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Which is best: solar thermal or solar PV with a immersion switch?

Posted by Cathy Debenham on 8 December 2014 at 9:07 am

Q: My question is whether solar thermal or solar panels would be the best investment. We have a large unshaded roof, facing less than 5 degrees from due south. Its a 3 bed semi with solid walls and a new gas boiler with insulated pipes and a dual coil cylinder in the loft. We have cold fill appliances, thermostatic valve showers, 2 young kids and work from home (in an electrically heated garden office). Is solar thermal or solar PV with an immersion attachment the best option?. 

A: Firstly it sounds as though your roof is ideal for solar - whether it be thermal, PV or a combination of both.

However, this is not a simple question to answer as what is the 'best' investment differs for everyone. Is it simply a financial measure? Do you want the cheapest option, with the attendant risks of it going wrong, or would you like to get the best quality you can afford? Do you also want to take into account the relative impact on carbon emissions of different systems? 

There are also the practical questions: how big is your roof? Is there room for both solar thermal and solar PV? Is the second coil in your cylinder already in use (say for a back boiler) or is it available for solar thermal to plug into?

In an ideal world, if you have the roof space and the money, then getting both solar PV and solar thermal is the best option. 

However, for most people both space and cash are limited. The cheapest option is to install solar PV and an immersion switching device. This would also leave more room on the roof for solar PV, so your income from the feed-in tariff would increase. But it is also less green.

In your case, installing solar PV and solar thermal together could be cost effective. The scaffolding will be up for the PV, so that cost will be knocked off the average cost you see advertised for solar thermal. If that second coil in the cylinder is available that reduces installation costs even more. And you will get the renewable heat incentive for the solar thermal. This is based on heat demand, not installation cost, so the return would be better for you than it is for someone who installs a stand alone system. Ultimately, you'll need to get your calculator out and crunch some numbers...

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2 comments - read them below or add one


paul53Comment left on: 21 April 2015 at 8:37 pm

must be  something  wrong with your switch installation . they should be programmed to only use the power going to the grid not  from the grid so  should never increase your usage

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MortmanComment left on: 21 April 2015 at 10:38 am

Don't bother with immersion switching device we had solar pv for 3 plus years and elect use averaged 8.5kw/ year gas & electric £66 a month 

Then had Solic 200 professionally installed 

since installing Solic 200 electric up 4.79kw per day equals £300 extra a year almost cancel out pv advantage to electric bills still have fit true but for lower tariffs is a loser 

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