Passive House Calculator
Fifteen kilowatt-hours a square metre a year, which is about a ninth of an ordinary house.
The formula
annual heating = floor area * kWh per m2 per year
Fifteen kilowatt-hours a square metre
That is the Passivhaus limit for space heating, and it is roughly a ninth of what an ordinary existing house uses. The standard is defined by performance rather than by construction: it does not tell you what to build, only what the finished building must achieve.
Airtightness does most of the work
The 0.6 air changes an hour at 50 pascals is the requirement people find hardest, and it is where most of the saving comes from. An ordinary house leaks five to ten times that, and heating air that immediately escapes is the single largest loss in most buildings. Insulation without airtightness achieves far less than the figures suggest.
Which is why ventilation becomes mandatory
A building that airtight cannot rely on draughts, so mechanical ventilation with heat recovery is not an optional extra but part of the standard. It recovers 75 to 90 % of the heat from outgoing air, which is what makes the low heating demand possible at all.
How much to trust a footprint
Life-cycle figures for the same product routinely differ by a factor of two between studies, because deciding what to count — the farm, the packaging, the shop, the drive home, the disposal — is a methodological choice rather than a measurement. The numbers here are central estimates from published meta-analyses. They are reliable for comparing one option against another, which is what people actually use them for, and not reliable to the second significant figure.
The other thing worth keeping in view is scale. Individual choices matter and they are not where the emissions are: the difference between the best and worst quartile of beef producers is larger than most people's entire dietary footprint, and that is a question about agriculture policy rather than about shopping.