Due to a metric-standard measure conversion issue, I had accidentally reported the demand for EnergyStar clothes washers in L/cycle, not gal/cycle, leading to a much greater water demand than expected. History has a few cases where this doesn't work out so well: famously NASA's Mars Climate Orbiter which burned up in Mars' atmosphere, and an Air Canada flight that ran out of fuel halfway through its trip (the captain happened to be an accomplished glider pilot and successfully landed the plane). In my model, it significantly reduced the required water load. As such, there's a good 30% margin between water demand and supply. Here's the model if you want to take a look for yourself. For those of you reticent to download some stranger's Excel model, I've provided a summary:
- Cabin size (ft2): 545
- Cabin footprint (ft2): 375
- Occupancy: 2 continuously, 4-6 for weekend
- Rough cost ($300/ft2 + infrastructure costs): $210,000
- Solar PV demand (kWh/day): 11
- Solar thermal demand (kWh/day): 11
- Max solar insolation available (kWh/day): 2000 (excluding conversion losses)
- Weekly water demand (gal/week): 68
- Min weekly rainwater availabile (gal/week): 99
I've run out of things to be concerned about. I originally thought water would be an issue; apparently two people can live comfortably continuously on a 15'x25' water collection footprint, and 4-6 can for a weekend. There's ample power to run the various infrastructure systems. And there seems to be enough space. I'll add that blackwater (i.e. sewage) processing seems to be an open question. Septic systems and aerobic digestion systems still require periodic sludge removal and composting toilets aren't all that familiar or easy to use (I did this research while having dinner last night: not the best idea). I now understand why the Bill and Melinda Gates Foundation is funding 15 teams to reinvent the flush toilet; waste treatment still is a hard problem to solve.
Bottom line is living comfortably off of naturally locally available resources isn't nearly as hard as I had originally expected. With some good engineering and clever design, a very comfortable, modern, sustainable living can be made.