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During 6 days my heating system consumed 11,4 m3 of natural gas keeping its temperature at 20 degrees of Celsius. No human or other energy source (except for refrigerator) was inside the flat at that time.

Outside temperature was about 0 degrees of Celsius (but I can provide more accurate temperature data set).

Is it possible to calculate energy efficiency of that flat by knowing how much energy was consumed to keep temperature?

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    What heating source do you use? Does it have an efficiency rating? The main uncertainty in the calculation is the amount of heat leaving through the chimney/exhaust. – Jan Doggen Jan 26 '16 at 13:23
  • Ah indeed, I didn't take it into account. – Marian Paździoch Jan 26 '16 at 13:27
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    What do you mean by the "energy efficiency of the flat"? That could mean various things. Another way to approach the same question: how would you use the information? – EnergyNumbers Jan 26 '16 at 18:40
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    To measure the overall thermal resistance (R rating) you'd need to know the temperature difference and also how much air was moving in and out. But as @EnergyNumbers says, what would you do with that information? StackExchange works better with concrete problems. – Ⴖuі Jan 27 '16 at 7:04
  • Things with energy efficiency expressed as percentage are normally machines of some sort that use one type of energy and do some useful work, e.g. an electrical transformer or a winch. A flat isn't like that - it isn't built primarily to do physical work. – bdsl Jun 19 '16 at 9:43
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No, because with this limited data you cannot separately estimate the three components that make up a building's energy signature:

  • the base load
  • the base temperature
  • the energy efficiency

Even if we assume the base load (i.e. the part of the energy used for other things than heating) is zero, there's no way you can estimate the base temperature (i.e. the outdoor temperature above which no heating is required).

If you assume that there were no casual gains during that period, then you can probably assume the base temperature to be 20 C, and only then can you reasonably claim that the energy efficiency of your building to be 11.4 * 1000 / (6 * 20) = 95 liters of gas per degree per day. Natural gas holds about 11 kWh per m3, so that's about 1.045 kWh / day / degree. That's very good (Swiss houses are between 4 and 10), whatever the size of your building, but it doesn't take into account your heating system's (in-)efficiency.

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