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

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.