Tuesday, 1 October 2013

Solar Energy Redeemed

The Last Piece Falls into Place for the Domestic RHI

A hot prospect? 
Image courtesy Viridian Solar
MCS recently unveiled its new calculation methodology for predicting the energy benefits of solar heating, and its great news for the industry. The announcement was eagerly awaited not least because it will be the way payments are calculated under the domestic Renewable Heat Incentive.

What's Changed?

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

1.  The Number of Occupants

Arguably the single 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 in the house.

The previous version of the calculation followed SAP, which was originally developed for new build where the number of people using hot water in the building is not normally known.  The SAP calculation predicts the occupancy 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 abnormally high, it is underscoring the potential for solar energy greatly for the many households with more people than this. If this were to have been used for the RHI payments it would incentivise a solar installation in a mansion with a retired couple living in it over a smaller house with a family of five living there.  

The new calculation requires the solar installer to enter the actual occupancy of the property at the time of the installation, taking into account any part time residents (for example children that visit regularly but who don't count the house as their primary residence).  Installations will be subject to random audit by OFGEM, discouraging potential cheating, but for RHI deeming purposes, it was also decided to limit the occupancy to six full time equivalent.

This change is broadly neutral for smaller households of one or two people, and increasingly positive as the number of occupants increases. The new calculation will significantly boost the energy yield for larger households, rewarding those solar installations that deliver the highest energy savings. 

2.  Electric Showers

The SAP calculation assumes a certain hot water use per person. In the previous (2009) version, the hot water use was based on an average for all homes in the UK including those with an electric shower.  Electric showers are fed with cold water so don't use water from the hot water cylinder and this has the effect of lowering the hot water use in the UK average.

By separating out homes without electric showers, in the 2012 version, hot water use is boosted by around 30 per cent and the benefits from solar heating further increased.

Incidentally, this welcome change should see some of the daft (predominantly new build) installations disappear where solar water heating has been installed alongside electric showers.

3.  Backup Heater Efficiency

The previous calculation worked out the solar heat input to the hot water cylinder and this was the figure presented to the customer.  The new calculation recognises that the customer is more interested in their saving on energy bills, and 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 bought  is higher than the energy input to the cylinder.

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

It turns out that backup heaters have a summer and winter efficiency. Most are less efficient in summer than in winter - energy overheads from starting up are a larger proportion of energy delivered when you're only heating water and not heating the whole house too. Of course, most solar energy is delivered in summer when the back-up heater is 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. The figure is expressed as an "additional fuel saving" so should be added to the deemed renewable heat to arrive at the total energy saving.

4.  Irradiation

Where the previous version used a single irradiation for the whole of the UK, the new version divides the country into 21 regions (see my earlier 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 standing losses

A standing loss for the cylinder is added to the hot water demand increasing the total heat load that the solar system can contribute towards.

6. Other Minor Changes

The second order heat loss coefficient for the solar panel performance is taken into account.

Homes designated water efficient under Approved document G have their hot water demand reduced by 5%

Believe it or not, there are solar thermal panels out there with efficiency lower than the SAP default. A loophole where these could use the SAP default values has been closed.


The Effect of the Changes


Let's run a few examples through the previous and new calculations. To keep things simple, we'll use the same high quality solar thermal panel for each situation, as well as keeping the shading (none or very little) orientation (south) and pitch (30 degrees) the same for all options.  We’ll also locate the panels in Sheffield (which receives the UK average irradiation) and remove electric showers from the house.  We'll assume the house has a modern condensing gas boiler.

For the previous calculation, I have used a house size of 85 m2, an average sized town house with three bedrooms.  For the new calculation, a range of occupants between two and six is considered, with a panel area and cylinder size chosen appropriately.

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

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

The new calculation gives energy savings between 50% higher and 142% higher than the old calculation.  The deemed renewable heat is calculated to be between 14% and 85% higher.

Many solar installers would have been disappointed by the tariff level of 19.2p set for solar heating under the forthcoming domestic Renewable Heat Incentive.  This improved calculation method provides just the boost that solar heating needs, and radically changes the economic attractiveness of solar heating within the domestic RHI.
The announcement of the RHI deeming calculation for solar heating is the final piece in the jigsaw.  Finally, solar installers can get out there and start presenting the financial benefits of the domestic RHI to potential customers.  In fact with a £600 upfront grant available for installations before the start of the scheme, there's every reason to install solar now.


  1. No doubt, the published MCS calculation (MCS024) gives more optimism than SAP ever did. And I appreciate the work need to get it thus far.

    But by continuing to link RHI payments to a clunky SAP regional radiation database gives rise to anomalies. By rights, those in the higher (sunniest) locations should be gaining considerably more than those in duller locations; all else being equal. Unfortunately some SAP regions cloak over 20% variance in annual irradiance under one average value. Regions 13 & 17 Wales and Scotland are examples. Commercial RHI will pick this up because it’s metered but this form of domestic ‘deeming’ creates artificial winners and losers.

    A similar consequence is that this method will suggest additional fuel savings that in the duller locations will not appear. The solution would have been to allow installers to refer to any one of the homogeneous postcode-linked databases that the trade already uses to design systems. Why oblige MCS installers to present customers with such clunky predictions when so much better is already in use ?

    In addition, there is no response to the wide variety of diurnal DHW profiles – as you say the total volume is important but for tiny UK sized storage cylinders, the time-of-day has strong effect. And does anyone without specialised training ever get the four shading choices correct ?

    I do hope the domestic RHI brings about a recover of solar thermal but we do need to streamline our certification methods to encompass the wider variety of applications other than just DHW.

    1. Chris

      Good to hear your comments on this.

      The structure of the calculator is such that it would be relatively easy in future to take it down to a resolution of the 'outcode' (the letters and first digit) of the postcode. We should work towards that outcome.

      For now though, the changed calculation is a big step forward from the previous one (which used a single irradiation figure for the whole of the UK - Sheffield), and it was really important that DECC was comfortable with the changes, for all its shortcomings, sticking close to SAP did that job.


  2. It's great to see more accurate supply-side calculations; great job helping (understatement I'm assuming...) to draft those.

    I still have reservations about the demand-side calculations. It's great SAP is finally starting to dis-aggregate data to recognise the difference between DHW demand with electric showers vs baths vs mixer showers. I don't think they've got it anywhere right though, leading to poor decision-making from an environmental and economic point of view.


    -Whilst it prevents the lunacy of solar thermal + electric shower in new-build, SAP 2012 has now opened the pandora's box of deemed DHW demand. Double edged sword for solar thermal: I can see the point of use heater lobbyists seeking to render this more accurate to their benefit in future.

    -The "background demand" of miscellaneous sink draw-offs in SAP is too large and the "shower/bath demand" in SAP is too small. This has a significant effect on solar thermal:

    (1) Efficiency would be higher if the large morning/evening draw-offs were larger and the miscellaneous draw-offs throughout the day smaller. (per Chris's comment on DHW profiles)

    (2) Total usage in a home with modern appliances will be lower than SAP estimates and will limit the contribution made by the solar system. I've yet to see studies that contradict Viridian's early studies: DHW demand in modern homes is significantly lower than what SAP (and therefore MCS/RHI) assumes.


    -By basing the SAP score on "efficiency of provision" rather than "efficiency of use" and making no allowances for the behavioural influence of the installed technology we're incentivising profligacy as much as we're encouraging efficiency. In a rational world 100L/day @ 200% seasonal efficiency would not be encouraged over 50L/day @ 100% seasonal efficiency.

    Wearing my capitalist solar thermal hat this is quite a nice state of affairs thank you very much - I'll tolerate the draw-off profile in exchange for deemed generation subsidy based on very favourable total demand numbers. Risk of point-of-use folks successfully changing the total demand number

    The environmentalist and socialist in me would prefer to see improvements in both efficiency of provision AND reductions in DHW demand. The current schemes aren't great in this respect. They're socially regressive too - grandma and the skint postgraduate students with an electric shower and the heating off except in Dec/Jan will have crunched their DHW related CO2 emissions just as much as the wealthy home counties family with a power shower and solar panels on the roof, yet receive none of the financial recognition for this/end up subsidising it through various taxes and levies just as they subsidise the FiT fiasco. Not ideal from a social point of view.

    Working through the SAP DHW calculation:

    Take two properties. One with an electric shower, one with a standard shower. The calculations spit out 23L/day + 16L/person-day for the electric shower, and 46.8L/day + 32.5L/person-day for the standard shower.

    This implies a "background demand" of 23L/day + 16L/person-day. "Showering" apparently uses 23.8L/day + 16.5L/person-day.

    Appliances are all cold-fill only. (thanks to the wrinkles in the energy efficiency labelling scheme)

    This implies "kitchen and bathroom sinks" use 23L/day + 16L/person-day and the "shower" uses 23.8L/day + 16.5L/person-day.

  3. What does this translate to in usage, based on say CSH 3/4 fittings?


    Bathroom sink is 4L/min. Kitchen sink is 6L/min. Shower is 8L/min. Assume 10C incoming mains and 60C hot water. Assume 60C at the kitchen sink, 30C at the bathroom sink, and 40C at the shower.

    The effective flowrates for hot water are Flow_total*(T_blend-T_cold)/(T_hot-T_cold)
    1.6L/min for the bathroom sink. 6L/min for the kitchen sink. 4.8L/min for the shower.

    This implies the kitchen tap runs for 3.8min/day + 2.7min/person-day. Or the bathroom tap for 14.4min/day + 10min/person-day. (or some undefined mixture of the two) The "shower" is clearer and SAP assumes 5min/day + 3.5min/person-day.

    Is this remotely accurate or is it a load of aggregated nonsense?

    1 person runs kitchen tap for 6.5min/day or bathroom tap for 24.4min/day. 2 people are 9.2 and 34.4min/day. 3 , 4, 5, and 6 people are 11.9/44.4, 14.6/54.4, 17.3/64.4, 20/74.4 mins/day respectively.

    Economies of scale in cleaning/cooking are plausible. Though I don't think that linear is necessarily appropriate the number of houses in high occupancy is low and it probably doesn't matter in the grand scheme of things. The total usage masks huge variations though, and thus adversely influences the design of new buildings and retrofit programmes.

    In my house of skint postgrads? There is no mains hot water. Kitchen and bathroom sinks cold* only. Use the dishwasher (cold fill only) for what it's designed for and chuck the odd kettle of water in the sink in the event that the water you boiled the veggies in isn't enough to do the washing up. Wash your hands and brush your teeth in cold in the bathroom; shave in the [electric] shower and use it hose down the bathroom. Said kettle is 1.5L @90C capacity and might get used twice a week for cleaning w/4 adults sharing. Let's say it gets used daily to be generous. Subsecond electricity metering says the weekly bathroom hosedown is <10 minutes of hot water. (40C @ 5L/minute) Let's say 20 minutes/week to be generous.

    Being generous I make that 3L/day (60C equivalent) in the kitchen, and 8.5L/day (60C equivalent) in the bathroom, for occupancy N=4. Reality is more like 5L/day. SAP thinks that number ought to be 87L/day. Is the model an order of magnitude out?

    *With a low flow aerator by the time hot water arrives from the cylinder you've long since washed hands/brushed teeth and left. Seeing as the hot water doesn't sanitise (old wives tale: anything hot enough long enough to sanitise takes your skin off) and cold is acceptable (except on the odd occasion

    Who would you need to balance this? How about the neighbour. Combi boiler running old 22 mm pipework from the previous gravity system (just to maximise draw-off before you get hot water) and no dishwasher. N=4 (2adults/2kids) They would have to wash up for 30 minutes @ 50C (kitchen tap) and run the bathroom tap for 40 minutes @ 30C each day in order to balance out my consumption. That's about par for a totally uninformed household that's just had a new efficient combi boiler installed, isn't on the water meter, and doesn't make the connection between hot water and their gas bills. (the boiler makes the house hot; hot water is an aside/cheap... ...right?)

    Who is SAP about right for? Said neighbour prior to fitting the combi and a shower. Good old non-condensing system boiler, zone valves for heating and hot water, 150L hot water cylinder with an averagely scaled up heat transfer coil; say 3kW output given how often the 12kW boiler cycled. Why was the SAP model right? It's difficult to be wrong when the installed equipment limits the influence of user behaviour. ;-)

  4. How about that shower? We get 8.5, 12, 15.5, 19, 22.5, and 26 mins for N=1 through 6. More relevant is showering time. This is 8.5, 6, 5.1, 4.75, 4.5, and 4.3 minutes per shower, depending on occupancy.

    This *is* bollocks. The studies I've seen, and my own records, say that T averages 8 minutes unless hot water is limited. Therefore with combi boiler or electric shower T should = 8 minutes. Where there's a tank T(total) = 8N minutes or tank capacity, whichever is the lesser.

    SAP is again fairly accurate in a 2.4 bedroom house with 2+2 occupants, a traditional boiler, and a 150L hot water tank. For everything else it's quite far out, though not as far out as it is on the demand at the sink.

    Part of me would like to think that this is a deliberate fudge. BRE runs the numbers, realises that if they make the split between "background" and "shower" accurate it'll result in electric showers and electric point-of-use heaters in all new builds rather than storage tanks and solar thermal, and decides to fudge the numbers/fail to update them in order to prevent this. I suspect it's not so deliberate though. ;-)

  5. Any idea how this will work with a thermal store + plate heat exchanger rather than a DHW cylinder? This has the potential to reduce fuel consumption for space heating particularly in spring and autumn. When I had a Green Deal Assessment it was tricky to decide on the Total Volume/Solar Volume answers.

  6. @ john_connett - at what height does the boiler input / output go into the store? Is the plate heat exchanger for DHW take off or for boiler input?

  7. @ Ben Whittle - Gas condensing system boiler directly connected to top and middle of store (Sonnenkraft PSR 350); space heating directly connected to store with bivalent, 4-port valve blending from top, middle and return; solar coil low in store; plate heat exchanger for DHW take off.

  8. The dedicated solar volume is the volume that can only be heated by solar and nothing else, so in your case the volume below the bottom of the boiler coil.

    The only thing that would change it is if there is an immersion heater lower than the boiler coil, but if you aren't using the immersion heater other than as a back up for emergencies it could reasonably be discounted in my opinion.

    So, If you have a 500 litre store, and the bottom of the boiler coil (or direct input) is exactly half way up the store, you would count dedicated solar volume as 250 litres. There is a more complicated way of working it out under some methods that take into account the timing of the boiler to make a more accurate estimate of the dedicated solar storage, and it may well be that SAP2012 would give a different figure (sorry I havent read it! but there is another solarblogger article about it somewhere if you want to read up on it), but for GDA I suspect this simplified method would be what they would use.

    I have a GD guy coming to my house next week so I'll try and pick his brains if I remember...

  9. Geraint Williams17 October 2013 at 13:03

    I'm new to this site. I have no expertise in this field and am not a solar installer but I was wondering if someone could answer a couple of my questions regarding the RHI which is meant to be starting in spring 2014. I had 40 Thermomax Evacuated Tubes connected to a 480L thermal store installed in 2010 making sure that the installer and tubes were MCS registered as at that point the RHI was meant to be imminent! If I follow the blog and the comments correctly my system is well designed; we have plenty of hot water for a family of four (including 2 teenage daughters) from April to October; the size of the thermal store means we do not need a back up; we have disconnected the electric shower we had and replaced it with two run from the thermal store; the system even helps with space heating via radiators in the spring and summer.

    Question1. The new calculator would appear to be great for us as there are four of us, we have a big tank, we have no electric shower and we purchased high quality efficient tubes. Is the new calculator retrospective?
    Question2. By chance we have just paid a green deal assessor £180 to get the green ticks required to allow us to claim the RHI if it happens in the spring. He is preparing the report as I type and appeared to have no knowledge of this new calculator. Has the new calculator become operational or will my green deal assessor be completing an 'old' SAP?
    Question3. Will I have to get another Green Deal Assessment to take advantage of the new calculator?

    I hope someone can help. Thanks.

    1. Hi Geraint

      I'm sorry that the new calculator is not retrospective. When you apply to join the RHI, the deemed energy will be the energy estimate shown on your MCS certificate as issued at the time of the installation.

      Green Deal assessments will be done with GD SAP, so some of the changes to SAP will make their way into the GD calculation, but the RHI is claimed against the MCS energy estimate and not the Green Deal energy estimate for solar.


    2. Geraint Williams23 October 2013 at 20:19

      Hi Stuart,

      Thank you for this information, it makes things a lot simpler.

      I'm glad solar thermal is finally having a chance to come out of the 'shadow' of solar PV.

      Keep up the good work.


  10. Hello, this is great information.

    Can you give a few WORKED EXAMPLES right through to actual RHI payments, for "typical" domestic setups?

    Does the Gov't work out the energy produced over 20 years, but then pay this over the first 7 years?