GOAL: so viel wie nötig, so wenig wie möglich
(as much as needed, as little as possible)
Units: Kilowatt Hours
This one's remarkably easy- if you've paid attention to your building materials and envelope. From here out, it's just math. How much heat your building requires depends on how much it loses, as well as how much heat the buildings' occupants add to it while going about their daily lives.
Recently a client asked me about an ecological heating system. I had to pause, because the term isn't necessarily defined; what might be ecological to one person might not be to another. What we ended up choosing together was a heat pump (brine-brine) with a coefficient of performance close to 5 (five times more efficient than heating with electrical resistance). Because the family lives in a sunny region, much of the electricity required for their heat pump can be sourced from photo voltaic panels if desired.
Other strict Passive House owners insist of infrared heaters, which heat instantly and only in the rooms being occupied at that moment. To each their own.
For others, solid fuels such as wood or wood pellets fit the bill for an ecological heating, wood being the wonderful renewable resource it is.