Since the reactive house is autonomous, it must produce most of the energy consumed in it. Most, but not all: notable exceptions include considerable embodied energy in many 'imported' goods consumed in the house (food, clothing, etc). It is well beyond the apsirations of the reactive house to guarantee all the long supply chains providing necessary goods be carbon-neutral, though this is a worthy goal.

And of course the occupants themselves metabolically produce carbon in respiration, so there are some exceptions to the carbon-neutral requirement in the narrow literal sense.

However, the systems that deliver basic services to the house, such as space heating or cooling, ventilation, hot water, waste management, potable water, electricity for lighting, appliances, etc, all need energy, and in modern homes, mostly in the form of electricity. The reactive house locally produces this energy without emitting more carbon than it intakes. (There is no 'net-zero' escape hatch here, since the house is not grid-connected - see principle 1).

This is an extremely daunting and difficult requirement for a number of related reasons:

  1. Energy demand varies significantly diurnally and seasonally.
  2. Most current carbon-free means of generation (PV, wind, hydro) are intermittent, and their peak production typically does not align with peak demand.
  3. Economical means of locally storing dispatchable energy from intermittent sources to offset the imbalances noted are hard to come by.

For these reasons, conventional wisdom for off-grid houses is to supplement low-or-no carbon generation with 'emergency'/'backup' high-carbon sources, to be used only when needed. For example, a propane heater (or wood stove) could be used on the coldest days.

This advice, while pragmatic and useful, recalls St Augustine's prayer: "Give me chastity and continency, only not yet". Reactive house design is less concerned with how to bridge gaps when renewable/carbon-neutral sources are not sufficient, but with the limits of habitability within a no-carbon budget. A number of important design directions and restrictions flow from this principle. One example: since most carbon comes from combustion, the reactive house will be combustion-free: no fossil fuel used, no furnace, fireplaces, gas cooktops, etc. While contributing to neutrality, a combustion-free home has numerous other benefits: it drastically reduces fire hazard, improves indoor air quality, and more.