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Small super insulated house – grid tied or off grid?

user-1072251 | Posted in Energy Efficiency and Durability on

Where is the $$ line today between grid tied and off grid? I’m talking to a retired nurse with a limited budget who wants to build an 800 SF, super insulated net zero home on a lot (in NH) which requires a long driveway (and therefore an expensive connection to the grid) . She will have low electrical plug loads. Does the Tesla Powerwall battery influence the decision?

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  1. charlie_sullivan | | #1

    If by net zero, you mean heating with a heat pump and using PV to produce the same amount of electricity, that's easy to do for an 800 sf house with a grid connection but really hard to do off grid. NH can have 2 weeks of cold and cloudy weather, so you'd need to either size your battery to be big enough to supply the heat pump in those weeks, or you'd need to size the panels to be able to supply the needed electricity even when it's cloudy. Either way that's expensive, so you are likely better off grid connected unless the connection cost is pretty extreme.

    On the other hand, if your concept of net zero includes heating with a wood stove using wood that grows on site, or perhaps trading tomatoes grown on site for firewood grown off site, and your client is frugal with electricity use, you can set up off-grid system that powers an efficient fridge, LED lights, etc. at a reasonable cost.

    Martin lives off grid in this region so I'm sure he'll have useful comments.

  2. GBA Editor
    Martin Holladay | | #2

    There are almost no advantages to living off-grid. Electricity is more expensive for off-grid homes than grid-connected homes. In New Hampshire, a PV system is close to useless from Nov. 1 to Feb. 1, so this retired nurse would have to become familiar with maintaining a gasoline-powered generator or a propane-fired generator, and would have to pay attention every day to the voltage level of the batteries. Lead-acid batteries need to be periodically topped up with distilled water, and replaced every 6 to 8 years. Tesla batteries are vapor-ware: promised but not yet delivered, with uncertain specs.

    You can't heat an off-grid home in New Hampshire with a ductless minisplit. Space heating options include a wood stove or a propane-fired space heater.

    Even if the local utility wants to charge $40,000 to extend the powerlines to the house, I would recommend that the retired nurse pay for the powerline extension, unless she really likes to tinker with internal combustion engines and is very good at electrical troubleshooting.

  3. Dana1 | | #3

    The total lifecycle cost per kwh of grid tied PV is about 20-25 cents/kwh (unsubsidized).

    The full lifecycle cost of off-grid PV is 35-50 cents/kwh.

    Only under the most egregious and abusive rate structures (or very large hook-up charges) does off-grid become the most cost-effective option. AND with off-grid you become your own power systems tech and utility manager. in a NH location keeping the snow off the PV in winter would be essential, and at any monthly power use it takes more panel to go off grid than annual net-zero grid tied.

    The Tesla Powerwall isn't particularly well suited to fully off-grid use unless you cut your power consumption WAY down, to less than 200 kwh/month, (or buy multiples.) A 10kwh Tesla is going to set you back more than $5K installed, and production is already sold out through the end of 2016 due to higher than anticipated demand in Australia & Germany.

    So, how big a charge are we talking about for hooking up to the grid, and what is the anticipated power use for the months of December & January?

  4. Dana1 | | #4

    Tesla's grid batteries have been tested & vetted in 100s of installations in CA and the early prototypes have been hooked to the grid for well over a year now (over 2 years?) They are way more than vapor-ware, if not available off the shelf at the local box store.

    Full production isn't really there until the huge Tesla/Panasonic batter factory under construction outside of Reno NV gets running, but deliveries from other production sources will begin in late Q3 2015. My guess its that almost all 2015 production will be installed at SolarCity client sites in CA, many of whom have been wait listed for something like a year now. The bulk of 2016 production will be going to German & Australia.

    That's not to say that it will ever make sense for going off grid in lieu of grid-access in NH, the way it might become in parts of HI if the NextERA buyout of the Hawaii Electric Company falls through. Even after the natural-gas crunch wintertime pricing of power was in place, the residential retail cost of power in NH was still under 20 cents/kwh, compared to 31 cent/kwh average in Hawaii (where they get a lot more sun year-round, for higher yield per PV watt.)

    Going off grid in NH more than doubles the price of power at this winter's grid-retail, and roughly triples the cost of power at the recent 5 years' average.

    Most of the sales in Germany & Australia are to existing grid-tied PV, due to regulatory & net metering rules that make self-consumption more economic than exporting PV power to the grid, even when the lifecycle cost of the battery is factored in. Retail electricity is over 30 cents in Germany & Australia as well, and they don't do simple net-metering in those countries. As lucrative feed in tariffs expire (as has already happened for early adopters) any millisecond-by-millisecond exported power going onto the grid is compensated at the spot wholesale price at best, and in many cases zero, making batteries a much more attractive proposition to installations with a decade or more of useful lifecycle on the PV.

  5. AlanB4 | | #5

    "A 10kwh Tesla is going to set you back more than $5K installed, and production is already sold out through the end of 2016 due to higher than anticipated demand in Australia & Germany."
    In a way i am glad to hear this, tesla has made some big bets and if they don't pan out the company could easily go under. On the flip side it would be nice if they were selling well but still available, and some of those structures are not the most efficient way to do things.

  6. Dana1 | | #6

    They're only sold out through 2016 because the gigafactory in NV isn't scheduled to start production until Q1-Q2 2017. It remains to be seen just how big demand will be in 2017, or whether it will it will grow fast enough support the long term investment. Competitors never sleep, and short-term inability to meet demand can have lingering effects.

    That said, those betting against Elon Musk have had a poor track record in the past, despite the fact that Tesla was teetering on bankruptcy only three years ago, before they were finally able to make good on the Model-S. I think they'll make it, but it's far from certain.

  7. AlanB4 | | #7

    I agree with you Dana, and i also hope they make it

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