I live in a relatively small 3 bedroom detached house built in the late 1990s. Like many UK properties it had relatively poor insulation, basic double glazing, gas central heating. It was pretty much as it was built by the original builders. The family car was a small turbo-diesel with low annual mileage.

The household energy usage (house + car) was monitored as changes were made to the property. It was possible to determine the contribution of each change to the overall energy usage. To help achieve this a detailed heat loss model of the property was created and validated against historic energy use data.

Not all the changes in energy usage are part of the carbon reduction project as some reflect more gradual changes in family energy demand.


In Figure 1 we can see the effect of the various changes on the overall energy used. The results are split between the 3 different sources of energy used: gas, electricity and diesel.

Figure 1: Annual Equivalent Energy Usage

The change from incandescent lights to Compact Fluorescent Lights (CFL) was initially a more than halving of the electricity usage at the time, but seen in terms of overall energy usage was relatively modest as by far the largest use of energy was from gas for heating and cooking. Implementing some of the relatively simple actions as recommended by the energy saving trust included reducing the average room temperature by one degree (twice), first to 19°C , then to 18°C. Surprisingly, lined curtains and topping up the loft insulation helped reduce the gas usage by about a similar amount. The next energy saving measure was applying exterior insulation to around 10% of the property (where the walls were timber framed) at the same time as re-rendering. And as the garage is integral into the property, my heat loss model identified that the thin metal garage door was actually responsible for a disproportionate heat loss via the colder garage, so some multi-foil insulation was added to the back of the door. The next major change was the addition of solar Photo Voltaic (PV) panels and a diverter (using the immersion heater in the hot water tank) for summer hot water. This had a positive effect on the amount of electricity bought in and the diverter did reduce the gas usage too, however it had less of an impact on the total energy usage as that was still dominated by gas (exported generation is ignored here as this is about household energy usage). Creating an open-plan living space downstairs allowed internal wall insulation to be added to about a quarter of the property and as new windows were required upstairs for building regulation reasons, triple glazing was installed throughout. These building fabric changes were good at reducing heat loss. The addition of an all-year energy efficient conservatory had a detrimental effect on predicted energy usage with gas central heating so the decision was made to move to an Air Source Heat Pump (ASHP). The style best suited to the property now with its open plan downstairs was actually a multi-split air-to-air type Air Source Heat Pump. One small unit is in the conservatory and one large unit is in the main house. Now there was no need for downstairs radiators and we could reclaim the walls for furniture where radiators had once been. Initially we ran Gas Central Heating (GCH) on the upstairs radiator loop (taking 20% of the space heating duty) but as we found that the open plan stairs actually enabled heat from the heat pump to heat upstairs (heat rises!) we could progressively reduce the heat supplied by gas. Switching to a green tariff for electricity clearly had no effect on energy usage (REGO means Renewable Energy Guarantees of Origin). The next big change was the purchase of a second-hand electric car. Immediately that shifted a large chunk of energy associated with diesel use over to electricity. New diverters for the car and hot water allowed a greater proportion of self-generated electricity from the solar panels to be used. More confidence in the use of the Air Source Heat Pump allowed the upstairs to be heated passively throughout winter. The final aspect has been the re-insulation of a poorly insulated dormer to reduce heat loss as that was identified as the highest heat loss per surface area of the property.

The most effective reductions in energy use for us were (in order):

  • Installation of an Air Source Heat Pump (ASHP)
  • Use of an electric car
  • Loft insulation and thick lined curtains
  • Efficient lighting
  • Solar PV and diverter for hot water (excluding exported electricity)

Note that these reductions are in absolute terms at the time of the change. For example, insulation changes before the installation of an Air Source Heat Pump would save more energy than had they been done afterwards, because less energy is used for heating.

How does this translate to carbon dioxide emissions? Here we calculate carbon dioxide emissions equivalent and include the carbon cost of extraction, refining and transporting the primary fuels (gas and diesel) and the transmission and distribution losses for the electricity supply (due account is taken of the life-cycle emissions for the generation of electricity). Figure 2 shows the results:

Figure 2: Annual Equivalent CO2 Emissions

Generally the carbon dioxide emissions track the energy usage. The biggest drop in carbon dioxide emissions came with the installation of the Air Source Heat Pump (ASHP) even though the electricity used at the time was fairly carbon intensive (320 gCO2e/kWh). The drop in carbon dioxide emissions from using green tariff electricity are quite significant. The first provider used Renewable Energy Guarantees of Origin (REGO) to demonstrate purchase from renewable generators and the life-cycle carbon intensity was calculated from their published energy mix and applying the median numbers from the IPCC [1]. As REGO certificates can be purchased independently from energy purchase and because another supplier who uses PPA (Power Purchase Agreements) had a lower carbon intensity energy mix, the carbon intensity was further reduced by switching to that other supplier. The use of an electric car made the final substantial reduction in carbon dioxide emissions as we shift away from diesel to electricity. The choice to use electricity to boost hot water temperatures in winter rather than gas was a deliberate decision to reduce carbon dioxide emissions but it was an expensive change relative to the amount of energy involved.

The most effective reductions in carbon dioxide emissions for us were (in order):

  • Installation of an Air Source Heat Pump (ASHP)
  • Green tariff electricity
  • Use of an electric car
  • Efficient lighting
  • Solar PV and diverter for hot water (ignoring contribution from exported energy)

Note that these reductions are in absolute terms at the time of the change. For example, if green tariff electricity were in place before switching from Gas Central Heating (GCH) to an Air Source Heat Pump then the drop in carbon dioxide emissions would be even greater.

The reduction in energy use and carbon dioxide emissions have happened simultaneously with a reduction in spend on energy. The cost to purchase fuels and electricity has been tracked in Figure 3.

Figure 3: Annual Equivalent Energy Cost (inflation adjusted to 2020 prices)

The prices have been adjusted to 2020 numbers to account for inflation over the years. There is still a strong economic benefit for changing from gas for space heating to an electrically-driven Air Source Heat Pump, despite fairly low gas prices. As gas prices start increasing as predicted by the World Bank [2], the economic case for heat pumps is likely to become even stronger. Electric cars appear to provide a solid way of reducing energy bills. Finally the premium paid for the lowest carbon green energy is affordable once the savings from using less energy have been realised.

The most effective reductions in fuel bills for us, excluding tariff changes, were (in order):

  • Use of an electric car
  • Installation of an Air Source Heat Pump (ASHP)
  • Efficient lighting
  • Solar PV and diverter for hot water (excluding payments received for exported electricity)
  • Loft insulation and thick lined curtains

The average return on investment for these changes is expected to be around 11 years.

While latterly my focus has been to reduce carbon dioxide emissions, it is clear that quite a few of the changes offer the opportunity to reduce costs at the same time.

[1] IPCC 2014 carbon intensities: https://www.ipcc.ch/site/assets/uploads/2018/02/ipcc_wg3_ar5_annex-iii.pdf, p.7

[2] World Bank Commodities Price Forecast for natural gas in Europe from April 2021 shows 2020 wholesale commodity prices at a historic low of $3.2/mmbtu, with projections of $6.1/mmbtu by 2030 (mmbtu = one billion Btu = 263kWh), https://thedocs.worldbank.org/en/doc/c5de1ea3b3276cf54e7a1dff4e95362b-0350012021/related/CMO-April-2021-forecasts.pdf