Solar diverter - flawed return on investment calculations

Meatballs

Petriix said:

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 selv-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. 

I agree with most of what you are saying but our grid isn't solely powered by gas power stations, so you'd need to compare direct electric heating with the carbon intensity at the time with a gas boiler.

Average UK grid intensity was 182gCO2/kWh apparently last year. Gas boiler is rated at around 215gCO2/kWh.

So direct electric heating could well be cleaner regardless of whether you are generating solar yourself?

That said self consuming solar isn't really any better than just producing solar, it all decreases your neighbourhood co2 intensity whether you use it or not. 

EcoScruples

Petriix makes a very good point, heating the amount of water you actually need is the most efficient way, almost everyone with an iboost or eddi will heat their water to the max all day long whilst there's sunshine.
I never gave electricity savings a thought when I bought mine as I was trying to avoid using kerosene to allow me more oil in the winter for heating. 

Petriix

Meatballs said:

Petriix said:

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 selv-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. 

I agree with most of what you are saying but our grid isn't solely powered by gas power stations, so you'd need to compare direct electric heating with the carbon intensity at the time with a gas boiler.

Average UK grid intensity was 182gCO2/kWh apparently last year. Gas boiler is rated at around 215gCO2/kWh.

So direct electric heating could well be cleaner regardless of whether you are generating solar yourself?

That said self consuming solar isn't really any better than just producing solar, it all decreases your neighbourhood co2 intensity whether you use it or not. 

But the last kWh of generation (i.e. the kWh you would have removed from the grid demand by exporting it yourself) almost exclusively is generated by burning gas. So the overall grid CO2 is irrelevant unless all the gas power stations are shut down and you don't happen to live in a region with surplus renewable electricity. 

iotum

Reed_Richards said:

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.

My generation is 10% greater than predicted, I'm consuming 60% of what I generate and electricity prices are way higher than the assumed inflation rate. Yet in spite of all of that, I've saved 10% less than the provided estimate. 

But whether it's close or not is irrelevant; the point I was trying to make is that the estimates will always be wrong given they are assuming the diverter is replacing heating water by electricity when my water was heated by gas.   

Screwdriva

Meatballs said:

Average UK grid intensity was 182gCO2/kWh apparently last year. Gas boiler is rated at around 215gCO2/kWh.

Average grid intensity would be a bit misleading. During the peak PV generation months, grid intensity is higher, closer to ~200g CO2/kWh as wind dies down. Cleaner.... but without an increase in PV generation as part of the UK's mix, not by much.

Officer_Dibble

I was given ludicrous savings projections for iBoost when I had my system fitted last year. It must have assumed we were all having hot baths twice a day. Didn't go for it and fitted a smart spur for the immersion heater instead. That paid off last year, but will be of no use while Flux export rates are so good.

Reed_Richards

iotum said: 

But whether it's close or not is irrelevant; the point I was trying to make is that the estimates will always be wrong given they are assuming the diverter is replacing heating water by electricity when my water was heated by gas.   

I take your point about the false assumption about electrical heating of the cylinder but whether it is close or not is relevant because estimates are always wrong because they are estimates.

JKenH

Petriix said:

Meatballs said:

Petriix said:

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 selv-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. 

I agree with most of what you are saying but our grid isn't solely powered by gas power stations, so you'd need to compare direct electric heating with the carbon intensity at the time with a gas boiler.

Average UK grid intensity was 182gCO2/kWh apparently last year. Gas boiler is rated at around 215gCO2/kWh.

So direct electric heating could well be cleaner regardless of whether you are generating solar yourself?

That said self consuming solar isn't really any better than just producing solar, it all decreases your neighbourhood co2 intensity whether you use it or not. 

But the last kWh of generation (i.e. the kWh you would have removed from the grid demand by exporting it yourself) almost exclusively is generated by burning gas. So the overall grid CO2 is irrelevant unless all the gas power stations are shut down and you don't happen to live in a region with surplus renewable electricity. 

I agree with this but then the same argument applies about charging your electric car using your own solar. Had that solar PV gone to the grid instead it would have displaced marginal gas generation. So effectively when you charge your car you are charging it using gas generation or sometimes in the winter even 100% coal fired generation. You’ll never convince EV enthusiasts of this though.

Petriix

JKenH said:

Petriix said:

Meatballs said:

Petriix said:

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 selv-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. 

I agree with most of what you are saying but our grid isn't solely powered by gas power stations, so you'd need to compare direct electric heating with the carbon intensity at the time with a gas boiler.

Average UK grid intensity was 182gCO2/kWh apparently last year. Gas boiler is rated at around 215gCO2/kWh.

So direct electric heating could well be cleaner regardless of whether you are generating solar yourself?

That said self consuming solar isn't really any better than just producing solar, it all decreases your neighbourhood co2 intensity whether you use it or not. 

But the last kWh of generation (i.e. the kWh you would have removed from the grid demand by exporting it yourself) almost exclusively is generated by burning gas. So the overall grid CO2 is irrelevant unless all the gas power stations are shut down and you don't happen to live in a region with surplus renewable electricity. 

I agree with this but then the same argument applies about charging your electric car using your own solar. Had that solar PV gone to the grid instead it would have displaced marginal gas generation. So effectively when you charge your car you are charging it using gas generation or sometimes in the winter even 100% coal fired generation. You’ll never convince EV enthusiasts of this though.

The calculations for EVs are somewhat different because you're not replacing the charging with domestic gas usage. While it's absolutely true that exporting the solar during the day then charging the EV overnight would lead to lower overall CO2 emissions, for me it comes down to money saving:

I can currently charge overnight for 9.5p per kWh while my exports are deemed at 50% of generation regardless of how much I use. I am careful to record the saving at the cheapest rate I could have charged for.

For anyone on SEG payments, the savings would only be the difference between the cheapest charging rate and the best (compatible) SEG rate (with the caveat that you can only currently choose one of a cheaper charging rate or a higher SEG rate).

Deemed exports definitely skew things but it is rather perverse that the incentivised behaviour is collectively worse. Ideally we could have both the high SEG rate and the low EV charging rate, then we'd be encouraged to do the 'greener' thing with a commensurate financial benefit.

But, yes, getting a Zappi charger is unlikely to be a sound financial decision for anyone on metered exports for similar reasons to the iboost.

JKenH

My iBoost failed a couple of weeks before I went on to Octopus Flux but I had already decided I would be heating my water between 2 and 5am rather than with diverted solar PV as we use the water in the morning for showers and having it sat in the tank  for 12 hours cooling down is not dry efficient. 

My problem, now I am on Flux, is that every kWh of energy I use in the house when the sun is shining is no longer free - it could have been earning me 22p or even 35p. That is potentially the same for anyone who has solar panels, as whatever tariff you are on, you are missing out on the export payments you would be getting were you on Flux. 

Last week I generated 177kWh and imported 59.3 of which 83.1 was consumed in the house and 30.2 went into my son’s EV via the Zappi. I exported 122.8 kWh worth just over £30. My total import cost was under £20 so I was in profit for the week by over £10. 

Even had I had enough batteries to time shift all my import into the cheap 9.5p period I would, on Octopus Go, have paid out £5.63 and earned about £5.30 in deemed export so just about broken even but been about £10 worse off than on Flux. (I don’t have batteries so that was hypothetical.)

Go will work out cheaper for some people who do lots of miles and have batteries but for some people, even with both solar and EVs, Flux might still be worth considering. 

Edit: sorry if that has gone off topic but my thought process started with the iBoost and it is relevant to those who do have diverters and high DHW consumption, some of whom may be on Go. It would be cheaper for me to use the oil CH boiler to heat the water but only one of our HW tanks is connected to the boiler - unfortunately not the one we regularly use.