In the United States, a house built to the Passive House standard results in a building that requires space heating energy of 1 BTU per square foot per heating degree day, compared with about 5 to 15 BTUs per square foot per heating degree day for a similar building built to meet the 2003 Model Energy Efficiency Code. This is between 75 and 95% less energy for space heating and cooling than current new buildings that meet today’s US energy efficiency codes.
(from: http://en.wikipedia.org/wiki/Passive_house)

The Passive House Planning Package (PHPP)
is a clearly structured design tool that can be used directly by architects and designers.
Does energy-conscious design require sophisticated simulations?
This was indeed the case for the first Passive Houses that were completed in 1991. Calculating the energy balance of buildings with very low energy consumption is a demanding task – existing regulations, standards and prestandards lack the required precision. Nevertheless, we have identified the critical factors for preparing reliable balances – with tools that are simple to use and with acceptable effort in terms of data input.
(from the Passivhaus Institut: http://www.passiv.de/07_eng/phpp/PHPP2007.htm,
find sources to order an English version at the bottom of the page,
find a free simplified version here, in German only)