Earth Notes: Saving Electricity and Gas (2011)

2011

2011 data (kWh: e used 1578, e gen -3998, g 3936).

See the daily PV generation logs for 2011 as .csv and .png chart.

I aim to continue to meet or beat the targets set at the end of 2010:

• Electricity: 4.5kWh/d+0.2kWh/HDD12
• Gas: 5kWh/d+3kWh/HDD12

In August I set up an account with [archived] The European Citizens Climate Cup to see how it compares with iMeasure. After a few minutes' data entry (and fighting some glitches such as the site not understanding 'net metering' ie meters that run backwards), I was able to look at some pretty diagrams. One is below, reinforcing my belief that we use less than half the typical UK electricity and gas for our size and location:

Also in August I reviewed the AlertMe energy-monitoring kit for The Register. Amongst other things, it has convinced me that our "always on" electricity load / baseload is ~70W, mainly the fridge I suspect.

As of iMeasure's upgrade in September, with a HDD12 base, iMeasure reports:

Baseload = 27.82 Slope = 2.89 R = 0.942

The near-1.0 R/correlation figure implies that we are efficient, using heating based on actual external temperature. If our DHW and cooking were separate this might be even closer to 1.

The baseload seems to be a weekly kWh figure, ie ~4kWh/d for DHW/cooking, which is about right.

The slope of 2.89, presumably kWh/HDD, is again fairly close to the 3kWh/HDD measured and used as a target. Possibly I should lower our target a little, especially as the data used goes back to 2009 and we've been improving! Note that the (high-heat-demand) points at the right-hand end are all from around the exceptionally cold start and end of 2010.

(The following day, possibly as a result of on-going bug-fixing, the numbers were shown as Baseload = 31.38 Slope = 3.01. So slightly higher but still including values from 2009 of course.)

Having gone back and entered data for previous meters I now have a 4-year energy chart from autumn 2007 to end-summer 2011.

• Heating demand has dropped each year even though winters have not been getting milder.
• Peak heating demand has dropped from ~63kWh/d in 2007-12 to ~38kWh/d in 2011-01.
• The electricity meter from 2008-02 to 2009-09 was unratcheted and iMeasure objects to readings going backwards so that part of the chart is dubious (subtracting our next export from PV generation for the period gives ~3.1MWh consumption for that period, not including what was used locally).
• The extra historical data has adjusted our performance-line heating numbers to Baseload = 52.33 Slope = 3.17 R = 0.810, ie the old data makes us look worse now!
• I discovered that I have lost all meter readings for 2009-07: annoying!

An implication of this, with 3kWh/HDD12 and a typical 1070HDD12 year (3.21MWh), and the fact that our current gas combi is actually only ~80% efficient, is that our true heat demand for CH is now ~2.5MWh/y (and falling). That's somewhat over the heat storage capacity of one mythical buried milk-tanker (20kl/20t) at a delta-T of 80K (~90kWh/t for a delta-T of 80K, ~1.8MWh/tanker). A couple of tonnes (~60kWh/t at a delta-T of 50K, thus ~120kWh) could carry enough from mid-summer into autumn to postpone using gas for DHW for an extra month or so.

Note that the SheevaPlug stayed off-grid all year after the battery upgrade. (And some more tweaking of software to use less energy in the darkest periods.) And I haven't seen the controller light on amber or red even in December.

Carbon footprint for heat'n'light this year indicates, mainly due to the milder weather, that we're slightly carbon-negative (circa -300kgCO2 for the house for 2011), which is good.

That's assuming 0.43kgCO2/kWh electricity for both consumption and generation. At 0.5kgCO2/kWh we're at about -500kg for 2011 by the looks of things. (We're currently a net exporter until I can get ASHP in.)

Definitely not Passivhaus. But with heating at ~50kWh/m^2/year (ie gas use at <4MWh/y cf national average of 18MWh/y) I don't think we're doing too badly.

Disappointingly our gross electricity consumption has gone up a tiny bit.

(Note that we no longer have any incandescent bulbs other than in the oven and (oddly) fridge—we're all CFL and LED energy-saving lamps—nor any other easy low-hanging fruit in this area. Indeed we may have to use a bit extra to dehumidify the house in winter.)

HDD12 for year 820, much lower than previous years, with gas consumption ~3936kWh. So if baseline (DHW+cooking) is still 5kWh/day that leaves 2111kWh for space heating, or a little under 2.6kWh/HDD12. That can guide next year's target (down from 3.0kWh/HDD12).

As of 2011-12-31 imeasure.org.uk is claiming Baseload = 50.46 Slope = 3.17 R = 0.810 under Weather Analysis.

Electricity targets could be tightened a little too. Maybe more like 4kWh/d+0.15kWh/HDD12 (with mean consumption currently ~4.3kWh/d over the year).