I spoke with a LEED guy once about this.

Heating efficiency is more a function of heat escaping the house through poorly sealed doors and windows, poor insulation, etc. You lose heat from every side/top of the house. The heat gained inside a house from winter sun on a roof in a cold environment is negligible. The best strategy for maximizing heating efficiency is to get better windows/doors and insulation.

This compares to the summer, in which the majority of heat is gained by solar radiation on roofs and walls, with a much smaller amount being obtained through heat leaking into the house or radiating through doors/windows (assuming somewhat modern doors/windows).

You are forgetting some heat sources already present that can have a big effect:

Humans - yes humans

From http://aheatingguysblog.blog.com/2009/09/23/human-body-btu-o... :

"The human body at rest will produce about 250 BTU while sleeping, At idle about 400 BTU, doing light work about 650 BTU, and with heavy work up to 2400 BTU. That means 25 people dancing in a 2000 square foot home with temperatures of -10 degrees outside would never need a furnace. This is why bars and misc other places tend to have swings in temperatures more frequently."

Electronics and lighting

Incandescent and halogen lighting while dropping in usage is still huge. Computers or any other machinery.

Insulation

Not a heat source but slows down the heat transfer.

As someone who has mostly lived in hot climates, I have never understood why all our homes here in the South (Gulf Coast) have black roofs. I want to paint mine white badly, but I am afraid it will look terrible.

I'm also confused by my HVAC return air duct opening at floor level instead of ceiling. I only use heating a few weeks out of the year. But the climate is such that A/C is needed often for dehumidification as well as temperature control.

For one thing, because the sun is lower in the winter time, the solar gain deflected in the summer is greater than that lost in the winter.

Heat gain/loss through the roof and other opaque parts of the building shouldn't be an issue because insulation is cheap and about 200mm of modern insulation is all you need before air tightness and AC efficiency become the major factors for energy loss. Also, cubicle farm style office buildings are typically in cooling mode, i.e. they are rejecting excess heat for at least 3/4 of the year even in a temperate climate because of the heat generated by people, IT, lighting etc. Therefore reducing solar gain through windows is pretty critical to improving energy efficiency by reducing the load on the cooling systems. So using solar panels as part of a brise soleil to provide shading to windows would be the optimum solution but this is a more expensive facade structure to build and more expensive and less safe to maintain, so it's typically only done on high spec buildings which need some eco bling.

Residential buildings have different factors in play. Residential efficiency is all about retaining as much heat as possible in winter through air tightness, winter solar gain through windows and insulation. With good insulation and solar shading, summer heat gains are not so much of an issue for residential in temperate climates; for example most people in Northern Europe don't have AC because there are only a few weeks of the year when you would have it switched on.

I think it depends where you are, but the extra insulation could help:

> For example, in winter, the panels would keep the sun from heating up the building. But at night, they would also keep in whatever heat accumulated inside. For an area like San Diego, the two effects essentially cancel each other out, Kleissl said.

If you are a homeowner, you want your attic to be as similar to the outside air as possible, especially in winter. A warm attic and a freezing outside is a recipe for ice dams.

The solar heat gain is close to 1kw per square meter. The wood stove that heats half of our house is only 6kw.