2025 Data Analysis & Review

Introduction

This year I made no big changes:

• The first full year of solar power
• The first full year with a smart meter

How does consumption compare to last year? Have the savings been maintained?

 

I have covered the solar ROI separately  Solar#10: 12 month ROI Sept 2025 (100% with a smart meter). This also covers the limitations of the GivEnergy native scheduler.

 

During the year we had a couple of new issues:

• Honeywell Evohome controller 'crashes'
• GivEnergy system reporting spurious energy figures

I will cover these issues as well as the customer support provided by the two companies.

 

Data Source: See notes at the end of this post.

 

Comparison with Previous Years

My electricity bill is now slightly negative - more on that in this post: Solar#10: 12 month ROI Sept 2025 (100% with a smart meter). It is worth noting that my consumption from the grid in £'s was less than last year (the blue arrow on the chart), but actually most of this went to the battery. Our home consumption was more or less the same as the year before - fed from the battery and direct from the solar panels.

 

Gas consumption was similar in the last 3 years. In my analysis of 2024 I put forward the theory that the best comparator year for lifestyle/heating demand is 2020/21, when we were both (working) at home during the pandemic and now we are both retired and at home. In both cases we had the heating on all day for much of the year. My theory is that the combination of better insulation (attic and pipes) and weather compensation on the boiler accounts for the saving in cost for heating our house to about the same temperature, but over the full day:

 

	kWh	£'s	Energy saving measures	Notes
2020/ 21	35,800	£2,556	Smart radiator controls	At home during the pandemic
...
2023/ 24	19,500	£1,400	Pipe and attic insulation and weather compensation installed on the gas boiler	At home retired
2024/ 25	18,500	£1,336	Nothing new	At home retired

 

My theory is a £1,200/17,500kWh saving in gas from a combination of chiefly attic insulation and weather compensation, with a bit of help from insulation of pipes. I also think that additional insulation has helped in the savings from weather compensation by allowing lower flow temperatures.

 

Note: 21/22 and 22/23 do not fit neatly into this pattern, but are perhaps explained by (partially) improved insulation and my wife being at home while I am at the office (& she was more frugal with heating in the daytime!)

 

Do Winter Holidays Have a Significant Impact?

In 2020/21 we were not allowed any holidays in the winter (COVID) - we did manage a week in Scotland in the summer but that would have had negligible impact on gas consumption. So essentially we were heating our house all winter.

 

In 2023/24 we had no winter holidays, again essentially no impact on heating bills.

 

In 2024/25 we had about 12 days away in November '24 and another week away in late January:

 

Month	Days Heating Off	Average daily gas saving (kWh)	(very) approx house warm up energy (kWh)	Approx energy saved (kWh)	Approx £'s saved
Nov '24	11	85	70	883	£59
Jan '25	6.5	108	108	583	£39

 

 

The total 2024/25 holiday gas savings of 1466kWh (883+583) translates to £98, ie if we had not been on holiday then our gas consumption in 2024/25 would have been just over £1,400.

 

It surprises me that the costs between the two years are so close, after holidays are accounted for. I would have thought that other differences (weather, gas fire use, cooking, etc) would have been significant. My expectation is bigger differences in future years, after accounting for winter holidays; we will see.

 

Return on Investment

I have calculated the return on investment for my solar system (see Solar#10: 12 month ROI Sept 2025 (100% with a smart meter); how does  this compare to other investments?

 

Grouping loft insulation (£1,160), central heating pipes insulation (£120) and weather compensation (£205):

• Total costs: £1,485
• Total saving: £1,200/year

(Note: this savings figure is a comparison of gas power consumption 20/21 Vs an average of 23/24 and 24/25, using a single, recent tariff rate to convert kWh to £)

 

I used the same methodology as for my solar system, ie what if, rather than spending money on insulation/energy saving, I invested this money in a financial product, eg a stocks and shares ISA, what return would I need to make to pay the additional gas bills? See Solar#9:12 months ROI for more details on the methodology. I also provide a payback period (in my view this is less useful, but some people like it…)

 

	Invest-ment	Annual Saving	ROI %age	Payback period (years)	Maintenance / Lifespan
Solar power with battery	£13,500	£1,200	7.9%	11.7	£5,000 at year 12 (replacement battery and inverter) and I assumed a system life of 25 years.
Insulation and weather compensation on a gas boiler	£1,500	£1,200	83%	1.2	£205 for new weather compensation with a new gas boiler at year 12. I also capped the 'system' life at 25years, but I guess that insulation should last longer than that.

 

What About CO2 Emissions Reductions?…

 

My solar system installer quoted 770kg/year of saved CO2 emissions. However, I am not sure how this was calculated. Does it assume that each kWh of power produced saved a kWh of gas burning at a power station? Maybe more because the efficiency of converting gas power to electricity and then transmitting that to our house is <100%. Maybe less because a good %age of electricity generation is from non-CO2 producing ('green') means. Or perhaps you consider that, with a battery, our home consumption is pretty much all from solar and off peak electricity and so mostly 'green' and at peak times (when electricity production is least 'green'), we are often exporting 'green' electricity from our battery. In the end, I don't think that I can calculate a meaningful figure for CO2 emissions reduction, and can only make a qualitative statement, that our solar system is contributing to reducing CO2 emissions from electricity production and (I assume) reducing peak demand on the grid.

 

Reducing the amount of gas we burn is different. Google gives very specific numbers for CO2 production from burning gas in the UK - an average of 0.184kg/kWh. The £1,200 saving that we have seen is based on a use of £0.067/kWh, ie we have reduced our gas consumption by around 3,300kg.

 

Is this significant? I see different figures quoted for the average UK CO2 emissions per person:

• 4,420kg (2023) https://en.wikipedia.org/wiki/List_of_countries_by_carbon_dioxide_emissions_per_capita
• 10,000kg (2023, includes aviation, shipping and imports/exports) https://www.carbonindependent.org/23.html
• 6,000kg (2023, excludes aviation, shipping and imports/exports) https://www.carbonindependent.org/23.html

 

So with two of us sharing our reductions in emissions, our comparable savings have been:

• 385kg/person/year from solar system based on solar installer's figures
• 1,650kg/person/year from reductions in gas consumption

 

My view: a solar system gives a reasonable return on investment and gets noticed by your neighbours, but boring things like insulation and weather compensation for a gas boiler, in comparison, give an astonishing return on investment and reduction in CO2 emissions.

 

Other changes

In the press I see discussions/questions like:

• Should you keep your heating on all day or turn it off when you are out? (eg https://www.moneysavingexpert.com/utilities/energy-saving-myths/)
• Should I turn off the heating in rooms that I do not use? (eg https://www.express.co.uk/life-style/property/2152619/heating-home-stay-warm-winter)

One source of information that makes a lot of sense to me is heatgeek.

 

First Some Simple School Physics:

Our house is close to a simple cuboid shape - we heat the inside and heat escapes though the outer surface:

• The rate that heat escapes is proportional to:
1. The external surface area
2. The amount of insulation on the outside surface
3. The temperature difference between inside and outside.

And ultimately what we are paying for is the heat that escapes from our house, so the less heat that escapes then the less we pay on our gas bill.

• The time that the house takes to warm up is proportional to:
1. The level of insulation (a better insulated house warms up quicker)
2. The level of heat input (weather compensation reduces the radiator flow temperature so purposefully reduces the heat input to the house, but increases the operating efficiency of the boiler)
3. The heat capacity (ie the amount of air and stuff, like furniture and walls, that we have to heat up)

 

Turn the heating off or leave it on all the time? Well the less that you heat your home then the colder it will be on average and the less heat will escape (rule c above). However, if your house takes a long time to warm up and you are not out for very long (and if you want to keep comfortable and return to a warm house) you will not let it cool down much and you will not save much. So what do we do:

• Before 2022 our flow temperature in the radiators was ~70°C and the insulation in our house was OK but not great. Even on a cold day the house would warm up in a few hours. If we were out for most of the day (eg at work), then the heating was turned off and timed to turn on before we returned (to a nice warm house).
• From 2023 we have left the heating on all day (it is off at night, 10pm to 6am), with mostly lower flow temperatures, and only switched off if we are away for more than one night. When we return from holiday we switch it back more than 24hours before we return. You can see from the graph in the Introduction that this costs less than the on/off approach in 2017/18 BUT, this relies on much improved insulation and weather compensation that controls radiator flow temperature between around 20°C and 66°C, so that our condensing gas boiler is operating much more efficiently (ie it is mostly in condensing mode as opposed to at  ~70°C where it is probably not in condensing mode).

 

In summary: if you want to heat your home all day (eg you work from home or are retired) then low flow temperatures are a good idea. If you go out to work, then, my experience, is that high flow temperatures and the heating off for most of the day costs more (less heating but the boiler is now working less efficiently), but in my case we also had less insulation with the on/off approach. If you switch of the heating during the day, and you have low flow temperatures, then your home will take longer to warm up. This means you will need to switch the heating back on well before you get home to allow the house to warm up, ie this is the cheapest way to run your heating system, but the additional saving from switching off the heating, for not very long, is not much.

 

Heat all the rooms? The big sell, for me, of the Honeywell Evohome system was that we need not heat all of our rooms. We are empty nesters in a typical 4 bedroom, 1970's UK house, so that seemed like a good opportunity to save money. Having  seen material on heat geek and not having seen (much) saving from this approach (summer 2020 to summer 2025) . Here is my logic/understanding of the physics:

 

Our house is approximately a cuboid. This has quite a low (good) surface area for the volume/space inside. If we stop heating one bedroom (~¼ of the upstairs) then the volume is reduced (see rule 3 above - the house has a lower heat capacity), but the surface area has not changed:

 

Our house approximates to a cuboid with 4 rooms on each floor, each room is roughly the same size:
Heated space, heating all rooms	Heated space, not heating one bedroom. Heated space is reduced BUT the surface area of the heated space is the same

 

Not only is surface area of the heated space unchanged, but the surface now includes badly insulated internal walls/floor, so that rules b & 1 comes into play and we actually increase the lost heat from the rooms that we are heating and slow the rate of heating.

 

Last summer (2025), I increased the temperature in the unheated rooms so that they are the same as the rest of the house (in fact it was not that extreme, the unheated rooms were set to 16°C or 18°C and are now set at 20°C). It is too early to see if this changes our heating bills, but it has made the house more comfortable/consistent temperature and we make better use of the once chilly, unheated rooms. Early results show that we are using a bit more gas this year (eg ~12% more in December), but it is still hard to tell with holidays, etc. So perhaps, based on this early data, not heating some rooms does save some money, but maybe less than I would have anticipated; it does not cost much more to heat all rooms and get the extra comfort and utility. I assume that savings depend on the heat insulating qualities of internal walls and if the external surface of the heated space is reduced (in the extreme if you stop heating an outbuilding then you will make savings!).

 

It does also look to me that our central heating system is not well balanced with some rooms heating up very quickly and others taking much longer. An experiment for next summer is to see if I can get closer to ideal radiator balancing where all rooms take the same time to heat up. My expectation would be a small reduction in energy consumption and increase comfort - we will see.

 

In the end, I think that not heating a room does give some saving, but much less than I had hoped for. 

 

Honeywell Evohome controller 'crashes'

Since writing up my experience with the Evohome heating control system and a more technical commentary in 2024 I have experienced a few 'crashes' on the Evohome controller:

 

Evohome Controller after a crash. All the room temperatures show as "-.-". The LEDs on the relay boxes (BDR91) also showed red indicating a communications fault.

 

I learnt by trial and error that it was easy to recover from a crash by removing the batteries from the controller to force a hard reset. It then took around 30mins to re-establish communications with each TRV. The frequency of crashing is around every 2-4 weeks. This meant that after a holiday we sometimes return to a cold house.

 

I raised a support request via the Evohome website. The support from Evohome was fantastic: Within a day or two I was on a scheduled call with Claudia (based in Romania, but speaking perfect English). She ran through a few possible issues and things to try (eg new batteries in the controller) and was able to confirm that my firmware was up to date. The crashes continued and we spoke again. Her suggestion was to do a factory reset and run through all of the system settings as if it were a new installation…

…this was the middle of winter (24/25) and we agreed to do this in  the summer (25) when we could live without heating if there were problems.

 

Summer (July) 2025: Claudia spent 1 hour, 10minutes with me on the phone as she guided me through a complete system set up, including all the radiator TRV's etc.

 

At the time of writing (Feb 2026) there have been no more crashes & we have just returned from holiday to a nice warm house. I assume that the issue was a parameter that I had adjusted in error trying to recover from another fault (as I was learning how to operate/support the system myself).

 

GivEnergy Battery & Inverter System Reporting Spurious Energy Figures

I have had two issues with my GivEnergy solar/battery system, in both cases support has been poor:

• After the 1st few weeks' of experimenting (~April 2024) the disabled battery issue became clear. The support desk were not aware of this issue and in the end told me that I needed to build my own home automation system to resolve this (at odds with their marketing materials on the GivEnergy website that waxes lyrical about how the system is optimised and no mention of needing a DIY automation system). My installer told me that I had gone into this in more depth than other customers and were not able to help, other than lodging a request with GivEnergy support. After a dead end with GivEnergy support, I spoke with senior management at GivEnergy, I was put onto one of their senior engineers, who told me that this was a known issue and was on the list for their development team to resolve. He gave the impression that it was not high up the priority list and, now, around 12 months later, it is still not resolved.
• Currently the GivEnergy system is giving spurious figures in energy graphs. I reported this to GivEnergy in December 2025 and they promised a 1 week response to my e-mail. I heard nothing and phoned in January; the response to the phone call was pretty good. The customer support representative acknowledged the issue and said that he was 'speaking with the Technical Team the next day'  and would get back to me. I have heard nothing more (mid-Feb). Looking at the GivEnergy online forum it seems that others have the same issue and there is even a proposed solution from GivEnergy that I (and others) have tried and does not work for me (& others).

 

General Comments on Support

Both GivEnergy and Evohome are selling a relatively expensive piece of hardware, with embedded software. This is sold via an installer and the installation is a significant part of the cost to the consumer.

 

Both products have a fairly complex configuration. The documentation for both is limited:

• GivEnergy: the assumption is that the user-configuration/scheduling is self-explanatory; however, while some great software is self-explanatory and does not need a manual (eg Google search), the GivEnergy product is not in this category.
• Evohome: The documentation for the user to schedule their heating is fine; however, system settings and trouble-shooting are not covered.

 

Both suppliers assume that you will make the installer your first port of call for issues. This seems unrealistic. In my case my feeling is that both of my installers have limited knowledge of the embedded software and in most cases I assume that, at least after the 1st 12 months, the installer would want to charge for their support work. This is not unlike someone who sells you a washing machine, or a car except:

• For both cars and washing machines there is a manufacture sponsored repair network and also independent repair shops:
• We have a local independent kitchen appliance company who charge £60 for a callout and I have used them to fix our washing machine and dishwasher in the last few years. In both cases they knew what they were doing and fixed them quickly on the 1st visit with just the call out charge and in one case and extra £10 for a part.
• We have used both car manufacturer franchises (Nissan, Jaguar, Mini…) and independent repair shops and have mostly found that they are knowledgeable on the product and are able to make a repair.
• There is both manufacturer and third party materials and spare parts available if I want to attempt a DIY fix. 
• For Evohome and GivEnergy, based on the warranty period support and expertise with the software, I would not trust either installer to be able to fix these issues within a sensible time period or cost.
• Both the GivEnergy and the Evohome issues have their root cause in poor documentation and (in  the case of GivEnergy) code in the system software. These are not issues that the installer can readily resolve. Both kitchen appliances and cars seem to be more  robustly engineered with complete documentation/training that enables the service engineer to quickly resolve issues. I suspect that relatively limited market adoption/maturity for Evohome and GivEnergy products is really behind their support limitations.

 

My view: if 'green' technology is to be universally adopted, then suppliers will need to get to the level of washing machine, dishwasher, car suppliers (Bosh, Neff, Nissan…); making products that run without issue for several years and, when they do go wrong, repairs can be quickly made and at a reasonable cost (relative to the purchase price) without any significant effort or knowledge on the part of the user.

 

Notes on graph data

The data in this chart has a variety of sources:

• Gas consumption: calculated from the kWh consumed in the period and my 1st Oct 2024 Octopus tariff - kWh from my meter readings and for Sept 2024 - Sept 2025 from Octopus website data download for my account.
• Electricity consumption to September 2024: calculated with the kWh consumed and the 1st Oct 2024 Octopus import rate (kWh from meter readings to March 2024 and then from the GivEnergy app - home consumption)
• Electricity (import and export) Sept 2024 onwards from my Octopus bills with the actual rate at the time

The logic here is that for gas & electricity without solar, then my only control was via energy saving and it is better to normalise the tariff to better see the impact of energy consumption changes.

With the addition of the solar and battery system then there are options for tariff choice and scheduling that are as important as any energy consumption (total kWh) changes, so I am using actual £'s charged.

See 2024Data Analysis for details of the October 2024 tariffs.

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).