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New solar thermal calculation for domestic RHI increases returns

Posted by Stuart Elmes on 28 October 2013 at 9:26 am

MCS recently published a new calculation method for predicting the energy benefits from solar water heating. The announcement has been eagerly awaited by the solar industry because it will be the way payments are calculated under the domestic Renewable Heat Incentive.

As we’ll see, the changes are far-reaching and significantly increase the calculated energy savings, especially for households that use more hot water.

What's changed?

The old version was based upon the government's Standard Assessment Procedure (SAP), and so is the new version, but with the following changes: 

1.  The number of people in the house

The biggest factor impacting the solar heating energy yield is the amount of hot water used, and in domestic properties this is almost entirely down to how many people live there.

The old version of the calculation followed SAP slavishly. Unfortunately, this was originally developed to help builders show that they are meeting building regulations for energy use. Of course in a new build the number of people who will be using hot water in the building is often not known, so the SAP calculation predicts it from the floor area of the property, with an increasing number of occupants as the property size increases.

Because this SAP calculated occupancy peaks at around three people for homes that are not exceptionally large, it greatly under-estimates the potential for solar energy for the many households with more people living in them. If this calculation were to have been used for the RHI payments it would encourage a solar installation in a mansion with a retired couple living in it over a smaller house occupied by a family of five.  

The new calculation requires the solar installer to enter the actual number of residents in the property at the time of the installation, taking into account any part-time residents (for example children who visit regularly but who wouldn’t describe the house as their main home, or kids away at college in term time).  Installations will be subject to random audit by Ofgem to discourage people from cheating, and for RHI deeming purposes, occupancy will be limited to six full-time equivalent.

The change is about neutral for smaller households (one or two people), and increasingly positive as the number of people increases. The new calculation significantly boosts the energy yield for larger households, rewarding the solar installations that deliver the highest energy savings.  

2.  ‘Boiler’ efficiency

The old calculation worked out the solar heat added to the hot water cylinder and this figure was presented to the customer.  The new calculation recognises that the customer is more interested in their saving on energy bills, and that this is not the same thing.  Why? Because the boiler that would have heated the water if the solar system wasn't there works with an efficiency.  So in most cases the fuel energy saved is higher than the energy input to the cylinder.

For example, a boiler operating with 80% efficiency would burn 1,250 kWh of gas to produce 1,000 kWh of heat. (1,000/0.8 =1,250)

It also turns out that back-up heaters have a summer and winter efficiency. Most are less efficient in summer than in winter – the energy costs from starting up the heater (warming up the heater and any connecting pipes) are a larger proportion of energy delivered when you are only heating water and not heating the whole house at the same time. Most of the solar energy from a solar water heater is delivered in summer, the time when the back-up heater is at its least efficient.

The calculation takes this into account and a "solar efficiency" figure is used to calculate the energy saving as well as the renewable energy.

3.  Electric showers

The SAP calculation assumes a certain hot water use per person. In the old version, the hot water use was based on an average for all homes in the UK, and this included those homes with an electric shower.  Electric showers are fed with cold water and so don't take water from the hot water cylinder.  This had the effect of lowering the hot water use taken as the UK average. (See my solarblogger article on SAP 2012 for more detail.)

By separating out homes without electric showers, hot water use is boosted in the new version by around 30 per cent and the benefits from solar heating further increased.

4. Sunshine

The previous version used a single figure for the light levels irrespective of where in the UK you were. The new calculation divides the country into 21 regions (see my solarblogger article on SAP 2012). The UK average remains the same, but some regions are lower than the average and some higher.

In addition, the irradiation is calculated for the absolute angle and orientation of the panel instead of using a look up table. 

5. Cylinder heat losses

A standing loss for the cylinder is added to the hot water demand and this increases the total heat load that the solar system can contribute towards and boosts the calculated energy.

The effect of the changes

Let's run some examples through the old and new calculation. To keep things simple, we'll use the same flat plate solar thermal panel in each situation and keep the shading (none or very little) orientation (south) and pitch (30 degrees) the same for each option. We’ll locate the panels in Sheffield (which receives the UK average light levels) and make sure we’ve removed electric showers from the house.  We'll also put a modern condensing gas boiler in the house as the back-up heater.

For the old version of the calculation, I have used a house size of 85 m2, an average size for a town house with three bedrooms. For the new calculation, we look at a range of occupants between two and six, with the area of solar panel and volume of the hot water cylinder chosen to suit.

The old calculation ignores the actual number of people who live in the house and uses 2.7 residents in all cases, a figure derived from the floor area.  The results highlight the way that the old calculation under-sold the benefits of solar water heating in situations where more people live in the property.

In addition, the energy saving presented to the customer was the solar heat input to the cylinder, and ignored the efficiency of the boiler.  The new calculation takes the boiler efficiency into account. 

The graph above shows this. The black line shows the old calculation method.  As the number of occupants increases in this house the energy increases slowly due to the increasing size of the solar system but because the assumed hot water demand is fixed by the size of the house, the increase in energy is limited.

The orange line shows the new method, with a much greater increase of solar energy as the number of people using water increases and the system size increases to suit this. 

The red line shows the energy saving taking into account the ‘solar’ efficiency of the boiler, in this case a modern gas condensing boiler.

The new method calculates energy savings that are between 50% higher and 142% higher than the old calculation.  The deemed renewable heat is between 14% and 85% higher.

What this means for householders

If you have already installed solar

The new calculation is for solar installations and cannot be applied retrospectively to solar installations that have already been uploaded to the MCS database. For these installations, the figure that will be used to calculate the domestic RHI payments is the figure that is on your MCS certificate, calculated with the old method.

If you are considering installing solar

The new calculation method is available immediately for all new MCS registered installations. People installing today will be able to join the domestic RHI once it begins next spring as if they had installed on the date they joined. There’s also a £600 grant available for households that install solar thermal between now and the start of the domestic RHI.  Although this grant will be deducted (top-sliced) from your domestic RHI payments, it’s still better to have the money now rather than in seven years’ time, plus you start saving on your energy bills immediately.

There’s never been a better time to install solar heating.

More information on solar thermal and the domestic RHI on YouGen 

YouGen guide to solar thermal

YouGen guide to the RHI

Solar hot water panels: 7 things to check before you install

Case study: Solar thermal, solar PV and a biomass boiler work together at Devon vineyard

Photo: Viridian Solar

About the author:

Stuart Elmes is founder and CEO of Viridian Solar, a UK manufacturer of beautiful roof integrated solar PV panels and matching solar thermal panels.

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

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4 comments - read them below or add one

Stuart Elmes

Stuart Elmes from Comment left on: 4 November 2013 at 9:22 am

Hi Paul555


good to hear you're so pleased with your solar heating system.  The domestic RHI is available retrospectively for installations installed between 15th July 2009 and the start of the scheme next spring.


Based on yours being 9 years old, I gues this means it was installed some time in 2004, so unfortunately this puts it outside of the qualifying period.



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paul555Comment left on: 4 November 2013 at 8:58 am

I had a solar array for hotwater fitted 9 years ago - I wonder if there is any way I can get this registered for the scheme. My system is not currently registered, nor did I get any assistance with installation costs. It is a large array to heat my swimming pool in the summer so when it is just heating domestic hot water in the winter months it still covers all my hot water needs for almost all the time.

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Stuart Elmes

Stuart Elmes from Comment left on: 30 October 2013 at 9:07 pm

Hi Ed 

The RHI is paid against the 'renewable heat', so 19.2p x the solar energy. The energy saving is the saving on bills.

The south of England will get up to 5% more than the UK average energy. I reckon reading would be about 3% higher than the figures I've shown. 


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ed1066Comment left on: 30 October 2013 at 6:59 pm

 This is an excellent article Stuart, clear and great news. Thanks

In the worked example, will the RHI be  paid on the basis of energy savings?  I guess its yes, but the term 'renewable heat' for the lower increases in calculation confused me as its similar to renewable heat incentive.

What is the further increase for a home counties location? (Reading area)

Thanks again


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