It's just shy of 29 degrees outside, the wind is from the north at a blustery 15 - 20 mph and yet it is 71 degrees in the house and the heat is off. That is passive solar heating making itself obvious. Although our south and west-facing windows are gathering solar energy in milder weather, on days like today they really shine, in combination with our R-34 wall and R-60 attic insulation, and fairly extensive sealing of heat-robbing cracks and holes in the house framing and finishes.
Passive solar is truly our unsung hero; it is why our home is net zero energy, and it is why it is so comfortable year-round. Our home gets about 25% of its space-heating energy from the sun coming in through the windows, and this energy requires no net-metering because it is delivered at the time that it is needed, and appears to our utility as conservation. That is one disadvantage of passive solar; it isn't metered, and often goes unmentioned. Another barrier to wider use of passive is the fact that much of the existing housing stock has limited or no solar access to the windows during the heating season. But, any home with large windows that have access to the winter sun will be passively heated. Some homes will retain that heat, and others won't, but it is a resource that is present in many homes. As house 'envelopes' become more efficient, passive solar becomes a larger percentage of heat energy; a house with a tight envelope will hold the solar heat longer and will have a lower total heat load. This is important if a homeowner's goal is to reduce carbon emissions, because even if the utility serving that home uses coal to generate electricity, a sun-tempered home with an efficient heat pump will keep many tons of CO2 out of the atmosphere.
A passive solar remodel can be simple or it can be extensive, and even a simple one will be much more invasive than simply installing solar panels, but the reward is great. In most cases a new home can be designed to use this low-tech but powerful resource without much additional expense, but sadly not enough are. If you are planning a remodel or new construction of a home, do yourself and the planet a favor; hire a designer or architect who has a portfolio of passive solar designs and go passive. It will pay for itself many times over, and on days like today your home will be a snug harbor.