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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthortychwarel
    • CommentTimeMar 10th 2021 edited
    I have 10 years of records of my houses energy performance, and I would like to see how it performs relative to the EPC given at completion. how would I do it.

    mean annual consumption 7500 kWh gas, 2500kWh electricity floor area 150 sqm.
    • CommentAuthortony
    • CommentTimeMar 10th 2021
    67kWk/m2/a is not bad, half of normal

    I would give it an A but there are a lot of variables occupancy temperature regime and more
    • CommentTimeMar 10th 2021
    Your records don't count for much unless you have religously kept the house at exactly the temperature used for SAP.
    The formula is in SAP2012

    You need to multiply the energy used by the 2012 prices listed in SAP Table 12

    Gas 7500kWh x 3.48p plus standing charge £120 = £381

    Electric 2500kWh x 13.19p plus standing charge £54 = £384

    (Check - different figures for economy 7 etc, and deduct if you have PV) (or a wind turbine... See other thread...)

    Total cost = £765 in 2012 money

    Then formula on page 35

    ECF = 0.42 X £765 / (150m² +42) = 1.65

    SAP rating = 100- 13.95 X ECF = 77

    Rating band in Table 14: that is a high C77, nearly a B. Very good!

    Please check my figs..

    It likely doesn't match your EPC and the interesting question is why not, is it a usage factor like Tony suggested or has the house performed differently than the original assessment.

    Or was the original assessment garbage like mine... other thread again.
    • CommentAuthortychwarel
    • CommentTimeMar 10th 2021
    Thanks for the replies. The thermostats are set at 20 and it pretty much flat lines at that all the time.
    I can't remember the exact number given but I do remember being told it just scraped a B. So I guess the original assessment which was done off the construction drawings was correct.

    Which leads to the question without either a heat pump or PV is it possible to get an A ?
    EPC ratings are about energy *costs*, not energy *usage*.

    For your size house, every £100/y you save on energy costs, gains you 3 SAP points. You score 100points if you have zero energy purchases. You need to get a score of 92 to get an A rating, so you'd need to spend less than £270/y on energy, including cooking, DHW, lights, standing charge etc

    You could reduce energy costs and get a better EPC by using more gas and less electricity, SAP treats gas as much cheaper than electricity per kWh. (Not good for CO2 anymore, but SAP2012 intensity figures are way out of date).

    Or by using economy 7 to get cheaper electricity. SAP in 2012 hadn't heard about smart meters.

    Or by heating with coal (!!) instead of electricity or gas, very cheap per kWh and no standing charges.

    Or by having solar thermal, PV and a wind turbine so you purchase less of the energy that you use. SAP only cares about energy bill costs, not about capital or maintenance costs. (Again not good for CO2, would be better to have all that gear and export the energy while reducing own usage)

    Sounds like you are already well insulated and much of your energy usage will be hot water and cooking, so more insulation prob wouldn't help much.

    Heat pump probably wouldn't help with costs, as it is substituting expensive electricity instead of cheap gas, even with the CoP it doesn't save costs. But does save lots of CO2.

    Your original EPC might have been done with SAP2009 which gave slightly different scores. It seems to be based on 21degC when the heating is on, cooler the rest of the day.
    • CommentAuthortychwarel
    • CommentTimeMar 11th 2021
    Thanks all. Yes it would have been SAP 2009 as it was completed June 2010
    • CommentAuthorJeff B
    • CommentTimeMar 12th 2021
    Posted By: WillInAberdeenEPC ratings are about energy *costs*, not energy *usage*.

    It is not quite as straightforward as that in our case. Where we are the cost of wood pellets is usually approximately on a par with the cost of heating oil (1Kg pellets is about 25p and 1 litre of oil 50p; N.B. 1 litre of oil has approx twice the energy content of pellets). The EPC assessor rated us as D because he used a 66% efficiency figure for the wood pellet boiler. If we changed back to oil the oil boiler would be rated at 90% and would give us a B rated EPC apparently! Although on paper our annual energy cost for oil or pellets would be the same, the "low" efficiency of the wood pellet boiler makes the difference. In reality the pellet boiler is 92% efficient, so at least the same, if not more efficient than an oil boiler!

    At the moment I am not too bothered but if we ever decided to sell the house then I would be insisting on a higher efficiency figure for the pellet boiler and getting that B rating!
    It makes no difference what the real-world cost is where you are, unfortunately. The SAP assessment is based on standardised energy prices from 2012, the same price is used wherever you are, heating oil is priced at 5.44p/kWh, about 40p/litre. TBF the price of oil has been haywire this year, so if they used real world prices then the EPC would change every few months.

    Some boilers (with well-measured efficiency) have been included in the database by their manufacturers. If yours is not, then the assessor has to use the default values, s/he cannot use other values even if the homeowner produces documents. So SAP will assess that you consume more pellets, so your energy costs will be higher, so your EPC will be worse.

    A change of efficiency from 65% to 90% would take 30% off your heating bill, that wouldn't get you from a D to a B unfortunately - you would need to halve your energy bills to do that, including for lighting cooking etc

    Time the system was revamped....!
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