# Cost-Benefit Calculation - Pay back time on Solar Tubes



## onq (7 Aug 2011)

I spoke to a colleague the other day who gave me pause for thought.

€7,000 estimate for installing the tubes with and expected replacement cycle of 7-10 years.

That's a write down of between €700 to €1,000 PER ANNUM for heating your water a few degrees.

Has anyone ever done a cost benefit calculation on Solar Tubes?

TIA

ONQ.  

[broken link removed]  

All advice on AAM is remote from the situation and cannot be relied upon          as a defence or support - in and of itself - should legal  action    be      taken.
Competent legal and building professionals should be asked to advise in          Real Life with rights to inspect and issue reports on the  matters    at      hand.


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## PaddyBloggit (7 Aug 2011)

I haven't but I've always thought that the installation costs were too high to expect any kind of sensible return in investment.

I'm currently working on a two-up/two-down farmhouse restoration (in the hope of downsizing efficiently) but have totally dismissed solar tubes.

I'd love to install a wind turbine ... but again, it doesn't make financial sense.


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## colm5 (7 Aug 2011)

i did one a year or so ago, you need to pay about 2k for them to be worth while if you have access to natural gas. Anything over that is a waste of money.
Its pretty easy to calculate what they are worth
They provide 50% of your hot water (at best)
How much how water do you need ~ 6000kWh on average per household, 50%= need about 3600kWh of gas in a boiler for 3000kWh of useful heat

cost of 3600kWh @ 5 cent per kWh (gas) = 175 euro per year savings

if you heat with oil ~ 10cent per kWh ~ 350 euro per year savings ~ so you can pay more.

One advantage is that you are partially hedging against fuel price increases for the next ten years


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## quentingargan (8 Aug 2011)

Two assumptions here that I wouldn't agree with;
*
€7,000 for a system replaced with tubes replaced after 7 to 10 years*

Most companies are doing installation for about €4K to €5K, often with a grant knocking that back to €3,200. Most tubes are expected to last 15 to 20 years, with some companies offering a 20 year warranty. Flatplate systems are expected to last 40 years or more. 

*Gas at 5c per KwHr*

This may be true if the boiler is operating at 90% plus efficiency, which it may do in winter (when the system heats the water at the same time as it is heating the radiators). However, in summer, most heating systems are much less efficient, running for short periods at a time, usually heating an outside boiler, all of the pipework between it and the house, just to  bring a 120L cylinder up to 60C. At this time, solar works best, and your central heating is at its lowest efficiency.


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## colm5 (8 Aug 2011)

I mean the cost of gas is 5cent/kWh, the Eff is accounted for in 20% more gas. (3600 vs 3000kWh).
Typically, when dwellings are analysed, half of the energy is consumed in space heating and half in hot water. roughly. If this changed by 10%, it doesn't really alter the economics significently.

7k is not an uncommon quote, I know a guy who was quoted 12k!

Its important to know just how much these technologies will save you in a dwelling, which would be roughly 1/4 of your heating bills.


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## RMCF (8 Aug 2011)

As someone who has thought of solar, and who has read countless articles on solar and wind, most of the 'experts' seem to think that these things will never pay for themselves, as they will need replacing, maintenance or parts before they pay for their initial cost. And for that reason I think they will continue to be a niche market.

I think they are a great thing if you are Eco-friendly, but you can heat a hell of a lot of hot water for €7k.


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## onq (8 Aug 2011)

Thank you for all your replies, its certainly given me pause for thought.

It strongly suggests that designing a well-insulated building to benefit from passive solar gain and using massive thermal stores within it would be a reasonable way to go as opposed to using the panels.

The purists will argue a both-and-solution, but most of my clients don't have an unlimited budget and my job is to advise them no their best return on investment.

No-maintainance insulation with appropriately orientated and sized windows and rooflights seem to be the way to go for best pay back over time, with anything that needs a pump or an electricity supply at the second tier in terms of payback.

Look forward to hearing any more examples of actual costs and thanks for those that are already posted.

ONQ.  

[broken link removed]  

All advice on AAM is remote from the situation and cannot be relied upon          as a defence or support - in and of itself - should legal  action    be      taken.
Competent legal and building professionals should be asked to advise in          Real Life with rights to inspect and issue reports on the  matters    at      hand.


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## colm5 (9 Aug 2011)

onq said:


> It strongly suggests that designing a well-insulated building to benefit from passive solar gain and using massive thermal stores within it would be a reasonable way to go as opposed to using the panels.
> 
> .


 
Agree, the current economics suggest they fall into the category of eco-junk!


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## serotoninsid (9 Aug 2011)

colm5 said:


> Agree, the current economics suggest they fall into the catagory of eco-junk!


+1 -  like you, I've read up on solar and wind - and try to keep tabs on them (in the hope that they will become cost effective) - but the payback simply isn't there right now as far as i'm concerned.


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## kbie (10 Aug 2011)

Also considered solar panels but decided, on cost, to heat the water on the low night rate tariff electricity. It is timer to come on for 3 hours before 6 am and has enough water heated to last the day.


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## Firefly (10 Aug 2011)

serotoninsid said:


> +1 -  like you, I've read up on solar and wind - and try to keep tabs on them (in the hope that they will become cost effective) - but the payback simply isn't there right now as far as i'm concerned.



I'm thinking the same...otherwise countries which get a lot more sunshine than us would be using them extensively.


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## Shawady (10 Aug 2011)

ONQ, I was under the impression that the problem with using solar energy in Ireland is that there is no facility to sell excess electricity back to the national grid. As far as I am aware this facility exists in the UK. So at times when it is particularly sunny, you are not wasting energy because you can make money by selling it to an electricity provider.


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## Leo (10 Aug 2011)

Shawady said:


> ONQ, I was under the impression that the problem with using solar energy in Ireland is that there is no facility to sell excess electricity back to the national grid.


 
The solar systems under discussion here do not generate electricity, they heat water directly. 

Grid buyback is available in Ireland, you'll need photovoltaic panels to utilise solar energy to generate electricity. These are even more expensive than solar water heating panels.
Leo


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## Slim (10 Aug 2011)

Leo said:


> Grid buyback is available in Ireland, you'll need photovoltaic panels to utilise solar energy to generate electricity. These are even more expensive than solar water heating panels.
> Leo


 
Could you point me to more information on the 'grid buyback'? Slim


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## Hoagy (10 Aug 2011)

Slim said:


> Could you point me to more information on the 'grid buyback'? Slim


 
[broken link removed]


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## serotoninsid (10 Aug 2011)

As quentin has articulated over on boards.ie - a feed-in tariff exists - but the rates are paltry and are only guaranteed for 5 years.  If there was any foresight, they would be looking at higher rates and a longer-term guarantee.  People need to be secure in the knowledge that they can achieve payback.

There is a very generous feed-in tarriff in the UK (with tax-free earnings and a long term guarantee on the tariff) - which makes the decision to run with solar a no-brainer.


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## onq (10 Aug 2011)

Shawady said:


> ONQ, I was under the impression that the problem with using solar energy in Ireland is that there is no facility to sell excess electricity back to the national grid. As far as I am aware this facility exists in the UK. So at times when it is particularly sunny, you are not wasting energy because you can make money by selling it to an electricity provider.



I wasn't sure about this but I see Leo has done his usual excellent job in succinctly supplying all relevant information - in three sentences!

I'll have to keep practicing LOL!

ONQ.


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## onq (10 Aug 2011)

serotoninsid said:


> As quentin has articulated over on boards.ie - a feed-in tariff exists - but the rates are paltry and are only guaranteed for 5 years.  If there was any foresight, they would be looking at higher rates and a longer-term guarantee.  People need to be secure in the knowledge that they can achieve payback.
> 
> There is a very generous feed-in tarriff in the UK (with tax-free earnings and a long term guarantee on the tariff) - which makes the decision to run with solar a no-brainer.



(nods)

Does anyone know how to store electricity in large quantities for rainy day apart from 

(i) batteries (hazardous materials, leakage and loss of charge over time and 
(ii) pumping it up to a lake (evaporation and mechanical losses)

I'm thinking chemical storage of separated hydrogen and oxygen in fuel cells as possibly being one useful way of doing things, but the manufacturing process is necessarily risky and the storage medium is expensive.
One benefit is that it isn't dependent on a national supplier or large infrastructural works and its relative secure once stored - minimal leakage or degradation.
One downside would be the mechanical losses incurred if you tried to use the chemicals for electricity as opposed to say motive power.
Still, nothing's perfect.

ONQ.  

[broken link removed]  

All advice on AAM is remote from the situation and cannot be relied upon           as a defence or support - in and of itself - should legal   action    be      taken.
Competent legal and building professionals should be asked to advise in           Real Life with rights to inspect and issue reports on the   matters    at      hand.


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## colm5 (10 Aug 2011)

Check this guy out for some ideas and technologies on energy storage
[broken link removed]


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## chipclub (12 Aug 2011)

There are two adults and two small children in my house.  For five months a year we use electricty to heat water and the rest of the time the oil fired central heating provides all we need.  

The summer months make the calculations very clear.  A typical weekly consumption would be:

8.5kW shower.  Two * 5 minute showers per day = 10kWh per week
3 kW immersion for children baths.  45 mins per day 4 times per week = 9 kWh per week
3 kW kettle to boil water for dishes and cleaning.  4 mins to boil used 3 times daily = 4kWh per week

Total for one week = 23kWh.

I have no idea of the winter oil useage for the same amount of hot water but assuming a similar cost to the electricty we're looking at approximately €200 per year for hot water.

Based on figures above (3.2K to install a system that provides 50% of hot water), I'd be looking at a 32 year payback!  No thanks.


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## quentingargan (14 Aug 2011)

According to most energy authorities, (SEAI included) the average household uses a lot more energy to heat domestic water than you do. 

Fair play to you, and clearly solar isn't for you, but the average is 2,000 Kw Hrs per year for hot water.


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## jcash (14 Aug 2011)

am i right in this ,
with solar system ,you have a bigger water cylinder, hence in winter your oil boiler has more work to do in heating this cylinder before it puts any heat into the rads ???


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## quentingargan (14 Aug 2011)

jcash said:


> am i right in this ,
> with solar system ,you have a bigger water cylinder, hence in winter your oil boiler has more work to do in heating this cylinder before it puts any heat into the rads ???



I can see where you would draw this conclusion, but there are two coils in the cylinder, one near the top is supplied by your boiler and this will only heat the top 120L or so of hot water that you would always have needed with a standard system. It won't heat water below its level. 

The lower coil is supplied by the solar panel and heats the whole cylinder. And even in winter, your solar panel will often enable this coil to bring the contents of the bottom of the cylinder to 40 degrees. So effectlvely your boiler actually has less work to do.

Finally, most solar cylinders have about 50mm of insulation, and are better at storing the heat, so there is less heat loss in the winter. This is often a forgotten benefit of installing solar water heating. In a good cylinder, if it has been brought up to 60 degrees the night before, in the morning the water temperature will only have fallen very slightly, so in fact your boiler will have much less work to do on its daily rounds to the radiators. :~)


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## Barney Magoo (16 Aug 2011)

We looked at this last year as part of an energy efficiency upgrade to our house, and talked to several potential suppliers and 'experts'. Most were more interested in the sale than doing a personalised cost-benefit analysis for our situation and all appeared ill-informed about their technology. 
We found the linked article very helpful and unbiased. Decided not to fit water-heating panels after all as it did not make economic sense to us.

[broken link removed]


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## quentingargan (16 Aug 2011)

Barney Magoo said:


> We found the linked article very helpful and unbiased. Decided not to fit water-heating panels after all as it did not make economic sense to us.


I beg to differ. To my mind, that is a bit of a rant, suggesting that it might cost €5K to €8K to install a system that would meet as little as 15% to 20% of your domestic hot water needs. The article also totally misunderstands zero loss efficiency in tests conducted by SPF on vacuum tubes (by simply ignoring IAM). The author favours DIY panels over commercial products, and DIY installation over professional installation. It isn't exactly unbiased, but then neither am I.


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## Leo (16 Aug 2011)

quentingargan said:


> The author favours DIY panels over commercial products, and DIY installation over professional installation. It isn't exactly unbiased, but then neither am I.


 
The article favours 'an indigenous, localised solar thermal industry producing low cost easy to install solar collectors for mass deployment in a manner similar to Austria.' Not exactly DIY panels. 

DIY installation makes sense if you're trying to reduce the overall system cost to reduce the payback period.
Leo


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## onq (16 Aug 2011)

Hi chipclub,

I see no reason to doubt your figures and what I see is more of the same empirical data coming back - not great returns - and you will have to replace the system well before three decades + 2 years expires.

ONQ.


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## onq (16 Aug 2011)

quentingargan said:


> Finally, most solar cylinders have about 50mm of insulation, and are better at storing the heat, so there is less heat loss in the winter. This is often a forgotten benefit of installing solar water heating. In a good cylinder, if it has been brought up to 60 degrees the night before, in the morning the water temperature will only have fallen very slightly, so in fact your boiler will have much less work to do on its daily rounds to the radiators. :~)



Agreed but this latter point is a sore one with De Wife.

She uses the hot press to part dry the clothes in really inclement weather when you cannot get them fully dry outside and she say she could never do that if the cylinder was fully insulated - go figure.

ONQ.


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## quentingargan (17 Aug 2011)

Leo said:


> The article favours 'an indigenous, localised solar thermal industry producing low cost easy to install solar collectors for mass deployment in a manner similar to Austria.' Not exactly DIY panels.
> 
> DIY installation makes sense if you're trying to reduce the overall system cost to reduce the payback period.
> Leo


Ah, I was talking about the author rather than the article. The author is an advocate of homemade systems. I don't mean to slag that off by the way - I made my own first solar water heater, and as long as you know what you are doing, DIY is a valid way of cutting costs.

A DIY system using old radiators, or clip fins from the Centre for Alternative Technology ([broken link removed]) for example, will be extremely cheap to make, but its performance would not compare with modern collectors using the latest absorber materials. 

There is a market for this sort of DIY system, but the vast majority of installations will be certified product manufactured to standards. Whether that can be done as part of a low cost indigenous industry remains to be seen, but producing quality solar panels anywhere requires heavy investment and a sizeable market.


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## Leo (17 Aug 2011)

quentingargan said:


> Ah, I was talking about the author rather than the article. The author is an advocate of homemade systems. I don't mean to slag that off by the way - I made my own first solar water heater, and as long as you know what you are doing, DIY is a valid way of cutting costs.


 
Ah OK, thanks for clarifying. I was unfamiliar with his work/advocacy outside of that article. I'd agree that DIY panels will never yield great efficiencies. Even the article above states that the low-complexity Solar Twin option yields poor results, so I was wondering how the use of DIY panels would fit with that. It's unlikely anyone going down the DIY panel route will use intelligent control mechanisms.
Leo


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