Jason Hughes, also known also as wk057, who from time-to-time uses his hacking skills to experiment with Tesla cars, shared his insights after the first year of owning a 44.4 kW off-grid solar system.
The solar PV system is combined with an energy storage system - perhaps not surprisingly build from Tesla Model S batteries.
Electricity is used both for the house and for two Tesla Model S. It is clearly not an economical option, but rather a proof of concept.
Despite the installation was designed to be off-grid, sometimes there still was a need to use electricity from the grid.
Some details on the project (via: wk057's SkieNET)
"As of noon today my house has been powered primarily by my off-grid solar setup for one full year (a leap year, too). In celebration, I figured I would share some aggregate stats from the past 366 days.
- 43,929 kWh generated by solar array and utilized (Average 120 kWh/day, usable AC power)
- 20,576 kWh discharged from the batteries
- 114 equivalent cycles on the storage batteries
- 11,259 kWh used for Tesla Model S charging (over 33,000 miles)
- Best production day: 2016-03-21 @ 260.7 kWh (Charged three Model S that day)
- Worst production day: 2016-01-22 @ 0.02 kWh (All panels covered in several inches of snow)
- On 222 out of 366 days (60.7%) the storage batteries reached full capacity before sunset and some amount of incoming power was wasted.
- 703 kWh bought from the grid (1.6% of total usage) Some reasoning behind the switch-to-grid events:
- November 2015 - First shortage due to 9 straight days of bad weather. Power needed for driving.
- December 2015 - Two grid usage events needed for driving power. Cold weather caused significant power usage.
- January 2016 - Snow and bad weather caused little to no generation for an extended period. Preemptively switched to grid power for a couple of days to keep reserve in battery bank in case of grid outage.
- February 2016 - Above average driving power needs during moderate bad weather period.
- May 2016 - Error in custom predictive algorithm caused a switch to grid power with 25% SoC remaining, resulting in about 40 kWh of unneeded grid use.
- July 2016 - Error in custom predictive algorithm caused a switch to grid power with 25% SoC remaining, resulting in about 30 kWh of unneeded grid use.
Suffice it to say, it's been a great year.
Something to keep in mind is that my generation numbers can not take into account power that would have been generated on the 222 days where the storage batteries reached full capacity and all incoming power was not utilized. A goal for this year is to put that excess/wasted power to better use. Rough estimates would say that I've wasted somewhere around 15 MWh of power over the last year due to this. This is not a bad thing, generally, since it means I've made enough power for my needs and is part of having an off-grid system. Would be nice to do something productive with that power, though.
The cycle wear on my storage batteries comes out to the equivalent of about ~29,000 miles of driving in a Model S. It worked out to a bit more cycling than I would have preferred. Some of this is due to needing to charge one or both of my household's Model S in the morning before sunrise in order to have power for driving during the day when the car isn't going to be at the house to charge. Also, some of the cycling happened prior to the creation of my interface that keeps the Model S charge rate equal to incoming solar power when a fast charge is not required... resulting in no cycling of the home storage pack. Overall, I expect the next 12 months to have less cycling.
Also, my home is fully electric. Electric heating and cooling (air source heat pumps). Electric cooking. Electric vehicles. No natural gas, oil, etc.
For year one, I think having 98.4% of my power come from my own little power plant definitely makes it a successful year."