Earth Notes: Saving Electricity and Gas (2008)

2008

2008 data (kWh: e used 2036, e gen -901, g 8883).

See the daily PV generation logs for 2008 as .csv and .png chart.

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.