As October rolls around, and I start rummaging around the attic looking for the Halloween box containing various front yard ornaments, its time to look back and reflect on the home battery system. Overall we went into this with two main goals. Energy security and economics. We have succeeded on both fronts, with the home battery system keeping the lights on during two brief outages, further we have managed to save some money on our power-bills, helping offsetting some off the costs.
Last October, we installed a Tesla Powerwall 3 (PW3) in our garage. While the system can be installed entirely on the side of the house, for various regulatory reasons, placing the system in the Garage was the better choice. Next spring, we added solar to the PW3, but lets take a closer look at the PW3, and see how its particulars work with our situation. The capacity of the PW3 is 13 kWh, while it can discharge at 11.5 kW, and charge at 5 kW.
I configured the PW3 to use 80% of its 13 kWh capacity (~ 10 kWh), which has proven plenty to get us through the afternoon peak (4pm to 9 pm on weekdays where I live in Toronto, ON using the ultralow-overnight tariff (ULO)), even in the winter with the heat-pump going and my daughter on a baking binge. Its usually not enough to make it through the whole day, but it will do the peak.
Its discharge capacity of 11.5 kW was derated slightly to 10 kW. Where we live, our net-metering program’s regulations are easier to navigate if the “inverter” is less than 10 kW (source). While larger projects are possible, my contractor strongly suggested we derate to 10 kW. To be fair, that extra 1.5 kW is not really going to move the needle, as long as I am not charging the car, the rest of the house does not really get above 8 kW.
In terms of financials, the PW3, set me back $25k CAD installed, and permitted. If I were to use the full 10 kWh, I might save $2.5 per day (assuming charging at the ULO 2.8 c/kWh, and displacing 28.4 c/kWh, for the full 10 kWh). Assuming 250 working days in a year (source, with 10 Holidays), that’s $625 per year, or a payback period of 40 years. A bit steep, but there were some savings on the solar side, and the energy security is worth something to me.
In terms of upgrades, I have given some thought to two options: Add a PW3 DC expansion pack, or put panels on my garage. As discussed above, owning to the 10 kW inverter limit, getting a second PW3 makes little sense for me. The PW3 DC expansion pack would help with off-grid operation, if we had an extensive outage, such as the weeks-long outage back in 2003 (source), it would be helpful. Financially the case is a bit iffy. I did get a quote from my installer for about $15 k CAD, a payback period of 24 years, using the assumptions above. But alas, that all assumes I have a power bill at all, which after the battery-solar upgrades, may no longer be the case.
Similarly, putting panels on my garage, would help mostly on cloudy days, as my 10 kW export limit, would cap production on sunny days, as in full summer sun, I do hit 10 kW with my existing panels. I estimate I could perhaps get 0.8 kWh/W peak of annual solar production, compared with 1.2 kW/W peak for my main solar system, something, yes, but runs into the same problem as before, namely that I may no longer have much of a power-bill to offset those costs.
Looking back, the PW3 app reports 13 outages. Most being 5-10 minutes long. Of note, 10 of these 13 were manufactured, e.g. me testing the system, but three were real, and it is nice not having to re-set the clocks in my house, something I have only had to do following service to the PW3.
In summary, the PW3, with its 13.5 kWh capacity, and 11.5 kW inverter is plenty for our home. The capacity is plenty to get through the afternoon peak, more capacity in the form of a PW3 DC expansion pack, would help with operation during extended power outages, something I have yet to experience. My PW3 along with a solar system, has greatly reduced my power-bills, possibly all the way to zero, but that is the topic of another post, once I have had a year or more of solar.
Electric car adventures 