Prius Lessons: Building an Off-Grid House

There are a few Passive Solar homes in my region.

In our township, nearly every home has at least one generator. Most homes have two generators. [one big and one small] The only homes here without generators, are those homes that are solar-powered.

I applaud your desire for Passive Solar, but unless you have a lot of experience in this field [or your finishing a Doctoral thesis], you are simply not gong to have enough background and physics to draw on to make the end house work right. There are a lot of attempts at this, that later become fixers.

To strive for Passive Solar, and then accept fossil fuels, is kind of counter top the movement.

Go Active Solar instead.

 

Long-time Prius guru Hobbit up in New England did an extensive writeup of energy mods to his house a couple of years ago, with a vast amount of technical and practical information:
Front gate

That page needs a minor bit of user input to get to the detail pages, which Hobbit requests visitors perform to avoid spidering and indexing of that section of his site (the link above is public). Please honor his wishes by not publishing the direct link to his house pages.

 

How much seasonal difference in daily production do you expect, at your likely location?

For my location and weather (i.e. typical interference, based on local recording stations), NREL's PVWATTS calculator (an old one, not the updated version posted now) expected a 6 to 1 ratio between July and December. NREL's monthly average is the gray stepped line on this chart, superimposed on my daily production chart (courtesy of Enphase's online monitoring tools):

My all-electric house also has a very strong seasonal demand curve, nearly 180 degrees out of phase with this production pattern. Because of that, off-grid won't work for me. If sized for yearly annual production and load, production would be drastically short in winter, while the large summer excess couldn't be sold to the local utility or others to help defray its original cost (or collect the state incentives). So I need the on-grid net metering to make things average out over the seasons.

Your CO-NM location should have a less drastic seasonal production variation. And a passivHaus with combustion backup should have less load variation than my house. But I'm guessing that the mismatch will still be significant.

(I'll try to add more feedback as time permits. A not-so-easy attic insulation and infiltration upgrade is in progress.)

 

^^ I'm jealous. Only a 22% reduction in expected energy production at the depth of winter, compared to the 84% reduction expected at my house, and 89% reduction actually observed (PVWatts doesn't know about neighboring trees). Having always lived in cloudy portions of the PNW, it is easy to forget that some other areas get excellent winter sun.

My attic upgrade is now done, after a week of prep work, an afternoon blowing in a new insulation layer, and a couple days finishing up loose ends. The house was built with nominally R-30 above the ceiling. I had previously sealed many gaps around junction boxes underneath the overhead light fixtures, but knew that more deficiencies remained, including several small patches of uninsulated ceiling. The energy auditor last year suggested boosting this to R-60 as a second tier, deep retrofit project. (First tier was to complete the in-progress solar PV, upgrade to heat pump water heater, and fix significant air leaks around door frames and heat pump wall penetration.)

I didn't figure the R-60 boost alone would do much, but wanted to address the other issues too. And the overhead fire sprinkler needed better freeze protection. It had less insulation above than below, and that worsened every time the attic was disturbed, especially so during last year's re-roofing project when the tubular skylights and exhaust vents were all rebuilt to correct other problems. The night before last year's first serious cold snap, I threw a fiberglass roll over the worst portion. A couple weeks later during the next cold snap, my house's twin (mirror image, same builder at same time) suffered a burst sprinkler, just a couple months after the new owners moved in. They had to move out while repairs took most of the winter. And during our winter ski trip, we stayed in a hotel undergoing major repairs in the other wing for its own frozen sprinkler disaster.

It turned out that far more ceiling was uninsulated, about 80 to 120 ft^2, all hidden in corners and next to the outside walls where lack of headroom and distance from the access hatch prevented ready inspection. All previous inspections (my home purchase inspector, me, and even energy auditor) missed it. Some wire bundles strung through that space had blocked the flow during the original blow-in operation, so I had to use a long stick to push old material under and beyond those bundles. A remote IR thermometer verified the success of these changes even before the weekend's blow-in of new stuff. And some more air leaks were revealed and sealed.

The sprinkler system was uncovered and 'tented' to prevent insulation from falling underneath, creating a thermal connection from pipes to warm living space. Then the newly added layer of blown in insulation greatly increased the coverage on top.

Heat pump energy use (sub-metered circuit) is already down compared to the same time last year, but it is too soon to separate any improvements from the current unseasonably warm (but soaking wet) weather.

Because most heat comes from the single-zone heat pump on one end of the house, and the house was already reasonably efficient, this project might never pay back from raw energy savings alone. But I do expect to reduce supplemental electric heat at the far end, lower the outside 'balance point' temperature at which the heat pump needs supplemental help, and reduce the outside unit freezing over by reducing its load during the most susceptible damp outdoor conditions.

And I'll also save energy by not having to raise the inside temperature to protect the sprinklers during the coldest outside conditions. This will help most during winter travel absences. The reduced worry factor itself is worth quite a bit.

Also changed out more light fixtures, replacing them with surface mount LEDs. We prefer the nearly flush appearance (almost as flat as the bezels of recessed can lights) over the older hanging globes. This is mostly a styling change, the old fixtures already had LED bulbs. The units I put in last year wouldn't fit into our particular 4" junction boxes, requiring laparoscopic ceiling surgery to replace with larger volume j-boxes (still 4"). It was easier to change the rest from above than below.

 

Sage, have you seen / checked out THIS guy's setup? Very cool (Norway joke not intended):

My Nissan Leaf Forum • View topic - Use of Nissan Leaf battery in Solar installation.

In a few more years, I might not be tossing our pack for the puny $1,000 core after all

.

 

This is definitely the future or a great business opportunity for those with knowledge. The way our electric bills keep increasing at about 10% a year, it won't be long before it is viable to be off grid.

 

For some rural construction, off-grid has been financially viable for a long time, due to high per-foot charges for running the lines. At $10/foot in some areas even before this century, someone building a mile away from the nearest power line would be offsetting a $50k connection charge. Today's more efficient construction choices, lower equipment costs, more efficient appliances, and these better battery systems, are greatly reducing the distance to where the choice to be off-grid makes immediate financial sense.

With an existing house of older construction, already connected, in a less winter-PV-favorable climate, I cannot foresee a time of cutting my grid connection. But between conservation upgrades, my own production, and state production incentives, my all-electric house currently has no electric bill. (The last non-zero bill was $0.60, paid in June.) That will change sometime after 2020, when the incentive expires and the accumulating credits are consumed.

But even then, purchased energy use will be less than 20% of what I was buying before my conservation binge started. And 8 more PV modules would put this house to net-zero energy use.