66F is ridiculously cold to me, and I live in Canada where it can reach -40(F or C) in the winter. I would find that very uncomfortable and elderly people would be shivering constantly and highly susceptible to respiratory illness.

I have a modern cold climate air source heat pump which essentially needs to run 24 hours a day to maintain a stable 20C when the outdoor temperatures reach -15C. Below that, the heat pump shuts off and the furnace kicks in to provide emergency heating. My thermostat is a modern one with full time-of-day and day-of-week scheduling for heating and cooling, but it doesn't matter because the heat pump by itself is not able to swing the temperature up (by even half a degree) on its own, so this causes the furnace to kick in every time the schedule calls for a higher temperature, defeating the entire purpose of time-of-day scheduling.

I will also add that where I live (Southern Ontario) the sky is overcast 90% of the time during the winter. Solar panels, even somehow free of snow and ice, are going to produce almost nothing on those dark days. Add in the need to keep the panels free of snow and ice (presumably with heating, since nobody is going to be climbing around on their roof in the winter), and you'd likely reach energy net-negative trying to make use of them.

In the northwest corner of Massachusetts I converted an old school into an apartment building. I installed 2" of polystyrene on the outside and about a foot of cellulose in the ceilings. We relay on heatpumps for HVAC. I also installed a 50kW solar array. We don't start paying for heating until Nov/Dec and stop paying in Apr/May. Our Electric usage goes through the roof in Jan/Feb/Mar. Our weak point is that the exterior walls are about 40% windows. I hope to install better thermal shades which will cost about $80k. We also last fall installed a solar thermal array to for hot water and heat the hallway which is radiant floor. I would like to think we could achieve net-zero but I will likely need to expand the solar array by about 200%.

Thermal curtains are more effective than good windows. Good windows are minimally helpful.

Those brutally cold temperatures are really not compatible with most human beings

Net zero. But not effectively zero. They sell energy during the day when no one needs it and buy it an night when we all need it. If we all switched to solar and heat pumps there would be blackouts and an energy crisis

At 66F, I struggle to do job because my fingers go numb and I can't touch-type well. If others have that problem, a small heat-lamp (like for a reptile cage) can locally heat just the area above the keyboard cheaply.

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