Solar Energy to Immersion diverters.

Alfa438

Just fitted a Solic 200 to our house to act along with existing Solar panels. Cost £140 Fitting £165 both inc Vat. Its simple to fit although there are a few requirements prior to fitting so please read the info. However basically it moniters the electricity feed 100's of times a second and if there is energy being diverted back to the grid from your solar PV aray and you dont have a battery back up it will divert the energy (only the energy being created) back to the immersion heater for your water tank. This has allowd us as a family of 4 to completely stop heating the water using the gas boiler. The boiler is a 2 year old efficent Worcester 24KW version. It used to run for 1 hour a day on a Hive system every day which now it does not. Saving roughly 24Kw per hour X 7 days a week X 52 weeks a year.  April 1st Gas/ KW is 7p I think. so 24 x 7 days = 168 KW per week x 52 weeks = 8736 KW per year x £0.07 = a saving of £611.00 PA/ plus Vat so more like £642 and thats not accounting for all the extra boosts we used for additional baths. Even on cloudy days the tank water is in the mid 40s and with just a little brighness its maxed out at 65 deg C. 

MWT

Alfa438 said:

The boiler is a 2 year old efficent Worcester 24KW version. It used to run for 1 hour a day on a Hive system every day which now it does not. Saving roughly 24Kw per hour X 7 days a week X 52 weeks a year.  April 1st Gas/ KW is 7p I think. so 24 x 7 days = 168 KW per week x 52 weeks = 8736 KW per year x £0.07 = a saving of £611.00 PA/ plus Vat so more like £642 and thats not accounting for all the extra boosts we used for additional baths.

That is a long way off from the actual gas use though.

A modern boiler modulates the burner to much lower levels of power than it is rated for.

You will probably find your actual use was only ever a fraction of the 24kWh, we would typically use less than 10kWh of gas for water heating and that is with a lot of use throughout the day.

If your system produces the data, take a look at how much electricity you are diverting to water heating then divide by around 0.85 to increase that for the lower efficiency of gas and that is about the amount your are saving.   

jimjames

I guess after 12 months you'll be able to see how much your gas use has dropped assuming no other factors

QrizB

Alfa438 said:

Saving roughly 24Kw per hour X 7 days a week X 52 weeks a year.  April 1st Gas/ KW is 7p I think. so 24 x 7 days = 168 KW per week x 52 weeks = 8736 KW per year x £0.07 = a saving of £611.00 PA/ plus Vat so more like £642

Depending on the size of your hot water tank and the location of your immersion heater(s) you'll probably find that you can only put 10kWh or less into the tank (with mine it's closer to 6kWh). And you won't be able to do it 365 days a year; more like half that. Let's say 200 days as it makes the maths easier.

6kWh/day x 200 days = 1200kWh. At 7p/kWh that's a saving of £84 PA plus VAT = £88.20.

[Deleted User]

QrizB said:

Alfa438 said:

Saving roughly 24Kw per hour X 7 days a week X 52 weeks a year.  April 1st Gas/ KW is 7p I think. so 24 x 7 days = 168 KW per week x 52 weeks = 8736 KW per year x £0.07 = a saving of £611.00 PA/ plus Vat so more like £642

Depending on the size of your hot water tank and the location of your immersion heater(s) you'll probably find that you can only put 10kWh or less into the tank (with mine it's closer to 6kWh). And you won't be able to do it 365 days a year; more like half that. Let's say 200 days as it makes the maths easier.

6kWh/day x 200 days = 1200kWh. At 7p/kWh that's a saving of £84 PA plus VAT = £88.20.

Yep, there is nothing like creative ‘man maths’ to justify the cost of a solar diverter. For those on metered export, any savings need to take it account the loss of export payments. I have a 7kWp array and I would struggle to heat a kettle’s worth of water on a dank Winter’s day (350Wh).

FWiW, my average daily gas use for water heating (same boiler) was about 6kWh. I use about 3.5kWh of electricity.

QrizB

[Deleted User] said:

QrizB said:

Alfa438 said:

Saving roughly 24Kw per hour X 7 days a week X 52 weeks a year.  April 1st Gas/ KW is 7p I think. so 24 x 7 days = 168 KW per week x 52 weeks = 8736 KW per year x £0.07 = a saving of £611.00 PA/ plus Vat so more like £642

Depending on the size of your hot water tank and the location of your immersion heater(s) you'll probably find that you can only put 10kWh or less into the tank (with mine it's closer to 6kWh). And you won't be able to do it 365 days a year; more like half that. Let's say 200 days as it makes the maths easier.

6kWh/day x 200 days = 1200kWh. At 7p/kWh that's a saving of £84 PA plus VAT = £88.20.

Yep, there is nothing like creative ‘man maths’ to justify the cost of a solar diverter. For those on metered export, any savings need to take it account the loss of export payments.

I didn't want to mention that   but you're quite right.

We also don't know how big the OP's solar array is; a small one might struggle to generate 1200kWh/yr, and if it's genuinely large enough to export 8736kWh/yr s/he could be much better off selling those to Octopus on Outgoing Agile.

Merlin139

QrizB said:

Alfa438 said:

Saving roughly 24Kw per hour X 7 days a week X 52 weeks a year.  April 1st Gas/ KW is 7p I think. so 24 x 7 days = 168 KW per week x 52 weeks = 8736 KW per year x £0.07 = a saving of £611.00 PA/ plus Vat so more like £642

Depending on the size of your hot water tank and the location of your immersion heater(s) you'll probably find that you can only put 10kWh or less into the tank (with mine it's closer to 6kWh). And you won't be able to do it 365 days a year; more like half that. Let's say 200 days as it makes the maths easier.

6kWh/day x 200 days = 1200kWh. At 7p/kWh that's a saving of £84 PA plus VAT = £88.20.

My tank is 170 Litres and in 7 years and 10 months we have diverted 9.62 MWh out of 31.73 MWh of Solar through our iBoost. Today and yesterday it was just under 6 kWh each.

My tank is 20 years old and in a very hard water area. It needs replacing.

Alfa438

I guess interesting and all very valid points. What I can say for certain is this. Since the Start of Feb 22 when it was fitted we have not turned on the boiler to heat hot water. Prior to this if we were to boost the water on the Hive for 30mins during the winter months the water in the tank would not have reached the 60 deg set on the thermostat, but it was enought to have a bath. We have a 16 panel 4KW system on the roof that faces South. Last night both my wife and daughter had a evening shower and we manually washed up. Im still in the habit of checking and the tank water before bed at about 11pm was in the low 30's. This morning by 7am given we had only had a few hours of low light the tank was back up to 54 Deg C I think and both myself and son had showers. I may have the workings a little off but ultamatly it is working very well. We also are on an original FIT export tarrif so we get paid for every KW we produce and they do not monitor how much we actually export. It is taken as exactly half genorated. So even if we use all the power to run things for free, we still get paid to export it. Annually it pays back about £800 to £850 and is paid quarterly. 

QrizB

Alfa438 said:

This morning by 7am given we had only had a few hours of low light the tank was back up to 54 Deg C I think and both myself and son had showers.

This seems odd. Assuming you're in the UK, the sun will only have been up for 80-90 minutes by that time and there's almost no possibility of your solar PV having generated enough power to heat a tank of water by 14C in that period.

Are you still running your CH boiler for heating, or have you turned that off too?

If you are running your boiler, can you check that your two- or three-way valve that should isolate your tank coil hasn't stuck open?

outofcontrolmoaner

On current tariffs, unless you're only on electric, diverters look good for battery only installations. If you have PV and a gas boiler, current pricing doesn't make a diverter financially viable (unless you switch tariff for approx 3 months over winter, and even then, it's questionable). There are some marginal wear and tear benefits for the boiler.

I had one installed as I was told it would save £250 per year. But it can't - instead it will cost money. Current pricing of kWh for gas with Octopus is 6.99p, while export pricing with their Flux tariff is 16.51p off peak and 27.51p peak per kWh. (tariffs were higher when it was installed). Of course, my fault, I should have researched the tariffs more fully (as all installers recommend Eddi or iBoost it seems they haven't caught up with new products from energy providers and are giving poor advice based on the economics of energy pricing from 20/21).

To heat with diverted solar means you're effectively paying 9.5p off peak, or 20.5p peak due to the loss of export (peak will obviously only apply from around mid April - mid Sept as you won't be generating much after 4pm). Financially, it makes more sense to heat the water with gas. Emissions wise, you're still greening the grid so you're helping someone else use clean electric. 

Sure, you could run the diverter throughout the summer and mothball the boiler for 5 months or so, depending on where you live/CH needs/insulation. There's an argument that you'll extend the life of the boiler - but probably not by much (assuming you have a condensing boiler and your cylinder isn't scaled up). My HW needs use about 4kWh's per day. It's the CH that puts the strain on the boiler. You also can't avoid the standing charge while you're not using gas.

PVT panels are probably the way to go to get solar HW more efficiently, use roof space more effectively and not miss out on export (and so take less time to reach payback on the investment). Solar Energy UK don't mention diverters in their recent Solar Heat report: https://solarenergyuk.org/wp-content/uploads/2022/12/Value-of-Solar-Heat-report_Dec2022.pdf

You could switch from Octopus Flux to Agile through the winter and that gives the option to heat overnight with reduced (and sometimes negative) rates. But this is highly variable, meanwhile your export rates will drop through the day if the rates drop - so what you gain overnight, you lose in the day if you're exporting at less than the Flux rates.

BUT - if you're battery only it totally makes sense, you've nothing to export and you can take advantage of Agile pricing when it's available, and use that to heat the water. 

Installers need to update their advice for PV installations, because while some customers will be happy with the principle of using solar to heat their water, there's no financial benefit and current system designs end up costing customers money (as all installers I spoke to also recommended Octopus - and Flux had been released 5 months earlier. Ecotricity are due to be releasing a solar smart tariff soon, too). 

All this noted with the caveat that in a highly variable market these tariffs could be withdrawn and it could yet make sense to have it (if export rates drop below gas price). However, if there's a political change next year, I'd suggest if anything export prices will rise rather than fall (as well as geopolitics impacting base pricing, current turmoil makes that look pretty fragile). None of that gets away from the fact that it's misleading if an installer tells PV owners a diverter saves money - it doesn't, it costs (and at £500 for unit and installation - it's money you'd be better off putting into another battery).

Gerry1

You're muddling up kW with kWh which is never a good idea when working out energy costs.