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How much does solar thermal really save?

Posted by Helena Ripley on 21 September 2015 at 11:55 am

Solar thermal panels providing hot water for your domestic needs are a good way of reducing both your carbon emissions and your energy bill. Some households can use solar thermal to provide all of their hot water needs throughout the summer months. One SuperHomer manages to solely use solar heated water from March to November! But how much can they actually save?

This is a really difficult question to answer! It depends primarily on three things: price, energy and usage. The price of a solar thermal installation varies hugely, with estimates from £3000 to £6000. As this depends on the type of panels you choose, which installer you go for, and the state of your boiler, plumbing and roof, it is impossible to calculate a universal average cost. The amount of energy a solar thermal system will save you depends on the slope and direction of your roof, because these determine how much solar radiation hits your panels. And the monetary savings depend on how much hot water you use, what you use it for and when, and the fuel you'd normally use to heat it.

We’ve calculated, based on figures from the Department of Energy and Climate Change, that solar thermal could in most cases save you something in the region of £75. This assumes that solar panels will save on average 10% from your energy bill when you exclude electricity. To make the most of your solar heated water (and save the most money) you may have to make a few changes to your lifestyle: using a shower rather than a bath, and doing so in the evening instead of the morning. Other useful changes include using pre-heated water in your washing machine and dishwasher and using your boiler as a boost only after the sun has set.

The renewable heat incentive (RHI) is also available for solar thermal and is one of the few government subsidies that they don’t seem to be cutting at the moment. The amount you can earn through RHI depends on how much energy your solar thermal panels generate; the current tariff is 19.51p/kWh. The RHI is available for seven years, which is much shorter than the feed-in tariff, but it seems to be the right length of time to pay off your solar thermal system.

Let’s do the maths. A recent correspondent explained that their system would generate 1,800 kWh and was going to cost £4,200. They calculated that a 10% decrease on their energy bill would save £100.80 per year. The income through RHI would be £351.18. That means that their system would not have paid back within the seven years. However, there is another consideration: increasing gas prices. Between 2004 and 2014 gas prices rose by some 115%. Imagine what your gas bill will look like in 2025! Solar thermal systems last about 20 years and will probably be saving you more money every year. Electricity prices are going up by similar amounts so if you currently heat your water electrically the savings from switching to solar thermal are even greater.

A solar thermal system is an investment. The RHI can support its installation now and savings will be made for many years to come. There is very little maintenance required, so ongoing costs are minimal.

Photo credit: SuperHomes

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11 comments - read them below or add one


paul53Comment left on: 2 October 2015 at 2:17 pm

the  point  being  most people who  get  solar then  look at other ways  of making the  most out of them including insulation  and  energy  efficient appliances,until  cheap  storage is  available  ill  use  the  electricity during the  day  which is not  creating  any  carbon  at all and  the gas  which is ,during the  night

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Ian Smith

Ian SmithComment left on: 2 October 2015 at 1:14 pm


Lots of sources for my representation - try NEF's carbon calculator which uses upto date Government data -  

1kWh of electricity = 0.46kgCO2e

1kWh of natural gas = 0.18kgCO2e

Note too, that the electricity conversion includes provision for grid losses.  Not clear what the boiler efficiency assumption is for the gas but, like grid losses, you will get pipe losses.  It does not look like these have been included.  Irrespective of any reasonable assumptions about  grid or pipe losses, or boiler efficency, you are looking at between 2 to 3 times the carbon.  This is a big difference and raises ethical issues about diverters in my mind.

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paul53Comment left on: 28 September 2015 at 4:35 pm

how  can  you say using  a diverter  will increase emissions firstly  my next  door  nieghbour  may  be  using  my exported power to heat water  or  just waste  with lights  left on and  up  to  a third of  all power is lost  in transmission better  to optimise your  usage.

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Ian Smith

Ian SmithComment left on: 28 September 2015 at 11:16 am

For those considering diverters, if you normally heat your water with gas and you are motivated to save CO2 emissions, fitting one of these will increase emissions. The carbon intensity of the electricity that would be displaced from the exports you would otherwise make is still so high that it is better to heat the water with gas.

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Solar Wind

Solar WindComment left on: 28 September 2015 at 9:49 am

Hi Gary,

You should be able to do better than that.

I have a 3.76kW solar PV system and I made up Robin's Mk2 solar diverter from the plans at, total cost about £50.00 and have been diverting more than 1 MWh each year into an immersion heater and even a night storage heater if I can, when the water is hot.

So far this year 1,240 kWh. Normally I can go from May to September on free hot water and offset the power required the rest of the year.

Since the installation in December 2011, I have diverted 5.3MWh of surplus solar into hot water or heating as I installed a kWh meter in the unit.

It maybe that Robin's system is more efficient than the commercial systems as it will trigger right down to a single cycle of the 50 Hz supply.

Regards Howard.

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paul53Comment left on: 26 September 2015 at 8:49 pm

if  you want  to  encourage the  use  of  solar short  term  financial  gain is the  way  to  go .unless  the  govt  pays  us the same rate  for  exported  power as  imported power with a  small standing  charge or storage  is  made  much  more affordable

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Andy in Hawick

Andy in HawickComment left on: 26 September 2015 at 12:47 pm

If you're into short term financial gains, solar thermal may not make much sense but if you are looking to reduce carbon footprint, make effective use of available resources or are looking at the longer term, solar thermal makes much more sense than solar PV.

Using an automatic diverter like ImmerSun or such may seem to make financial sense if you only look at your household budget but using electricity (a high-exergy energy vector) to power a resistive heater (to produce low-exergy heat) does not make a whole lot of sense.

A square meter of PV will produce about 150W of electrical power with a modern, high-output panel in bright sunshine. A solar thermal unit will easily produce three times that output, even with a home-made unit and four or five times with a well-designed flat plate collector.

Even David MacKay reckons on 13kWh per person per day of usable solar thermal output versus 5kWh per person per day of [high quality] PV.

This is the strength of solar thermal; matching low-grade heat requirement with a low-grade source. Electricity should be better used for things that heat cannot be.

Prices of thermal units may be artificially high due to RHI. We paid £2k for the self-install kit for our SolarTwin unit that made sense long before RHI was around and also before PV, which certainly needed the FiT to balance the figures. Our boiler tops up the temperature during the winter and occassionally during the 'shoulder seasons'. We had to add an automatic thermostatic mixing valve to the non-mixer tap in the bathroom to avoid the danger of scaulding in the summer.

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Graham and Marion

Graham and MarionComment left on: 25 September 2015 at 7:31 pm

Gary - I agree. With just the two of us we turn off gas in March and rely on 2.5 kW solar PV's to keep us showered and clean every day.

So glad we kept our hot water cylinder and immersion heater though as iBoost takes care of any surplus electricity we produce. Have to admit after three days of cloud, water temperature drops to around 40 degrees C but still warm enough (we are not into cold showers!)

iBoost displays 122 kWh's saved this summer

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GaryComment left on: 25 September 2015 at 7:26 pm

But you can also power underfloor heating, oli radiators or storage heaters from you diverter, in addition to your immersion.  immerSUN has 3 outputs which switch sequentially - when device one turns off, two comes on etc.

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paul53Comment left on: 24 September 2015 at 4:37 pm

your  right gary  at the  moment  i  can see  no advantages  for  solar  thermal except maybe  under floor heating 

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GaryComment left on: 21 September 2015 at 6:51 pm

Interesting but why go for the expense and returns of solar thermal when you can achieve the same as a by-product of solar PV with the add-on of a diverter like the immerSUN or iBoost etc.

I have the former and turn my gas off as soon as the heating is no longer needed and turn the gas back on in autumn when heating is needed.  I save about £80 on gas over that period and probably get hot PV water on about 75% of the rest of the year. Quite how large the savings are from not sending hot boiler water round the hot water cylinder is a good question but maybe an extra £20 or so.

And the cost? £350 plus fitting for the immerSUN and a £100+ less for some of the others.  Build in the difference in FiTs and thermal looks expensive...

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