Solar #10: 12 month ROI Sept 2025 (100% with a smart meter)

 

Introduction:

The smart meter was fixed in Sept 2024 and has now been in operation for 12 months...

…so now we have full 12 months on Octopus Flux tariff and the inverter/battery schedule has been somewhat optimised for this tariff.

 

So what return are we getting?

 

I have used the same ROI calculation methodology as in my previous post Solar#13:12 months ROI

Octopus Flux Tariff

 

This changed each quarter during the period.

 

	Period	Oct-Dec 24	Jan-Mar 25	Apr-Jun 25	Jul-Sep 25
Buy 'Peak' (p/kWh)	4pm to 7pm	35.017	35.498	39.103	39.103
Buy 'Day' (p/kWh)	Outside peak & flux	25.012	25.355	27.931	27.931
Buy 'Flux' (p/kWh)	2am to 5am	15.007	15.213	16.758	16.758
Standing charge		49.983	49.983	49.623	47.341
Sell 'Peak' (p/kWh)	4pm to 7pm	26.197	26.557	29.254	29.254
Sell 'Day' (p/kWh)	Outside peak & flux	12.871	13.047	10.333	10.333
Sell 'Flux' (p/kWh)	2am to 5am	4.593	4.656	4.656	4.656

 

Baseline Tariff: "Flexible Octopus" (what I would have paid without solar)

	Oct-Dec 24	Jan-Mar 25	Apr-Jun 25	Jul-Sep 25
Buy (p)	23.49	24.15	26.65	26.65
Standing charge (p)	0.476	0.476	0.4789	0.4789

 

Schedule:

This was fixed through the year except for:

 

• One charge parameter - %age charge at the 2am to 5am cheap rate. See section below on weather and seasonal parameter adjustments.
• Discharge duration - see section below.

 

I adjusted these a few times over the year:

 

• AC 1 Charge  Upper SOC% Limit: 100% in winter, 20% in high summer (start and end times of 2am to 5am were fixed)
• Discharge 1 setting, Discharge 2 Setting - Start Time and End Time  (Lower SOC% Limit was fixed at 4% to avoid disabling the battery - see section below on 'Discharge Duration') - total discharge time between 30mins (winter) and 90mins (summer) of discharge

 

Unfortunately GivEnergy do not provide a manual that describes how these charging parameters function. In summary:

• Outside of charging and discharging I used ECO mode
• The battery was set to use the full range from 4% to 100% and to charge and discharge at the maximum rate.

 

Weather and Seasonal Parameter Adjustments:

With the Octopus Flux tariff it is important to set the overnight/cheap rate charge %age based on the next day's expected solar generation:

 

Why?

If you charge too much at cheap rate:

If you fully charge the battery at the cheap rate and sun shines you can end up exporting to the grid during the day time rate, you have in effect bought electricity 15p/kW/h lost around 10% in the battery and the re-sold at 13p/kWh

If you don't charge enough at cheap rate:

If you do not charge your battery fully at cheap rate ahead of a gloomy day and so end up with <100% charge at the start of the peak rate you miss the chance to export ie you could have bought for 15p/kWh, lost around 10% in the battery and then sold for 27p/kWh

 

In other words it is worth getting the level of overnight charging right based on the forecast for the next day.

 

Unfortunately, while the GE inverter does include a solar generation forecast in the cloud app this is not integrated with the scheduling function. I tried 2 approaches:

• Manual adjustment each evening of the "AC Charge 1 Upper SOC% Limit" parameter. For a predicted gloomy next day this should be 100% and for a sunny summer day this could be just 20% (just enough to power the house from 5am through to the decent solar production, say, ~9am in summer)
• Seasonal adjustment: just keep an eye on  the battery charge at 4pm
• if this is frequently more than 100% then the cheap rate charge should be reduced toward a minimum of (in my case 20%)
• If this is frequently  below 100% then increase charge at night toward 100%

After a few attempts at the daily manual method I gave up - it was just too time consuming and error prone (eg me adjusting the wrong parameter or the weather forecast being incorrect). I settled on the seasonal approach with:

• Winter: 100% charge at night
• Spring (around the spring equinox on 21st March) decrease to 60%
• Around May decrease to 20%
• Around autumn equinox (21st Sept) increase to 60%
• Around November increase to 100%

While this could probably be optimised, I used this for around 11 months and it seemed to work reasonably well and did not require too much effort.

 

Discharge Duration:

The GivEnergy systems behaves counterintuitively and makes it impossible to get a robust, optimum discharge schedule for Octopus Flux. 

The issue is what the inverter/battery does on discharge when the "Lower SOC %Limit" is reached. Consider these situations:

1. At 4pm the battery is fully charged, we are out during the entire peak rate with a fairly steady 300W is being consumed by the house
2. After a gloomy winter day, despite, a full charge the night before, the battery is at 50% charge by 4pm and we now are cooking lasagna in the electric oven and various other electrical devices, say 1.5 kW average consumption.
3. The same gloomy winter day and 50% charge at 4pm. We are out and come back at 6pm and stuff needs doing (dishwasher, oven, ironing) - we can easily max out the 3kW of the battery output.

When I was first attempting the schedule the inverter/battery I had assumed it worked like this:

 

My assumption for discharge state logic:  when end time or lower %SOC is met it returns to ECO

 

There is no manual and this operation seems intuitive and logical to me!

 

This would allow a series of, say, 9 x 20min discharge periods with progressively lower %SOC limits. If this were the case then in situation (a) the battery would progressively discharge through peak rate. However, in cases (b) and (c) the 1st few discharge periods would be skipped as they would be at the lower SOC% limit and then jump straight back to ECO mode. This would be very simple and robust to both the weather/SOC% at 4pm and also to in house consumption during the 4pm to 7pm peak rate period.

 

Unfortunately this mode of operation is just my dream. I have discovered by experiment (and confirmed with GivEnergy) that the operation is like this:

 

Actual discharge state logic:  if the lower %SOC is met then the battery is disabled before returning to ECO mode at the end  of the discharge period

 

What this means, is that, if I set the lower %SOC limit to anything above 4%, then I risk disabling the battery at peak rate - exactly what I don't want.

 

The answer (& it is not a good answer) is to do time based scheduling. One thing in our favour is the Octopus Flux tariff does help with robustness to some extent:

• If we have charge remaining at 7pm and use this in the house then we save importing at 25p/kWh
• If we had a more aggressive discharge schedule and exported this electricity at peak rate, then we would have been paid 26.197p/kWh

Ie, while it is slightly better to export the electricity at peak rate, it is super-important to have enough battery capacity to last through to the end of peak rate AND, even if we are away for the night we will use ~300W for 7hours before cheap rate starts (~2.1kWh), so not too much of an issue to use some remaining charge after peak rate. We just want the battery to be at or close to 4% by 2am so that we can buy as much as possible at the 15p/kWh cheap rate for the next day.

 

Summary: when setting up time based discharges then the discharge slot(s) should be biased to the end of the peak rate period and the level of discharge can leave up to ~26% (2.1kW/9.5kW + 4%) in the battery for late evening consumption, after 7pm, without significant penalty.

 

Quarterly Costs

The figures below are calculated from my Octopus bills.

 

I have then calculated what my costs would have been without solar and on the Flexible Octopus Tariff. For this I have assumed 10.18kWh consumption per day (as previous post)

 

Result

 

	Oct-Dec 24	Jan-Mar 25	Apr-Jun 25	Jul-Sep 25	Total
Estimated home consumption without solar (kWh)	936.4	916.0	926.2	936.4	3,715.0
Actual import with solar (kWh)	1,233.7	1,205.9	551.1	682.8	3,673.5
Actual export with solar (kWh)	529.4	781.5	1,743.6	1,221.1	4,275.6
Estimated cost without solar and on Octopus Flexible (£)	£280	£277	£305	£294	£1,156
Actual cost with solar on Octopus Flux (£)	£124	£98	£-160	£-75	£-13
Return in £'s	£156	£180	£465	£369	£1,169

 

Return as a paybackperiod: 11.7 years, but this is not a very useful figure. More useful is theequivalent return that I would get from a pension fund - around 7.9% return (see Solar#13: 12 months ROI from May 2025 for methodology).

 

The annual return has improved from 6 months earlier (6.6% to 7.9%) - this is to be expected as the smart meter has now worked for a whole 12 months and the period April through May 2025 was remarkably sunny…

…I will keep tracking this and see how we get on next year.

 

Note on Scheduling For Octopus Flux

The April 25 rates have made a subtle difference to previous rates:

 

Charge Battery Too Much at Cheap Rate:

• On a sunny day you end up exporting during the day
• Pre-April 2025: buy at 15p/kWh, lose 10% in the battery and sell at 13p/kWh: 3.3p loss for each kWh of charging that ends up adding to a solar export during the day
• Post-April 2025: buy at 17p/kWh, lose 10% in the battery and sell at 10p/kWh: 7.5p  loss for each kWh of charging that ends up leading to a solar export during the day

Charge Battery Too Little at Cheap Rate

•  Alternatively, on a gloomy day, if you do not charge your battery enough at cheap rate and so end up with <100% charge at the start of the peak rate you miss the chance to export:
• Pre-April 2025: buy at 15p/kWh, lose 10% in the battery and sell at 26p/kWh: 8.6p profit that you could have got from each missing kWh of charge
• Post-April 2025: buy at 17p/kWh, lose 10% in the battery and sell at 29p/kWh: 9.6p  profit that you could have got from each missing kWh of charge

In other words, for our system, before April 2025 it was best to be biased towards fully charging the battery. Only in the summer was it worth thinking about reducing the overnight charge to gain 3.3p/kWh as opposed to the risk of loss of 8.6p/kWh for not charging enough. In April 2025 this became more finely balanced at 7.5p/kWh loss for charging too much and 9.6p/kWh loss of profit for charging too little.

 

Improving on the Native GivEnergy Scheduling

My view is that the GivEnergy native scheduling capability is not optimised:

• Battery disabled on discharge
• No weather forecast feature to optimise cheap rate charging.

I have discussed both of these above. The latter point was made more challenging by the changes in the Octopus Flux tariff in April 2025, that makes the penalties (lost opportunity) for over and under charging about the same.

 

I am also considering buying an EV; EV tariffs add new opportunities and scheduling challenges that are further beyond the capabilities of GivEnergy native scheduling.

 

I see two possible solutions for this:

• Home Assistant installed on a single board computer; while there does not seem to be an off the shelf solution for this there are plenty of enthusiasts who seem to make a success of this approach.
• Wonder Watt: Cloud based scheduler for various inverters, including GivEnergy

Spoiler alert: I have recently got Wonder Watt up and running; once it has been running a little longer I will write up my experience and early results.

 

Conclusions:

In this same year I have had some better performing, but volatile, pension investments (eg S&P 500 tracker - 24.7% return in 2024) and some less volatile, but lower growth investments (eg Royal London Short-Term Money Market Fund - 4.56%)

 

but overall my pension investments have given a better return this year than the solar system (a bit more than 10%, after charges).

 

However, I am also told that a stock market downturn is expected. Perhaps the most comparable investment that I have is a UK government gilt that matures in 2034. If I keep this to maturity then the UK government guarantee that I will be repaid with an overall return of 4.79% per year. So with this as  a baseline the solar investment does look OK (but of course the risks and liquidity are not identical).