Zythryn wrote:I would suggest your estimates don't show how much it would cost to go off grid. They estimate how much it would cost to go off grid with those specific batteries.
Actually, this house has been nearly off-grid in the past. For nine months of the year the 2880 Wp array provided all of our electricity and about half during the other three months. The electric water heater remained on-grid. Heat was provided using a wood-pellet stove rather than a heat pump. The clothes dryer and oven ran off propane rather than electricity. That was until the house was hit by lightning and I lost an inverter. Also, we did not have an electric car consuming over 2 MWh of electricity each year back then.
No, this calculation is intended to see what is involved in eliminating the flow to the grid with our fully-electric house. (We still have the wood-pellet stove and can operate in the case of emergencies for long periods of time, so that is not my intention.)
I applaud your energy efficiency, but once again, you take the most expensive solution and state it as a general case.
Zythryn wrote:Ironically enough, just as I was getting started in Excel, our power went out!
That is ironic!
Whew, back up again after a couple of hours, temp in the house only went up a couple degrees
Zythryn wrote:I am planning to go back to November 1st up through today.
Unfortunately I don't have data going back prior to October 1st.
For reference, here is Virginia, my meter reversed direction around November 20, 2016. In 2015, it was around November 1. Yours likely reverses prior to that.
Our meter "reverses" direction every night.
Our first measured daily net loss was in early October, although I am sure there are days every month.
Our first measured weekly loss looks like it was the last week in November.
Monthly loss would be December.
The first monthly gain would be February, weekly looks like the first week of January and daily, well those are scattered throughout the winter.http://www.netzeromn.com/blog/the-envelope-please
Zythryn wrote:My first thought was to plot hourly kW usage (basically kWh) to get the worst case scenario.
That's roughly what the 1-hour column provides for me.
Yep, thank you again, this is a much better way to look at it than daily. The line between hourly data and power requirements for either battery backup or going off grid is much more direct.
Zythryn wrote:I could track net power use, but with such a short timeframe I'm thinking worse case is better.
This would put disproportionate weight on our car charging. One of the cars is mostly charged during sunlight hours.
So, I am tempted to subtract the car charging out of the data.
Don't you have a Tesla Model S? If so, does it have a 20 kW charger in it? That could cause you to draw power from the grid no matter when it is charged.
The standard charger in a Tesla is 10kW. The car can be set to charge at less than the maximum. I typically charge at 18A or about 4.2kW.
If I were going off grid, I would likely set them to charge at 12A/3kW. For our driving habits, that is sufficient.
I also try to charge when the sun is shining, which means the car has little impact on the net power use of the house.
Zythryn wrote:I also need to calculate the energy needed over a longer timeframe.
This is tricky, as the length of time varies depending upon the sunshine.
This is were net metering REALLY provides its magic: The last two years I have "stored" over 3 MWh each year in Spring, Summer and Fall. This year I am hopeful to store over 4 MWh before wintertime. If I do, the thermostat may go up a couple of degrees next winter!
Zythryn wrote:This should be fun, once the power comes back on.
Yes. We find electricity is very useful around here!
And now that it is back...
So our highest hourly power use between November and today was 4.4kWh.
So this can be taken care of by a single PowerWall. The duration is going to be the killer.
December 5th looks like one of the higher use days. However, while the hourly high use for 4.4kWh (around dinner time), our daily use that day was 26.7kWh.
We would need two PowerWalls to supply power that whole day without any changes to our routine, if we had no solar panels.
With the solar panels, we are down to a deficit of 17kWh for that day.
Power outages, and going off grid are very different though.
Being on the grid, we can bake, cook, charge our cars, etc, whenever we like.
In the event of a day, or longer, power outage, we can easily limit the amount of cooking we do and many other activities that take power.
Going off grid though, requires a more regular focus on power use. For example, I would not have a small fridge downstairs.
We would bake during daylight hours with lots of sunshine and store the results.
I'd rely a lot more on the microwave during dark hours.
And of course, this winter wasn't the coldest we have ever had. Non-the-less, I would estimate one PowerWall would be sufficient to get us through two days of no power at reduced capacity (no car charging, no oven, etc). A day and a half of the absolute worse case scenario of zero solar power (which is unusual).
Going off grid though, with no changes, would have required about 1200 kWh of storage (peak to trough of our production graph).
This would be about 95 PowerWalls! Or about $500,000. Very expensive indeed.
Pulling the cars out of the formula, it would be about 40 PowerWalls (the garage just might be able to fit that;)).
The above is all without the cars.