Retrofitting existing houses could be easier if, instead of running everything off of batteries and inverters, the various systems were separated. Average home's power consumption split by type of device:
Woops, this guy got torqued about hotlinking his pie chart. Click this link http://michaelbluejay.com/electricity/howmuch.html and you will see the one I am referencing.
First, lighting does not need 240/110v. 12v LEDs are coming into their own, even lower voltages can put out really bright light and they are available in 'warm' light output which is very similar to the light from an incandescant bulb, which most people like. So put a solar panel on the roof, add a bank of storage batteries and wire the LEDs completely separate from the 240/110v. With properly installed skylights, the house could be completely grid independent as far as lighting is concerned. There's roughly 11% of your power usage, according to the above chart.
If TVs etc continue on the LED path, it's conceiveable that in a few years they too could run off of this type system. Right now the average LED/LCD TV uses areound 110 watts. http://reviews.cnet.com/green-tech/tv-power-efficiency/ That might account for another 8-9% or so, again according to the above chart.
So there is 20% of the power consumption moved off the grid to a lower voltage self sustaining system.
Next is heating/cooling. With current technology, it's not feasible to run refrigerant pumps etc off of lower voltage (although that ORC cycle engine could possibly generate enough power). So that would probably have to remain on the grid but using the Earth's mass as a 'flywheel' to cut power bills has been proven to work all across the country. If heating/cooling are 43% of the power consumption and that is cut by 30% via geothermal, between the 12v lighting and cutting HVAC power usage there went 33% of your power bill. If a desuperheater is added to a geo system, this can help heat the household water thus cutting even more of the power bill.
Adding a solar mass storage system to the HVAC for winter use would help also. It's difficult to calculate the full benefit because it varies so much from area to area and even from day to day, but it would have a measureable impact.
Refrigeration can be a problem. During the summer, the refrigerator and freezer add to the cooling system's heat load, meaning the A/C has to work harder. This is reversed during the winter, it would actually be a benefit (however small) in helping to keep the house warm. So there's the challenge: develop a refrigeration system which can be used to best efficiency in both seasons. I sort of thought a simple duct system to draw outside air through a underground tube (like a geo system) could be run to the cooling coils of a fridge or freezer for summertime use, then the ducting could be 'flipped' to put that waste heat back into the inside environment during the heating season.
Same with a clothes dryer: its heat adds to cooling load in the summer and if just ducted to the outside is wasted during the winter. I would think a heat recovery system which did NOT vent the damp air to the indoors could be a benefit during the heating season.
It's not just one thing, it's a multitude of them which when used together could really cut the average house's electrical consumption without greatly inconveniencing the inhabitants.