A 32' R32 cube with no air infiltration would need 24h(65-48.7)5x32^2/32
= 62.6K Btu on an average Jan day in SF... With no internal heat gain,
it might stay warm for 5 cloudy days with 5x62.6K/(130-80)/2000 = 3.13
tons of water cooling from 130 to 80 F in a 3.13x2000/1024/62.33x12
= 1.2" layer under the 4th floor ceiling.

The 1st floor ceiling would be better, with less insulation above it and
less heat loss to the outdoors.

If A ft^2 of R1 sunspace glazing with 90% solar transmission over the R32
south wall gains 945A Btu/day and loses 6h(130-56.5)A = 441A and makes
62.6K Btu net, A = 124 ft^2, eg an 8'x16' sunspace or $200 patch of
polycarbonate solar siding.

ie 945A - 441A = 62.6K makes A = 124 ft^2.

Solar house heating is extremely easy in San Francisco.

Nick