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.