I Solar Immersion

Nellybee

Hello
During the summer months i have my gas, heating my hot water .During the Winter I have to have the gas doing it because it is linked with my gravity fed central heating ,in the summer i can have hot water only from the boiler but winter i have to have them both.So what i am debating is during the summer months using the solar panels to heat the immersion using isolar or similar .iguess the immersion heater will be 3kw but my panels only produce 2.5kw.Would it be cheaper to use the solar for hot water or having the gas boiler on for 1hr a day to heat my hot water during the summer.

tim_p

Assuming you already have an immersion heater fitted to the tank then you will have to factor in the cost of buying and fitting a device to divert the solar generated power to the immersion. The fact the immersion is 3Kw isn’t a problem as any device will feed it what it can given the conditions. Whether it will save you money is possibly debatable. I have one fitted and use next to no gas between March and Oct, only for cooking (hob). Take a look in the Green and Ethical sub forum and you’ll find loads of talk and knowledgeable people there to ask. I reckon mine has paid for itself over the 5 years it’s been feeding the tank.

JKenH

There are devices available costing £200 - £400 such as Solar IBoost+ or Eddi (there are others) that can detect the amount of spare solar PV available and send ONLY that amount to your immersion heater. Say you are generating 1kw and your house is using 300watts for fridge, lights etc then only 700 watts will be sent to the immersion heater. In practice it might be more like 600watts as the devices keep a buffer of say 100w. Your 3kw heater would actually be operating as a 600watt heater. If you are generating 2.5kw then with the same consumption you could send more than 2kw to your heater.

Whether it will be better to use solar pv or gas will depend on your circumstances but if there is spare pv then these devices do a good job of mopping it up rather than sending it to the grid. One thing to bear in mind is that your hot water cylinder will lose heat over time. If you shower in the morning then the hot water in the tank heated by solar will have spent 12 hours cooling before you use the water. I think a 140 litre tank can lose about 2kwh overnight depending on insulation.

If you have an alternative use for that power during the day, say, running a 1kwh heater for a couple of hours, it might be better just to put the gas heater on for a few minutes in the morning to heat the water. You can actually use these devices to operate other resistive heaters such as an oil filled radiator.

I have a solar IBoost heating two hot water cylinders and I also have Economy7 (no gas) and a couple of ASHPs (heat pumps) so in winter when I have limited solar pv it is better for me to use what solar I have to run the ASHPs during the day and heat the water by E7 for half an hour in the morning just before I shower.

I have diverted about 1600 kwh to my hot water tanks in the last year and so the payback time on the IBoost for me is quite short (about 2years based on me displacing E7) but your payback time might be much longer as you have less panels and maybe less demand for hot water.

If you are out at work all day and no other means of capturing the power generated by your panels then it might just be worth giving an IBoost or something similar a go. At least you will have the satisfaction of using the power you generate yourself even if it will take a while to get payback.

Hexane

This can also depend on your metering arrangement and your export payment arrangement. Most people are on a "deemed" export payment whereby it is assumed that you use 50% of the energy that your generation meter records, therefore you get paid (at a rate of about 5 pence per kwh) for the other 50%. This is why devices like the iBoost and eddi became popular - you can use more of the energy that you generate, without reducing the amount of energy that you are paid for (because they don't actually know how much you really use).

Increasing numbers of people are now on tariffs whereby they have a meter that is considered capable of recording export accurately, and an electricity supplier that is either willing, or insistent, on paying you for what you actually export, instead of paying you for a deemed 50%. As explained by someone elsewhere on this forum a while back, this could mean you end up in a situation where you are using 1 kwh of energy to heat your hot water, and thereby losing the 5 pence that you might have received for metered export of that 1 kwh, and by using it to heat your hot water you have only saved the (very approximately) 5 pence that you would otherwise have spent on heating it with 1 kwh of gas. In this scenario you gain approximately nothing financially, and are several hundred pounds out of pocket for the diverter device.

Some people have said enforced metered export is "coming soon for everyone", but I have opined elsewhere that the clutch of organisations that couldn't cope with getting smart meters rolled out anywhere near according to schedule, will be equally incapable of getting enforced metered export in place countrywide anytime in the next five years.

This morning by 8:45am my solar diverter had only managed to put 0.35 kwh in the hot water tank (it was a cloudy start to the day) so I manually turned on the gas water heating for twenty minutes or so. Over the course of the day my panels have generated another 15 kwh or so, most of which will have been surplus, so the hot water tank will now be up to max temperature.

Of course you could do all this without a diverter device, by means of firstly checking if it's sunny, secondly deciding if you think the water needs to be hotter, thirdly checking if your panels are generating at maximum output 2.5 kw, and if so turning the immersion heater on. And then remembering to turn it off at some point. The financial benefits will still be rather limited, however, as your panels usually won't be generating their potential peak output of 2.5 kw so you will be importing electric power at a cost of somewhere vaguely between 11 and 17 pence per kwh. Gas is much cheaper than imported electricity. And this is also all a bit of a nuisance.

mmmmikey

Hexane wrote: »

As explained by someone elsewhere on this forum a while back, this could mean you end up in a situation where you are using 1 kwh of energy to heat your hot water, and thereby losing the 5 pence that you might have received for metered export of that 1 kwh, and by using it to heat your hot water you have only saved the (very approximately) 5 pence that you would otherwise have spent on heating it with 1 kwh of gas. In this scenario you gain approximately nothing financially, and are several hundred pounds out of pocket for the diverter device.

Guilty as charged - this is becoming one of my hobby-horses but anyone is welcome to share! My crystal ball is telling me that these immersion heater controllers will soon become a thing of the past, although to be fair I have had problems with my ball in the past.....

Hexane wrote: »

Some people have said enforced metered export is "coming soon for everyone", but I have opined elsewhere that the clutch of organisations that couldn't cope with getting smart meters rolled out anywhere near according to schedule, will be equally incapable of getting enforced metered export in place countrywide anytime in the next five years.

Although I would agree with this, just because I don't think smart metering (and hence export metering) is likely to be enforced in the forseeable future, doesn't mean that it won't necessarily be a bad thing. You may well find that although you're not actually forced to have a smart meter installed, in choosing not to you will be unable to take advantage of the wider range of tarriffs they support.

My exepectation is that time of use tarriffs will be very favourable initially because they'll have to be to get over the intertia and get things started (how many people still don't change their energy provider, let alone the tarriff). So you'll have to weigh up anything you may (or may not) lose if choosing smart metering means metered export against what you might gain through more favourable supply tarriff.

Obviously this is all highly speculative, but the unfortunate reality is that however you look at this in order to assess the economics you have to predict what will happen in the future. To my way of thinking, planning on the basis of being on a simple, flat rate tarriff in a few years time is unrealistic.

Only time will tell

Hexane

It has just occurred to me that one additional advantage - not financial and possibly not even healthy - of solar diverters with immersion heaters, is that they support a slothful lifestyle. For example on a day like today I woke up at 6am and pondered whether to get out of bed. Knowing that in the absence of much sunshine, the hot water wouldn't be heated on "free" solar power yet, made it so much easier to decide against and just go straight back to sleep. When finally getting up several hours later, of course the steady trickle of a few hundred watts of heating had done its thing and the water cylinder was up to maximum temperature, no gas heating required.

One no longer needs to think "oh no, I've wasted nearly half a day, I could have been getting things done", one can instead think "by sleeping in I've saved both money and carbon emissions". All very good psychologically but perhaps not in other ways.

silverwhistle

Hexane wrote: »

they support a slothful lifestyle. .

Hmm. :-) I have a solar diverter and also use no gas from March to October. Unfortunately I have an electric shower so have to wait even longer for the sun to come up...

In practice, as I don't work (or online) I make do with a quick basin wash from stored heat. As I have just taken on an overgrown allotment I suspect it will be better to dash out and work up a sweat and _then_ come back for shower or shallow bath.

Incidentally, although my hot water tank has integrated insulation I thought it worthwhile to add a further cheap insulating jacket.

As for having a smart meter to take advantage of time of day tariffs I'm going to wait until getting an EV, or my gas boiler breaks down. I hardly use the latter anyway, so I'm not even going to get the crystal ball out until that time comes!