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Solar PV or solar thermal: which is best for heating water?

Posted by Paul Hutchens on 6 September 2012 at 9:46 am

When I started Eco2Solar in 2007 it was clear where the growth area was in the market. Fuelled by rising energy prices, increased awareness of climate change and fuel security, as well as new targets such as Code for Sustainable Homes or Merton Rule (10% of energy from renewables) for new build homes, the technology of choice was solar thermal heating; mostly for hot water.

Solar PV was very much a fringe technology; perceived as very expensive with a long payback; even with grants that were available.

The Feed in Tariffs (FIT) changed all that! PV became cost effective, financially attractive and even a bit "sexy". Prices of systems plummeted to reflect the enhanced competition and economies of scale - and rightly so. A solar PV system can now be installed for around one third of the price you would expect before the FITs.

This all but left poor old solar thermal technologies out in the cold (not literally fortunately!). However, solar thermal is still an attractive technology. It is extremely efficient - around 90% of radiation can be turned to heat. It is also appropriate for many applications in buildings; particularly for heating hot water or swimming pools. Heating water uses 25% of the energy in the average home. Therefore solar thermal should be considered first where it makes sense ie for heating water.

However, many homeowners and businesses are installing the largest PV system they can fit on their roof and then looking at ways to use the excess electricity to heat water. Whilst it makes sense to utilise as much electricity from a PV system as possible, if you are looking to heat hot water, doesn't it make sense to use a dedicated technology?

I looked at this in the light of current costs and what I discovered was surprising! An average solar thermal system of 3m2 of evacuated tube or 4m2 of flat plate would be specified to heat 180-200 litres of hot water; providing 50 – 70% of hot water. Such a system would cost around £3,750. To produce a similar level of energy from a solar PV system would require approximately 1.5kWp and cost – guess what – about £3,750.

The differences are that a solar PV system will take up around 10m2 of roof space whereas the solar thermal one would take up 3-4m2. So if roof space is limited then the solar thermal will provide a better solution. On the flip side, solar thermal only produces heat and a PV system produces electricity which can be used for many things. Also you get FITs now to install a PV system and you will need to wait for domestic Renewable Heat Incentive (RHI) and we don't know how much it will be.

To be fair this analysis is a little simplistic in that a solar PV system will need some tweaking and some kit to ensure that it can use the energy cost effectively to heat water.

Also, this only holds for small solar thermal systems. With larger arrays for homes with high hot water use, commercial buildings or swimming pools, the economies of scale mean that the solar thermal heats water much more cost effectively. The high cost of other components such as the cylinder and pipework do not increase in proportion as the system size increases. And, of course, non-domestic solar thermal systems are entitled to RHI. 

So, it’s up to you whether to install a small solar thermal or PV system. It really depends on roof space available and what you want to do with the energy once you have generated it.

Photo by szczel

About the author: Paul Hutchens is founder and director of Eco2Solar, which installs solar systems around the UK.

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

25 comments - read them below or add one

Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 31 January 2017 at 4:27 pm

So, what is the current situation, with the changing incentives and (presumably) cheaper prices? Is there much of a payback, and how long?   We are about to install a new flat roof on the top of our house. It would seem to make sense to install one or both at the same time. And it has been suggested that our current combi boiler (Vaillant Ecomax circa 2004, currently situated on the ground floor) won't cope with 2-3 showers (plus what will then be about 15 radiators).   We already have a 1.5kWp PV system on a rear extension roof, which was installed in Feb.2011 (highest FiT), so this would be a separate system should we get more PV. Though I wonder if they can be combined beyond the FiT meters for the purposes of the the kit to heat the cylinder?   Hi Niclee   Sorry for the delay in responding - this slipped through the net and I didn't see your question.   The incentives atre lower now but then prices have come down. A 4kWp system would be arounfd £5 - 6K these days leading to a return of about 8-10% in the first year.   Althought the systems would be separate from a FIT perspective, theye would combine to heat your cylinder.   Paul

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 31 January 2017 at 4:23 pm

Hi Rob

We do not fit solar thermal systems any more due to extremely low demand and have focused on solar PV. 

However, the ballpark cost of installing solar thermal tubes to heat a 60,000L pool woul;d be in the region of £10,000.

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kayrobin

kayrobinComment left on: 31 January 2017 at 4:14 pm

paul - interesting blog. i wish to heat my swimming pool by 8-10deg. surface area 40m / volume 60m. PV or Solar Pipes / approx cost of both pls ?

tks, rob k

 

 

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NicLee

NicLeeComment left on: 4 July 2016 at 9:30 pm

So, what is the current situation, with the changing incentives and (presumably) cheaper prices? Is there much of a payback, and how long?

We are about to install a new flat roof on the top of our house. It would seem to make sense to install one or both at the same time. And it has been suggested that our current combi boiler (Vaillant Ecomax circa 2004, currently situated on the ground floor) won't cope with 2-3 showers (plus what will then be about 15 radiators).

We already have a 1.5kWp PV system on a rear extension roof, which was installed in Feb.2011 (highest FiT), so this would be a separate system should we get more PV. Though I wonder if they can be combined beyond the FiT meters for the purposes of the the kit to heat the cylinder?

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 18 July 2014 at 10:37 am

Hi Shen

This is not really a meaningful comparison as 4m2 of solar PV would produce only about 0.5 to 0.75kW and not worth fitting after you have taken into consideration scaffold, labour etc.

For the same price as a 4m2 solar thermal system (producing 1.5 - 2kWP) we could install a 3kWp system for about the same price - £4,500 to £5,000.

This would need about 20m2 of roof space.

Kind regards

Paul

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shen

shenComment left on: 18 July 2014 at 10:31 am

Thank you Paul. That is clear.

As I am considering to install these systems, can we have a like for like comparison, how much electricity could be generated from the 4m2 area? I understand the differenct cost for cells due to their efficiency and brand, market image etc.

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 18 July 2014 at 10:19 am

Hi Shen

Thre issue with solar energy is the variability of irradicance (energy) at any particular time.

A 4m2 solar thermal system will produce a peak output of 1.5 - 2kW so I would expect the time to heat 180L of water by 30C to be similar - around 3.5 to 4 hours.

Paul

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shen

shenComment left on: 18 July 2014 at 10:07 am

Hi Paul,

Thank you for your post clarifying the pro and con between the two systems. Indeed, the option really depends on individual status quo, for which a survey would be required for a suitable solution.

What I have been researching is the efficiency of the systems, eg energy given in forms of either heat or electricity, would be generated as per certain area. That I believe is an important concern for the whole image of utilisation of solar power.

About the example you gave on comparison of solar thermal and solar PV, I did my calculation as well. My assumption is to heat up the 180L hot water up to 30 degree difference, it would need 22609.8KW.S, which equals to 6.3KW.H. If we install a 1.5KW solar PV system, it would take more than 4 hours to generate the equivalent energy. The question is, how long does it take for a solar thermal system to heat up the 180L water to 30degree difference? 

Thank you.

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DanielHolloway

DanielHollowayComment left on: 28 June 2014 at 11:25 pm

Just poring  PV into an old poorly insulated cylinder is sourly a tragic use of an energy which is capable of running computers and other such equipment.

Secondly, when a solar thermal system is fitted the cylinder is up graded with usually double the insulation and therefore much more energy efficient. Generating power is half the story and using appropriate energy is the other. When it comes to heating water then solar thermal is the obvious technology.

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 4 November 2013 at 2:10 pm

Dear Solar Fan

It may be appropriate to use solar thermal to heat water depending on the requirements of the building. Alternatively solar PV might be more flexible.

The Renewable Heat Incentive is available to provide an incentive for heat based renewable technologies.

If you would like to get in touch we could arrange for a surveyor to visit and advise you - or indeed any other experienced installer of both technologies.

Paul

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Solar fan

Solar fanComment left on: 2 November 2013 at 10:18 pm

Hi there,   I would like some advice if this possible. We are thinking of installing solar energy products within a Mosque to help reduce our costs and we have thought about installing solar panels within the Mosque. However, as there is a large use of hot water within the Mosque (for worshippers to perform ablution and wash themselves), I have thought whether solar thermal energy would be more suitable for the Mosque instead of Solar PV. For a building with high water usage such as a Mosque, what do you recommend?  Also, with solar thermal, is there any form of incentive from the government as I'm aware there is feed in tariff with solar panels?   I appreciate your help and advice greatly.   Kind Regards.

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 25 October 2013 at 10:53 am

Ellwood

Solar thermal will produce a maximum of 0.5kW per square metre depending on irradiance levels and hot water usage.

It would be sensible to have the property surveyed and your requirements assessed before providing further guidance.

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ellwood

ellwoodComment left on: 25 October 2013 at 10:12 am

Thanks Paul,

Yes, about another 20-25 m2 facing South. How many kw of electric energy or thermal energy will this roof space yield ? Presumambly more thermal.

 

 

 

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 24 October 2013 at 4:36 pm

Dear Ellwood

A bigger, better insulated cylinder heated by solar thermal woudl provide you with hot water. It would still need boiler or immersion back up though as the irradiance levels can be low - especially in the winter.

Do you have more roof space?

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ellwood

ellwoodComment left on: 24 October 2013 at 12:05 pm

I have a 10 kw pv system exporting electric and use an immersion heater on my hot water cylinder for hot water during the summer. I wondered if a larger better insulated cylinder possibly with solar thermal could give me hot water during the winter,

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 23 October 2013 at 2:43 pm

I think that RHI could change this situation; certainly in terms of the economics. It will make solar thermal more financially attractive and make it more viable; unless rooof space is limited and you want PV.

There is also a growing concern that using PV which displaces a highly carbon intensive energy source (electricity) instead of gas which has a lower carbon footprint. Environmentally the PV generated electricity is better exported.

Add this to some technical problems with immersion switches to divert PV electricity and maybe the pendulum swings more towards solar thermal!

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ellwood

ellwoodComment left on: 22 October 2013 at 6:23 pm

Paul,

Is it  possible for an update ? in light of recent announcements re domestic rhi.

 

Thanks 

 

 

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LionelM

LionelMComment left on: 29 September 2012 at 6:28 pm

edward50 was looking for some postings of users with PV- immersion switches.  I have installed such a device and am very pleased with the results. The power to the immersion heater is taken after the FIT meter and only uses 'excess' generated electricity. That is to say if my normal domestic load is 1kW and the PV panels are generating 2kW then I use the 'excess' 1kW to power my immersion heater. This is free  electricity  i.e zero cost as it has been generated by the PV panels and I am not drawing any power from the mains. In my book that is free hot water

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 28 September 2012 at 2:22 pm

Thank you all for your thoughtful comments. The correct advise for each situation will vary according to your infrastructure and usage and, as always, the "devil is in the detail". Your installer should be able to advise you  specifically.

I would temper Soltice Energy's analysis with a couple of things. Firstly the FIT export tariff and the price of gas currently and roughly equivalent - at 4-5p/kWh. The price of gas is highly likely to rise however - SSE have announced 9% for this winter - which will make it higher than the export tariff over time and increase the savings.

Secondly, smart meters are expected by the Government to be rolled out between 2012 and 2020. So you may be waiting 8 years (or more if the roll-out stalls) for your smart meter.

My assumptions may not be any more accurate but, as I said earlier, it is better to check the detail!

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edward50

edward50Comment left on: 28 September 2012 at 1:50 pm

Thank you Solstice Energy for your persuasive comment. Let's hope it provokes some discussion; it would be interesting to see some response from the advocates of PV-immersion switches.

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Solstice Energy Ltd

Solstice Energy LtdComment left on: 28 September 2012 at 12:11 pm

We've had a look at the financial benefits of directing PV energy to an immersion heater, and the conclusion is that as the cost of gas is less than the price paid for exported electricity, there is no point in heating a cylinder of water with an immersion heater if you have a reasonably efficient boiler (or even better, solar thermal and/or heat pump).

There is a 'but' though - Until such time as the electricity companies roll out smart meters which can read export values as well as import, you will be 'deemed' to export 50% of your power. So for that period, it seems at first glance to make sense to use that power for the immersion heater - you save money on your gas bill, but still receive the same (50% of generation) export payment for the PV's.
But let's imagine you are fitted with a smart meter in 3 years time (1095 days). In that time you might save 15p / average day* on your gas bill, so you will save 1095 x £0.15 = £164. This still does not cover the £300+ cost of a PV - immersion switch.

So unless you don't have a gas supply we're not convinced by PV immersion switches, and still think that the best use of PV power is to try your best to offset other electrical appliances during the daytime.

The carbon footprint of the house might be reduced by decreasing gas dependence a little bit, but exported electricity is never wasted.

* = according to our calcs - available on request. 

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Solar Wind

Solar WindComment left on: 28 September 2012 at 12:08 pm

I went for solar PV as I thought it would be more flexible, also I am all electric. Once I realised that there was so much spare generation I installed an automated system for using the spare solar to heat the hot water and this works extremely well. During this summer I was able to supply all my hot water most weeks and averaged 35kWh a week. Even now as we go into the winter the spare generation is replacing all the hot water used and I estimate that unto 1MWh could be used in this way each year.

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Plumb Solar LTD

Plumb Solar LTDComment left on: 7 September 2012 at 12:12 pm

Like you Paul, I specialised in Solar Thermal in 2006 and thought this would be the growth area for renewables - and it was until 2009/10 when some 'bright spark' in Government decided to subsidise a technology (Solar PV) that was only 15-18% efficient and which produced an energy that was difficult to store!

 Lets hope the RHI make thermal more attractive but with the past record of FiT's I'm not holding my breath....

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banjax

banjaxComment left on: 6 September 2012 at 5:51 pm

If solar thermal became subsidised to the degree that PV is, in the RHI. Then this would be more of a level playing field.

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catchercradle

catchercradleComment left on: 6 September 2012 at 2:39 pm

We have both, having fitted solar thermal before pv became affordable. In terms of financial benefit, there is no competition. Also pv will go on to the grid when you are on holiday and the energy can still be used usefully. Heat can only be stored usefully for a much shorter time even with good insulation. 

Another point is that you are quoting for a South facing roof. We are only marginally South of East. This seems to detract from the effectiveness of our flat panel thermal system far more than it does from our pv.

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