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Hi folks, my brother has a PV install, and has recently had a new (gas) heating system fitted with a large unvented hot water cylinder. His property has Solar PV, and i suggested he should get one of the solar iboost type devices to heat the hot water. Seems keen on the idea, but i'm looking for some pointers on what to actually use?

The Marlec Solar iBoost seems popular, but it has an RF sender for the CT and i'm not keen on that, i'd much rather have a physical wire.

Theres also the SOLIC200 and the eddi units on the market and probably some others i've missed.

Really i'm looking for any pointers on which to get? Peoples experiences etc.The eddi looks nice but bit more expensive and i think the app etc only works if you add the hub as well?

Thanks
 

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What's the payback period on these? Given gas is so cheap at around 2.5p/kWh not sure that these make much sense.
 
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Unsure on the exact payback, i guess theres an aspect of just not using as much gas though despite it being fairly cheap.

To make a bit of a guess, a typical solar install might generate 3000kwh per year with that heavily weighted towards summer. Its likely your exporting a good chunk of that on a typical weekday as theres no-one home (granted this year being a bit of an exception).

if you divert half of that 3000kwh into a water tank, thats a saving of just under £40 based on your 2.5p figure, which sounds fairly meh. However the 2.5p figure isnt perfect, because the boiler isnt 100% efficient, and likely worse so if its firing up from cold just to heat hot water, checking my gas bill we're also paying 2.8p so theres variation there too. Then on the other side theres the ability for the tank to actually absorb that much energy however its a fairly large tank. I suspect its pretty difficult to figure it all out exactly.

The fairly small savings ofcourse means you want to minimise the cost of the diverter too, the solic200 device is only £190 or so, whereas the eddi is pretty much double that.
 

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We have the Solic200 diverter. Works well. Bought it direct from the maker via ebay for about £150. We already had the circuit direct to the immersion heater via a timer, so just added it in to that circuit after turning off the mains supply.
Most gas boilers are only about 50% efficient when just heating water, so actual cost is about double the p/kWh of gas. Therefore, real cost of gas is about 5p/kWh for heating water, plus the higher CO2 emissions. As our average cost on Octopus Agile for all imported electric is less than that, when we have poor solar generation, we can still use the timer to heat up the water when Agile prices are low; usually overnight.
 

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We have the Solic200 diverter. Works well. Bought it direct from the maker via ebay for about £150. We already had the circuit direct to the immersion heater via a timer, so just added it in to that circuit after turning off the mains supply.
Most gas boilers are only about 50% efficient when just heating water, so actual cost is about double the p/kWh of gas. Therefore, real cost of gas is about 5p/kWh for heating water. As our average cost on Octopus Agile for all imported electric is less than that, when we have poor solar generation, we can still use the timer to heat up the water when Agile prices are low; usually overnight.
Do you have any source to say they are only 50% efficient?
 

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Unsure on the exact payback, i guess theres an aspect of just not using as much gas though despite it being fairly cheap.

To make a bit of a guess, a typical solar install might generate 3000kwh per year with that heavily weighted towards summer. Its likely your exporting a good chunk of that on a typical weekday as theres no-one home (granted this year being a bit of an exception).

if you divert half of that 3000kwh into a water tank, thats a saving of just under £40 based on your 2.5p figure, which sounds fairly meh. However the 2.5p figure isnt perfect, because the boiler isnt 100% efficient, and likely worse so if its firing up from cold just to heat hot water, checking my gas bill we're also paying 2.8p so theres variation there too. Then on the other side theres the ability for the tank to actually absorb that much energy however its a fairly large tank. I suspect its pretty difficult to figure it all out exactly.

The fairly small savings ofcourse means you want to minimise the cost of the diverter too, the solic200 device is only £190 or so, whereas the eddi is pretty much double that.
Plus installation cost if you're not qualified to do it yourself.
 

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I'd be looking for reliability. We have two immersion heaters in separate tanks and two power diverter The system installed with the PV had one hard wired and one connected with radio link. The radio link stopped working after a few years and the company who made it had closed down so I couldn't get it repaired easily.

I ended up getting an Eddi at a reasonable price but I only bought it as I already have 2 zappis and we don't have gas so not running the boiler during the summer is a big saving.

They are a very long time paying for theirselves when compared to using gas, but installing solar PV isn't all about the return.
 

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We may be selling our Solar iBoost but it wont be 'uninstalled' for a month. It saved us exporting 4600kWh over 4 years but is no longer active as we have moved to a 3-phase supply.
 

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They are a very long time paying for theirselves when compared to using gas, but installing solar PV isn't all about the return.
If Solar PV doesn't stand up by itself economically it doesn't have much of a future bar a few well off eco enthusiasts. Same with battery storage. Fortunately solar panels do make economic sense now for many as panel prices have come down significantly. Unfortunately these solar diverter devices don't make much sense for most people with solar and a gas boiler.
 
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If Solar PV doesn't stand up by itself economically it doesn't have much of a future bar a few well off eco enthusiasts. Same with battery storage. Fortunately solar panels do make economic sense now for many as panel prices have come down significantly. Unfortunately these solar diverter devices don't make much sense for most people with solar and a gas boiler.
I picked up used panels and did a DIY install. My payback will be soon enough. SolarPV is about clean electricity


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We may be selling our Solar iBoost but it wont be 'uninstalled' for a month. It saved us exporting 4600kWh over 4 years but is no longer active as we have moved to a 3-phase supply.
I’d be interested.


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Do you have any source to say they are only 50% efficient?
Best to look at the SAP rating for your installed boiler. My WB is about 85% efficient hot water only under that test but part of the design was that the chosen tank can accept more than the minimum modulation output of the boiler so there is minimal cycling during recovery. I’ve experimented enough to know it stays in condensing mode until switched off by the tank thermostat.

If you have a period of high demand that exceeds a tank full then recovery time also needs consideration. Our setup essentially never runs out so the choice of shower length/concurrency or bath filling is not restricted.

there’s always the option to set the gas to one temp and the electricity dump to a higher one too assuming you have hot water blending safely designed in.
 

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Best to look at the SAP rating for your installed boiler. My WB is about 85% efficient hot water only under that test but part of the design was that the chosen tank can accept more than the minimum modulation output of the boiler so there is minimal cycling during recovery. I’ve experimented enough to know it stays in condensing mode until switched off by the tank thermostat.

If you have a period of high demand that exceeds a tank full then recovery time also needs consideration. Our setup essentially never runs out so the choice of shower length/concurrency or bath filling is not restricted.

there’s always the option to set the gas to one temp and the electricity dump to a higher one too assuming you have hot water blending safely designed in.
So in general will be much better than 50% efficient for a modern condensing boiler (more like 80%+).
 

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I picked up used panels and did a DIY install. My payback will be soon enough. SolarPV is about clean electricity


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Doesn't apply to most people do who will need to pay an installer to fit their panels. Could still be be a reasonable payback for Solar PV but unlikely for solar diverters if you have to pay someone to install and have a have a gas boiler for hot water.
 

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Doesn't apply to most people do who will need to pay an installer to fit their panels. Could still be be a reasonable payback for Solar PV but unlikely for solar diverters if you have to pay someone to install and have a have a gas boiler for hot water.
Gas boilers have a limited shelf life.New builds are unlikely to have those starting 2025. Days of cheap energy are over unless you capture it.

True most folks will not install PV themselves.

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When doing the sums it's not about how much you divert, it's about how much import you prevent. There's no point diverting excess generation for the sake of it only to waste that heat.

Calculate exactly how much you spend on heating water: that is the maximum hypothetical saving. Then remember that you will only be able to divert enough for half of the year.

We spend around £60 per year on heating water so would estimate the maximum saving from a diverter at £30. Check the length of the guarantee (it's only 1 year on some of the cheapest diverters) and ensure it exceeds the payback period. Remember to factor in the cost of any ancillary components and/or installation costs.

Finally don't mistakenly believe that you are saving any co2: electricity exported in the UK will almost always directly prevent gas being burned at a power station (plus all the associated transport losses). Powering your neighbours fridges etc. is almost certainly better environmentally than heating your own water.
 

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Do you have any source to say they are only 50% efficient?
Maybe it is not quite as low as 50%; that was from looking up the spec for a combi boiler we had at our old house.
where the efficiency for just heating water was actually 48.4%
However, from looking at various models via that link, most appear to be at least 20% less efficient than the quoted overall spec (including CH) when just being used to heat water.

For example, this current model that is only 62.6% efficient when just heating water
 

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Gas boilers have a limited shelf life.New builds are unlikely to have those starting 2025. Days of cheap energy are over unless you capture it.

True most folks will not install PV themselves.

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Gas boilers will still be the dominant heat source for many years to come as the current stock will last for at least another 10-15 years. Solar plus battery will not be sufficient for all heating needs in the winter. Will likely need a hat pump as well. Unlikely solar diverters will make sense on new builds if they have heat pumps instead of boilers.
 

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Gas boilers will still be the dominant heat source for many years to come as the current stock will last for at least another 10-15 years. Solar plus battery will not be sufficient for all heating needs in the winter. Will likely need a hat pump as well. Unlikely solar diverters will make sense on new builds if they have heat pumps instead of boilers.
I agree we need Heat pumps for boiler replacement. PV diversion only makes sense if you don't have battery to charge.
I have SEG approval but no export tariff as I don't have MCS certification needed by Octopus

I try to run consume as much as possible but battery is needed. Diversion in my case would mean I use it rather than export for free

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Also, I would think that most electricity generated by gas is done at a higher efficiency than less than 70%, therefore, producing less CO2 than using your own gas boiler to heat water.
 
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