Solar diverter - flawed return on investment calculations

iotum

I've now had my solar panels installed for over 4 years and so I thought it would be a good point to see how the return on investment projections provided by the installer were measuring up to reality.  I was expecting the sums to reveal that I was ahead of the projection given the recent increase in electricity prices. However, as of October 22 I was actually £240 behind the projected savings.  This required some more investigation to identify the cause of the disparity.

The projection was based on an assumed yield of about 4MWh a year, FIT payments based on that yield, the electricity cost savings based on an assumed 50% self consumption and savings from the iBoost Solar diverter that heats water through our immersion heater. With 4 years of data I can see that we self consume approx 60% of what we generate and that we generate more than predicted - typically 4.4 MWh a year. So how could the projection still be higher than reality?

Our iBoost solar diverter uses approx 1300 kWh of solar generation per year. This amounts to 30% of what we generate.  When you subtract that 30% from the 60% self consumption you're left with a self consumption of just 30%.  i.e. My solar panels are replacing 30% of what I would otherwise be using electricity from the grid for and paying the relevant electricity rate.   The iBoost diverter is replacing heating by whatever method would normally be used to heat hot water which in my case is a gas boiler. Now my boiler is a condensing boiler and is getting on a bit so I've assumed it's only 70% efficient when heating water compared to the immersion heater.  When I look back at the prices of gas over the last 4 years and work out the savings from the diverter then I see a significant difference from the projected savings. e.g. in the first year I saved about £49 but the estimate said I'd save £105. For the 4th year I saved £109 thanks to rising gas prices but still not at the predicted £129 saving.

My installer provided me with the estimated savings along with an epvs (Energy Performance Validation Scheme) certificate.  This certificate states that "the installer has the competency to carry out accurate performance calculations, estimates and monetary savings within a reasonable range for your installation type, as set out in the Scheme rules." In other words the estimates provided to me were based on assumptions provided by epvs. So I contacted epvs. To cut it short, they assume that the savings from the diverter are based on the electricity rate at that time and not the gas rate.  So according to the scheme rules they can just ignore the fact that you currently heat water through your gas boiler and assume you have always heated hot water via your immersion heater. 

I think this is unreasonable given that the vast majority of homes in the UK have gas boilers.  Surely the estimate should take into account whatever method of heating hot water is currently employed in the house at the time of installation of the solar panels.  I've challenged epvs on this but they don't agree. Given they don't appear to have any accountability, I wonder what purpose they actually serve other than giving customers false reassurance.

Given the volatility of the energy market right now I'm not sure when, if ever, the iBoost will recover its costs.  If I was installing a new system right now I'd be looking very carefully at a battery and doing my own estimate calculations rather than relying on those provided by my installer whether they be "assured" by epvs or not.






  

Screwdriva

Batteries still do make sense if you have a heat pump or drive alot in an EV but for the average household, Octopus Flux torpedoed any remaining financial rationale for a battery. 

The hot water diverter's best days are behind it I'm afraid, unless you're using alot of oil to heat hot water. 

gefnew

Why did you not do that in the first place hind sight can be disappointing as you have now realised.

Krakkkers

My 4kwp system also sends about 1300 kwh a year to hot water but i am not disappointed with the savings as where would that 1300 kwh go if not to hot water?
Also the 1300 is not just 1 tank a day, it keeps on reheating the water through the day in a way you would not do with your boiler unless you were being very wasteful, also my immersion heats water to over 70 degrees where i would only use the boiler to maybe 55 max.
Had my iboost for nearly 8 years now.

[Deleted User]

Estimating the RoI on solar is something of a mug’s game. I have solar; battery and an EV. I could argue that I am saving 3500kWh/ year at the Ofgem capped rate but as I have been importing all my electricity at between 5 and 7.5p/kWh for the past 2 years, this assumption would be flawed. I have switched to Flux and I will be getting about 26p/kWh on the 2300kWh/year that my array exports to the Grid. As I write my solar diverter sits idle as I am only paying c.4p/kWh for gas.

Having installed solar and a battery, what is done is done.

ABrass

Solar diverters only made sense when you have the very odd FIT scheme with assumed export or absurdly low SEG rates.

Otherwise the cost to install isnt likely to be paid back.

Alnat1

Krakkkers said:

My 4kwp system also sends about 1300 kwh a year to hot water but i am not disappointed with the savings as where would that 1300 kwh go if not to hot water?

You'd sell it back to the grid for an average of around 25p/kWh and only heat as much hot water by other means as you actually use, saving money. Heating to over 70C will mean more heat is lost (wasted?) too.

Krakkkers

I don't have a smart meter so can't do the export, waste free energy? Lose heat on water you heated for free and will again.
major flaws in your thinking.

Petriix

I've always said that solar hot water diverters are (typically) a flawed investment for exactly the reasons outlined. Further to the points raised here, a common occurrence is for people to (inefficiently) heat their house all summer by diverting far more energy to their hot water than they actually use or need, then they claim it has somehow saved them money.

The maximum you can really say an iboost or similar has saved is the minimum cost of heating the water you actually needed, minus the maximum you could have earned from exporting that electricity. Today an Octopus Intelligent customer could heat their water overnight for 7.5p per kWh and export for 5p so the saving is just 2.5p per kWh.

Then people tend to claim something about CO2, ignoring the fact that exporting solar almost always saves more CO2 than self-consuming it. That's because a domestic gas boiler is more efficient than the round-trip efficiency of a gas power station.

Obviously there are exceptions and edge cases but, typically, these things are a bit of a con. 

Reed_Richards

I don't think you actually say what your projected savings were. @iotum .  If the £240 disparity was based on projected savings of £480 then it would a be a serious error.  If the projected savings were £2400 and you were only £240 off then I would say that was pretty close, for what must always be an estimate.

Mikeyboy01443

I have a hot water tank and was offered a hot water iboost thing on an few quotes but it seemed expensive to me...
So i bought a wifi switch for my immersion heater, i have previously been heating my water on gas but it seems expensive so ive now got my immersion on a timer to heat my water in the cheaper rate on flux...