Factors to improve energy efficiency/reduce energy use.
This is another tough nut. The EST % figures I believe represent a blended average view of an average house. The average EPC for houses in England is now 63 (D but borderline C), meaning that a great deal of energy efficiency measures have already been implemented (ref: 2018-19 English Housing Survey Headline Report, ONS). However, there remain a large number of houses (pre-1970) that have not been treated and have correspondingly lower EPC for whom the efficiency gains will be very much larger than the figures EST present. (Ref: UK Housing Fact File 2013, ONS)
So for instance a house needed solid wall insulation will get a much greater than 14% reduction in energy use - the EST in fact estimates that they will save (on average) 6,700 kWh (solid wall external cladding) - which is likely to be more like 30-50% of the heating energy for the house. I recommend reading this EST report which puts it in context (2013 figures, but the facts remain the same for kWh saving): (Ref: 2013/12/Review-of-potential-for-carbon-savings-from-residential-energy-efficiency-Final-report-A) link below.
There is a tool that EST developed for calculating the heat load needed for a house (intended for boiler installers, but easy to use) In this you can set the size of house and the type of fabric - becasue to get a true picture of the heat loss you need to model the U-value (the thermal resistance) of the fabric of the house (EPCs use the same calculation - called SAP Standard Assessment Procedure). (Ref: CE-54 domestic-gas_calculator from EST) link below.
I have done a model for my house and played around with changing the fabric settings. When I do this, for instance, for loft insulation I can save 37% of the heat energy in going from an uninsulated loft to 250mm of insulation. However, this is not what I have (I have a year 2000 built house): to go from 100mm to 250mm results only in a 3% saving (law of dininishing returns). Both these numbers are a lot higher than the table you have used. EST tends to state savings in terms of £/year saving rather than kWh, so will be dependent on the £/kWh spend assumption. I will email this spreadsheet to you.
Turning the thermostat down by 1 degree has a larger impact than stated. For instance in the case of my house model, the total energy demand is 248W/K (248 W of energy required for every 1 degree Kelvin © that I want to maintain the house above outside temperature). So to turn the thermostat down by 1 deg C saves 248W. If I heat for 13 hours a day and 180 days a year this equates to 581kWh, 12% of the total. Hence turning the stat down by 1 degree saves 12% of the energy.
Draughts are a bigger issue, again probably for the harder to treat, older houses. EST state that “Draught-proofing around windows and doors could save you around £20 per year* . If you have an open chimney, draught-proofing your chimney when you’re not using it could save around £15 per year.*” (link below) As ever they have stated this in money terms. But if you take the average gas price £45 per year = x.kWh * 0.044 (avg. gas price) therefore 1,022 kWh, so a saving of 8.5% (based on 12,000kWh average gas use). However, 20% of average gas use is for water heating so the saving is 10.6% on space heating. A hard to treat house, however, will have a higher starting point for space heating (another reason to ask for kWh originally for home energy rather than taking an estimate), then draughts represents potentially 10-20% of the heat loss. A very much larger number than in the table.
If solar water heating gives you 100% of your hot water demand, then the saving will be ~20% of your total gas energy (ref: English Housing Survey for proportion). Probably not the case in the UK, but even 50% will give a 10% saving on gas.
I think the problem with the EST figures is that they are not clear how they have been calculated and are based on “averages” - someone with a well insulated, modern house, won’t need cavity insulation or loft insulation, so these numbers don’t really work for them.
The challenge is how to convert all this into something meaningful, based on the type of house that you have. The EST report I have shared and the spreadsheet I will email may help steer this discussion.
Enough for today. Alex