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Off grid?

Off grid?

We are building a new home and want to know if we can be off grid with the Tesla solar panels and power walls? It would cost $30,000 to get the power company to extend service to our location. We are in Piney Point, MD. Thanks.

Kyle

greg | December 31, 2018

[part 1 of 2 - had to split it up into pieces]

@kylematthew78

First:
In theory you can do what you suggest.
But you have to size your solar and battery system well to match your total energy use. So you need to figure out what your likely average and peak demand for power will be [in summer and winter], and use that to size [and thus cost] the solar and Powerwall system.

But whether Tesla will let you run the Powerwall 2s like this is something you'd need to discuss with them first once you have an idea of the size of system needed.

I know previously they have said that PW2s are not intended for only off-grid use at the moment.
[They will work in a temporary off-grid fashion e.g. when your grid power is down].

However, they can and do change their position.

You may however find the size of the system you need will cost you way more than the $30K interconnection cost.

greg | December 31, 2018

[Part 2 of 3?]

Secondly:
The economics of staying off grid may be marginal, depending on your projected system size, how many solar panels you need (people talk of about $3.50 or so per Watt installed for good brand Solar Panels), so for s 5 kW system you'd be spending 5000 * $3.50 = $17,500 for the solar installation. And 5 kW would be the minimum I'd think you'd need?

Then you'd want one, maybe two Powerwall 2's plus the required installation costs and such, so allow up to $14,000 for all that if you got two of them

That gets you up to $31,500, then you';d get some form of Federal rebate off the solar - say 30%.
Maybe more depending on your state or other incentives.

And the Solar credit only applies to the Solar portion. So you'd likely get maybe $5,000 off the total cost.

Even so after credits you're now you've spent all up about $26,500

If you add even more solar, say double the solar to 10kW you'd end up massively overloading the Powerwalls in the summer with too much power, [Powerwall Gateway can shut down the solart when that happens] so the solar would spend most of its time shutdown then - as the power has nowhere else to go. Cant export it up the wire to the grid.

[unless you also get an EV as well to soak up some of the excess power as well] which could help with that issue.

But in winter time the opposite case may apply - 2 powerwalls would provide at most 27 Kwh of power reserve,
And once they're exhausted, you'd need to wait for the solar to fill them back up.

That may not be at all practical in your situation - 27kWh sounds a lot of power, but its not really.
And the difference between peak solar power in summer in the middle of the day and the same middle of the day in winter is about 3 times less -. so a 10 kW solar system in a fine day in winter at midday will be outputting about the same as a 3kW system would in summer. so it would take more than a day to refill the power walls back up, as the total.

Of course if you design and build a really efficient house so that 27 kwh would last you you several days and you'd manage the odd outage even so, it might work.

greg | December 31, 2018

[Part 3]

In summary, it really all depends.

To figure out the cost versus benefits for both option, you'd have to consider if you connected to the grid for $30K, then factored in 10 years say of normal grid power use at your local rates, then do a similar comparison for totally off grid operation.

Powerwall has a guaranteed lifetime of 10 years only, so thats why you use 10 years as the maximum.

You may find that the outlay on a 10 year basis is initially a lot higher for the off grid option, but you offset that outlay by not having regular power bills over the life.

Don't forget to allow in your "on grid" option for the fact that if your local energy company allows Net Metering or just pays you credits you for exporting excess solar - even if the credit/export rate is lower than what you buy/import it for, then the economics overall will likely swing heavily against Solar/PowerWall.

The cheapest battery system you can get - is the one you don't pay for - i.e.that is the grid.

If you can use the grid as your "battery" when you have solar panels you're usually better off to do that.

kylematthew78 | January 4, 2019

Thanks for the response, i'll do more research on both options. I'd be curious to know when Tesla will have the Off grid option available?

greg | January 7, 2019

I'd do some sizing of the likely solar system and powerwalls first, then give them a call.

[To do that at the very least, you need to work out your likely summer and winter peak demands, and also the daily average usage of your household and give them those figures and let them figure it out].

From that they can probably give you good idea of what it might cost, and also of course whether its possible to go off grid or not.

I realise you may not actually know what its going to cost to run your new house yet, but I'm sure your house builder can give you an idea based on similar houses in the same area, or maybe your local electricity company can tell you the average for other homes in your area. Allowing for the fact that your new home may be more or less efficient than the others nearby.

You may also find that even if Tesla do support off-grid operation the economics simply won't stack up to make it considering.

That doesn't mean you couldn't/shouldn't do it, even if so - but it means the decision is not a simple money in/money out one.