Energy Footprint

How to measure your own impact? It depends on what you measure.

For example, a solar thermal system’s annual savings for a typical 4-person house generates the energy equivalent of 260 litres of gasoline. Sounds like a lot, but in a large vehicle this may only mean about 2,500 km, which many people will drive in a month.

Some Real Data

Victorian Row (Terrace) House, 1905

The author has co-owned this 'downtown' row house since 1997. Only 15' (5m) wide, 3-storey, built originally to burn coal, then replaced with natural gas heating some time in the 1900's. Since moving in, project by project, the footprint has come down thanks to investments in building envelope, efficient appliances and renewable energy generation. And a major fuel switch has taken place to heat via an air source heat pump, not natural gas, due to a drastic de-carbonisation of the electricity grid that has taken place in the last decade. This data is to show what can be done!

Water

Notes: front axis washing machine replaced a top loader, low flow shower head (2 USGPM), one 6 lpf and one dual flush toilet (3/4.8 lpf).

Oh and a rain barrel.

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Electricity

Notes: 2016-on increase due to addition of air source heat pump

Matches to a reduction in natural gas (see below)

Biggest noticeable impact? Switch to LED bulbs from CFL and halogens.

2004 on - new born means lots more time at home.

Natural Gas

Notes: drop in 2008 (data for 2009-2012 missing) due to solar thermal hot water preheat. Eliminated about 15% of total gas consumption.

Jump in 2005 - cold winter and new-born arrives.

Big drop in 2016 with elimination of gas-fired heating.

Wood burning insert installed in 2016, minor use.

Energy Use Intensity

Notes: energy rates go up and come back down. Carbon intensity of grid electricity changes.

Energy in the base unit of consumption and is therefore the best metric long-term.

1 ekWh is one equivalent kiloWatt hour - 1,000W for one hour, or 3,600,000 J.

Renewable Energy Generation

The graph shows monthly generation for a 3.3kW solar PV system installed on the roof for 2018 (installed mid 2017)

The solar thermal system installed in 2008 generates a measured. 1,550 ekWh per annum. Data is recorded via flow meter and matching temperature sensors, connected to a Metrima energy calculator.

The system comprises two 4'x8' collectors and a 350 L storage tank, installed in a drainback configuration.

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Ways to Measure your Impact

Cost

There’s logic to determining how sensible an investment in your home is by how much it will save you. However costs go up and down – and it can be dramatic and shift over time.

Carbon Footprint

Logic is clear here – convert your gas/oil usage to an energy equivalency, apply the efficiency of your end uses and you can find out the cost saving in fuel for an energy investment.For electricity it is tricky as this depends on the ‘when’ and what is generating the power that you are currently using. The world's grids are rapidly de-carbonising, although the overall consumption grows as the world develops.

Energy

Convert it all to Joules, or ekWh (1 ekWh – 1 kW running for 1 hour – kW x h).This has the advantage of not moving over time, as #1 and 2 do. Apply conversion factors according to the energy content of liquid and gaseous fuels. Even wood.

Power grids can be connected across thousands of km and power generation ramps up and down depending on the grid’s power requirement at that time. We have to consider the local grid’s generation profile in order to understand the carbon impact of electricity savings. We have ‘base load’ (i.e. it runs 24/7), then ‘peaking’ plants that may only run a few hours a day to keep the lights on when everything is running.

There are even very short-run systems (hydro storage) where generation may only be a few minutes. Utility companies may also pay people to reduce power for a few seconds or hours to achieve the same thing – as can storage (hydro, batteries, compressed air…).

Certain technologies are suited to certain tasks. Nuclear is ideal as a base load – you can’t just turn a nuclear power station on and off every 5 minutes, whereas to solar electricity’s generation profile is naturally limited by the sun. Storage plays a great potential balancing role. Renewable energies can also become 'disbatchable' - output reduced or shut off at short notice to ensure large base generators don't have to shut down.