Earth Notes: Saving Electricity and Gas (2008)Updated 2023-09-22 18:22 GMT.
By Damon Hart-Davis.
2008 data (kWh: e used 2036, e gen -901, g 8883).
During 2008 gas consumption for cooking and water heating is ~8kWh/day. (From summer when there is no space-heat demand and we are not on holiday.) That totals ~3MWh for the year. This leaves ~6MWh as space heating, with 1061 heating-degree-days @ 12°C, ie about 5.7kWh/HDD12. By contrast, the equivalent for the first 3 months of 2009 is ~3.9kWh/HDD12, which implies 30%--40% improved heating efficiency as previously inferred. And indeed, given that 2008 benefited from efficiency measures at the end, comparing with the first 3 months of 2008 at ~7.4kWh/HDD12 more clearly indicates a near-doubling of efficiency over a year.
Our total exterior wall / ground floor / roof area is ~164m^2. So a heating demand of 3.9kWh/HDD12 (162W/K) as at the start of 2009 corresponds to an effective U-value of approx 1W/K/m^2 overall.
During the recent air-leakage and SAP investigation one of the parameters estimated was the U-value of the large blank north wall, at around 0.8W/m^2/K, so over its ~40m^2 that's 32W/K or ~0.77kWh/K/day. Or ~0.77kWh/HDD in the terms above, or about 20% of total heat loss. Modern building regs require a U-value of <0.4W/m^2/K, and retrofitting that wall to meet or exceed that would maybe save 10% or 0.6MWh/y in a harsh winter. Similarly improving the other exterior walls (~50m^2 - window area) could save that full 20% or ~1MWh/y, or half my 2009 target. This broadly agrees with the SAP calculations which suggest that ~30% of heat is being lost through the walls currently (including ventilation). Window, ground floor and ventilation losses all tie at about 20%. In general it would be good to exceed building regs by a 'best practice' margin of (say) 25% if it can be done to maximise the return given the disruption.
As of our June 2009 holiday the house base electricity consumption, in our absence with essentially just the fridge/freezer and server/ADSL running, was just over 2kWh/day. When I am here on my own (and my family is away) I tend to use about 0.5kWh/day extra.
We had a new electricity meter installed 2008-02-11 since the old one might have been unsafe. But the new one was unratcheted and thus ran backwards when we exported/"spilled" to the grid which apparently causes various accounting and reporting traumas. So we had yet another change (to an import/export pair) 2009-09-21!
The new import/export meter pair was installed in under an hour. (Unfortunately at solar noon, thus losing a prime hour's generation!) The meters are digital (mechanical rather than electronic). So we now have three digital kWh readouts: generation, import and export. Note that generation - (import + export) is whatever we consume internally.
The gas company's estimate of our consumption from 2009-01-02 to 2009-06-29 though a meter change, etc, is 3714.6kWh. My estimate from 2009-01-01 to 2009-06-30 is 3744kWh. Which is pretty-well spot on given the two days' difference.
|Month||Consumed kWh (/d)||Generated -kWh (/d)||Consumed kWh (/d)||Period||Notes|
|2008-01||214 (6.9/d)||1418 (45.7/d)||20071231T20:38Z to 20080131T20:51Z (31d)||Gas use >60kWh/day at the end of the month. No microgeneration (other than a tiny off-grid PV system). HDD12=140.|
|2008-02||195 (6.7/d)||-4 (-1.6/d)||1449 (50.0/d)||20080131T20:51Z to 20080229T20:06Z (29d)||Gas use >60kWh/day at various points. Import meter unratcheted thus able to run in reverse, so actual import readings totalling 191kWh shown with the 4kWh of the PV generation meter added back on. PV system operational for only the last 2.5d approx of the period from 2008-02-27 ~13:00 with first ~0.4kWh missed in the logs (|
|2008-03||187 (6.0/d)||-70 (-2.2/d)||1323 (42.7/d)||20080229T20:06Z to 20080331T18:36Z (31d)||Import meter unratcheted, so reading of 117kWh shown with 70kWh of the PV generation added back on. PV generation logs (|
Replacement of fridge/freezer with A+ model reduced electricity consumption by ~1kWh/day. HDD12=157.
|2008-04||165 (5.5/d)||-108 (-3.6/d)||882 (29.4/d)||20080331T18:36Z to 20080430T20:21Z (30d)||Import meter unratcheted, so reading of 57kWh shown with 108kWh of the PV generation added back on. PV generation logs (|
|2008-05||146 (4.7/d)||-135 (-4.4/d)||290 (9.3/d)||20080430T20:21Z to 20080531T19:18Z (31d)||Import meter unratcheted, so reading of 11kWh shown with 135kWh of the PV generation added back on. PV generation logs (|
HDD12=22. The central heating was turned off 2008-05-14.
|2008-06||146 (4.9/d)||-160 (-5.3/d)||264 (8.8/d)||20080531T19:18Z to 20080630T20:05Z (30d)||Import meter unratcheted, so reading of -14kWh shown with 160kWh of the PV generation added back on. PV generation logs (|
I estimate that our gas use for DHW is now a little over 6kWh/day. Cooking is the only other use at this time of year.
The electricity bill for the first six months of the year was just over £80, even on the premium 'all renewables' tariff.
|2008-07||139 (4.5/d)||-161 (-5.2/d)||204 (6.6/d)||20080630T20:05Z to 20080731T23:59Z approx (31d)||Import meter unratcheted, so reading of -22kWh shown with 161kWh of the PV generation added back on. PV generation logs (|
Not present to take reading due to holiday, so estimated by subtracting 4kWh from the import and 6.5kWh from the PV generation readings of 20080802T13:30Z (no gas was used at all while we were away).
|2008-08||147 (4.7/d)||-104 (-3.3/d)||230 (7.4/d)||20080731T23:59Z to 20080901T06:48Z (31d)||Import meter unratcheted, so reading of ~44kWh shown with ~104kWh of the PV generation added back on. PV generation logs (|
Adjustments for end-of-July reading taken into account.
|2008-09||152 (5.1/d)||-85 (-2.8/d)||271 (9.0/d)||20080901T06:48Z to 20080930T17:28Z (30d)||Import meter unratcheted, so reading of ~67kWh shown with ~85kWh of the PV generation added back on. PV generation logs (|
Electricity bill for Jul/Aug/Sep just over £15 and <1kWh/day net!
|2008-10||160 (5.2/d)||-53 (-1.7/d)||529 (17/d)||20080930T17:28Z to 20081031T19:47Z (31d)||Import meter unratcheted, so reading of ~107kWh shown with ~53kWh of the PV generation added back on. PV generation logs (|
Heating turned on this (fairly chilly) month, and we had had a couple of thermostatic radiator valves (TRV) fitted ready, with more due next month. The boiler broke down shortly after its service, so heating was erratic for a while.
|2008-11||191 (6.4/d)||-18 (-0.57/d)||828 (28/d)||20081031T19:47Z to 20081130T18:30Z (30d)||Import meter unratcheted, so reading of ~173kWh shown with ~18kWh of the PV generation added back on. PV generation logs (|
We had a new arrival in the household this month, thus extra demand for heat, washing, drying (that could not go outside on the line of course), etc! Green tip: have your babies in the spring...
|2008-12||210 (6.8/d)||-17 (-0.54/d)||1194 (39/d)||20081130T18:30Z to 20081231T18:09Z (31d)||Import meter unratcheted, so reading of ~194kWh shown with ~18kWh of the PV generation added back on. PV generation logs (|
We managed to meet our gas (heating energy) budget for the winter taking us up to <8.9MWh for the year.
The electricity bill to the end of this month was a little over £70, so ~£170 for the year, even at a premium 'green' rate, and the gas bill ~£280 to the year end: in total less than half the 'typical' £1000+.
|2008 total (Jan--Dec)||2036 (5.6/d)||-901 (-2.9/d)||8883 (24/d)||20071231T20:38Z to 20081231T18:09Z (366d)||1.29kWp grid-tied solar PV system operational from 2008-02-27, ie 57.5d after year start.|
See the daily PV generation logs for 2008 as
HDD12 for the year totals 1061.
(DUKES 2006 figures that suggest average consumption per GB household of natural gas is ~18MWh/y (400TWh/22M) and electricity is ~4.5MWh/y (118TWh/26.5M, though modal UK use is a little over 3MWh/y), so we are comfortably less than half of average for each.)
|kgCO2||875kg (2.4/d)||-387kg (-1.1/d)||1688kg (4.6/d)||~2.2tCO2 (~5.9kgCO2/d) gas/electricity emissions total for 3 people (4 from mid-November), or about 2/3rds reduction for the household from only a year or two ago, based on 0.43kgCO2/kWh for electricity (netted) and 0.19kgCO2/kWh for gas. This SAP energy performance certificate computed 2009-03 estimates 1.9tCO2.|
(Cf 2007-8 5.2t/person total footprint in China, 8.6t Western Europe, 19.4t US. David JC MacKay author of Sustainable Energy - Without the Hot Air in private communication suggests that in the UK public/government services alone emit ~5.5t/person/y as of 2008.)
Note that we have not flown this year (nor indeed for some years), we don't own a car and travel almost entirely by (usually-electrified) public transport (we drove ~80 miles in a hire/rental car in 2008).