The initial idea of a 1400 gallon storage tank in the main downstairs room appears today unpractical. The alternative is too have a tank in the water heater closet, and another one in the furnace room, which means two separate systems.
The water heater closet can accomodate a 3' x 3' x 4' tank, with a chamfer where the tank will come underneath the stairs. Total water volume is 75 gallons. Total energy storage, assuming a usable temperature range of 140F to 80F, is 10KWh. That is about 1 day of storage in winter.
The furnace closet can accomodate a 4' x 4' x 5' tank, with a water capacity of 270 gallons, and an energy storage (assuming 140F - 110F temp range) of 20KWh, equivalent to 1 day of heat in the winter.
The location of the furnace tank would make it possible to reclaim heat from the upstairs wood stove, which would be a significant advantage, since the wood stove easily overheats the upstairs living room.
Because the systems will be separated, they will also need separate solar panels. The furnace tank will use the garage South wall, with up to 150sqft collector area. The water heater system will use the deck rail, 75sqft collector area. An alternate location is the upstairs South wall, with also about 75sqft area.
Optimum solar fractions (not accounting for cloudy days) is 84% for hot water, and 72% for space heat. Total solar fraction (also accounting other uses than heat) is 39%. Yearly savings is $600. Actual savings will probably be between $400 and $500. With a total estimated cost at $2000, payback is between 4 and 5 years.
Our total yearly electric consumption will go from 25MWh to 15MWH a year. Further efforts on improved insulation (windows and doors are leaky, walls could be improved, siding needs resealing and repainting) may reduce that to 10MWh. Net zero would then be achieved with a 6KW photovoltaic system.