we topped up the batteries and charged the car. Additionally, we completed 14 power down sessions worth 78p. As autumn settles in, the output from our west panels has begun to decline significantly.
Solar Generation
Our total solar generation this month reached 390.00 kWh, which is a decrease of 228.80 kWh compared to the 618.80 kWh we produced in August. While it was a quieter month for production, we still managed to generate more than the house consumed on two separate days and exceeded our imports from the grid on three occasions. Our solar coverage for house consumption sat at 61.03% this month, reflecting the steady transition into the lower-production months.
Battery Performance
We utilised 249 charging slots this month, with 122 suspended slots where the battery stopped charging because it was more cost-effective to use the grid during those specific windows. The battery was charged by 456.60 kWh and discharged by 480.90 kWh over the month. The average grid slot cost came to 9.79p, with the cheapest import slot reaching 8.00p and the most expensive hitting 33.32p.
Grid Usage
The grid import seen by the inverter alone was 595.90 kWh, while the total measured by the Octopus smart meter including car charging was 677.29 kWh. Of this total, 85.95% was charged at the low overnight rate, 6.57% was charged during Smart Charge slots, and 7.48% was charged at the high peak rate. We also saw the heat pump usage rise slightly to 55.95 kWh, which is a 5.62 kWh increase compared to the 50.33 kWh used in August. Our car charger accounted for 97.97 kWh of the total imported energy.
Cost Savings Summary
Our exported energy reached 248.90 kWh, which is 163.20 kWh lower than the 412.10 kWh we exported in August. This drop follows our solar generation decrease, as house consumption actually fell by 67.50 kWh compared to last month; therefore, the reduction in export was driven primarily by the lower solar output rather than lower demand. Our export savings were £48.47, which is £2.08 lower than the £50.55 saved in August. When comparing our imported kWh to our tariff and the price cap, we achieved savings of £146.44, while our savings when comparing house consumption to that same baseline were £130.94. The monthly bill including export was £35.90.
Grid Usage by Rate
Electricity Usage By Kind
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.
For some reason it was a complex Octopus billing month so I have added all three
Octopus Energy bill
Octopus Bill
Octopus Bill
| 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 |
| 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 |
| Metric | Date | kWh |
|---|---|---|
| Best Generation | 5 September 2026 | 20.00 |
| Best front generation (pv2) | 1 September 2026 | 13.90 |
| Best back generation (pv1) | 5 September 2026 | 7.80 |
| Worst generation | 13 September 2026 | 6.60 |
| Worst front generation (pv2) | 8 September 2026 | 2.90 |
| Worst back generation (pv1) | 13 September 2026 | 2.70 |
| Metric | Date | kWh | Profit |
|---|---|---|---|
| Best Battery and Solar Export Profit | 6 September 2026 | 15.90 | |
| Best export profit | 22 September 2026 | £2.56 | |
| Cheapest Day | 22 September 2026 | -44p |
| Metric | Date | Time | Exported | Rate |
|---|---|---|---|---|
| Best Export Slot | 22 September 2026 | 18:30–19:00 | 1.12 kWh | 43.25p |
| Worst Export Slot | 17 September 2026 | 14:30–15:00 | 0.51 kWh | 2.06p |
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 2026
Showing generation, battery discharge, battery charging, export, import to use and consumption for 2026
Energy Breakdown
One of the best features of my Home Assistant setup is this level of visibility. This chart breaks down our entire home's power profile, from high-draw circuits like the immersion heater and car charger to specific devices like the TV and office UPS systems. It’s a powerful way to monitor our daily habits and see how different parts of the house contribute to our overall energy footprint.
Home Assistant Individual Device Usage and Energy Flow
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
Intelligent Octopus Go
The costs shown here do include the standing charge. Any comparisons are compared to the unit cost if we were not on IOG. 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.
| Standard Tariff | 26.11p | |
| Standing Charge | 52.19p |
| Cost from the grid | £68.19 IOG | £182.34 Std |
| Savings including export | £146.44 | |
| Monthly bill including export | £35.90 | |
| Exported (Agile Outgoing Octopus) | 248.90 kWh | £48.47 |
| Charging slots used | 249 | |
| Battery Suspended Slots | 122 | |
| Average unit slot price | 9.79 | |
| Cheapest slot | 8.00p | |
| Most expensive slot | 33.32p |
| House consumed | 639.00 kWh | |
| Car charger | 97.97 kWh | |
| Heat pump | 55.95 kWh | |
| Immersion | 5.37 kWh | |
| Air conditioner | 0.00 kWh |
| Solar generated | 390.00 kWh 2026 | 403.30 kWh 2025 |
| Solar vs last year | -13.30 kWh | |
| Battery Charged / Discharged | 456.60 kWh | 480.90 kWh |
| Percentage generated by solar | 61.03% | |
| Self-sustained days (solar > consumed) | 2 | |
| Self-sustained days (solar > imported) | 3 |
Here is the Octopus Watch report summary
Octopus Watch does not report smart charging costs at the cheap rate. It uses the full unit cost.
Octopus Watch Billing Month Review
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.