[Piwoslaw]

Nice result WD-40.
To Daox's post I'll add that in my case I use nat. gas for the stove and hot water, so by looking at gas usage for the non-heating season I know how much goes for those needs per day (about 0.5m3/day in my case). I multiply that by 365 and subtract from the year's total gas, and that gives me my gas usage for heating only.

If you want to be really detailed about this, then you should calculate how much hot water turns into space heating, same for the oven, refrigerator, hair driers, lights, electronics, etc. This is almost nothing for a large, inefficient house, but could probably amount to a large percent in a passive house. Maybe these things, plus the occupants' body heat, are taken into account when planning a passive house? I read that when some Scandinavians want to heat up their house they invite friends/neighbors over for a party.