Tesla is now marketing its Powerwall2 storage battery for domestic applications, claiming among other things that it can make your home self-powered and blackout-proof. Here I review Tesla’s claims using an existing rooftop PV array in the Arizona desert as a real-life example. Will a few Powerwalls allow the homeowner to go off-grid? Not a chance. Will they make the home blackout-proof? Maybe, maybe not. Will they save the homeowner money on his electricity bills? Not that I can see.

The example rooftop array is in Tucson, Arizona. I selected Tucson because if a solar-Powerwall2 combination won’t work there it won’t work anywhere in the US. Except for the area around Death Valley to the northwest the solar resource is about as good as it gets, the low (about 30%) seasonal solar range means that there is no large seasonal storage requirement and seasonal generation is not in antiphase to demand, as it is in some areas farther north:

Figure 1: US solar irradiance. Image from Arizona Solar Center

The real-life example I use is the array of panels on the roof of the residence of a Mr. Gary Bynum. Their listed capacity is 8.28 kW(p). Figure 2 shows the installation:

Figure 2: Mr Gary Bynum’s rooftop solar panels. Image from Sunny Portal

In the year I selected for review (2013) the panels generated 15,500 kWh (capacity factor 21.6%), and I assume here that this would have been sufficient to cover all household consumption durng the year had it been possible to store the surpluses for re-use. How many Powerwalls would have been needed to do this? According to the Tesla website

two Powerwalls would be needed to back up 15,500 kWh/year of consumption (42.5 kWh/day). It’s not clear exactly how much storage capacity a Powerwall2 has from the specifications shown in Figure 3, but I have assumed 12.15 kWh (13.5 kWh usable times 0.9 round-trip efficiency), so two Powerwalls would have 24.3 kWh of storage, enough to cover household demand for about 14 hours on average:

Figure 3: Tesla Powerwall2 specifications. Data from Tesla

Now to the data, solar first. The hourly data I downloaded from Sunny Portal are hard to read when plotted up, so Figure 4 shows daily averages:

Figure 4: 2013 daily average solar generation. Data are missing for January 31 through February 12

Figure 4 reproduces the seasonal pattern seen in other solar arrays in the Tucson area, as discussed in the misleading article on solar power post. Maximum solar generation occurs in the spring because skies cloud up during Tucson’s “monsoon” season in July and August, and sometimes in June and September as well.

Obtaining demand data was more difficult. No data are of course available for Bynum household consumption, so I constructed a synthetic load curve using graphics from sources here (Charts 7 and 8) and here, factored it to match Bynum household annual solar generation and used it as a proxy for Bynum household consumption. Figure 5 superimposes daily average demand on the Figure 4 solar data:

Figure 5: Daily 2013 daily average solar generation with daily average demand superimposed. The month divisions are not exact. Note that the daily means are the same in each month (I did not smooth them out between months) and that they will not show hourly demand variations. Hourly plots are shown later

Figure 6 shows the daily surpluses and deficits calculated from Figure 5:

Figure 5: Surpluses and deficits calculated from Figure 5 data.

The largest deficits occur during the summer monsoon when daytime temperatures drop somewhat but humidity increases significantly, leading to an increased demand for air conditioning. The question now becomes, how do Mr. Bynum’s two Powerwalls, with a combined storage capacity of 24.3 kWh, handle these deficits? As shown in Figure 6, not very well. It’s not obvious from daily data, but the Bynum household gets plunged into darkness for about 1,600 hours – 18% of the time – during the year:

Figure 6: Powerpack2 storage balance calculated from Figure 5 surpluses and deficits, daily average data. Note that the 24.3 kWh maximum Powerwall capacity is never achieved for an entire day.

Now we will look at hourly data for two selected months – May 2013, the month when the Powerwalls performed best, and July 2013, when they performed worst. Figure 7 shows the data for May. Because there were essentially no cloudy days during the month the Powerwalls failed to deliver for only 20 hours, and the fact that these all occurred in one-hour periods at 6am suggests that they are an artifact of the data. It is, however, clear that Tesla’s claim that “on a typical day, Powerwall and solar will meet all of your home’s energy needs” is optimistic. These were better-than-average solar days, and the two Powerwalls Tesla recommends only just made it through.

Figure 7: Solar generation, demand, surpluses, deficits and storage balance, hourly data, May 2013

Figure 8 shows the data for July. During this month the Powerpacks were out of charge for a total of 292 hours, or 38% of the time. The longest outage was 18 hours between 2pm on July 26 and 6am on July 27. These outages are not an artifact of the data. The Powerwalls were simply incapable of coping with the combination of increased demand and increased cloudiness:

Figure 8: Solar generation, demand, surpluses, deficits and storage balance, hourly data, July 2013. Note that the Powerwalls are failing to meet home demand when they are discharged, i.e. storage has fallen to zero.

A brief discussion of economics before concluding. Tesla lists the following Powerwall2 prices on its website:

A single Powerwall plus hardware costs $6,200 plus $800-$2,000 for installation, or $7,000 to $8,200, and then we have to add electrical upgrades, taxes, permit fees etc. I have no idea what the upgrades and permits might cost but when I last checked the Tucson sales tax was 8%, which will increase the total cost to $7,560 to $8,860. If we then take $8,000 as a good round number and apply it to my 12.15 net kWh estimate we get $660/kWh, and $590/kWh if we apply it to Tesla’s estimate of 13.5 kWh “usable”.

So Mr. Bynum’s two Powerwalls will set him back about $16,000. What financial rewards will accrue from this expenditure?

Probably none. As things stand his surplus solar generation offsets the electricity he imports from the grid, and while there may be complications resulting from Arizona retail electricity pricing and regulations his electricity bill should be zero or close to it. With the two Powerwalls he will store some electricity that would otherwise have gone to the grid, but he will import an equal amount less from the grid, so it will still be a wash. The only benefit might be the ability to weather the occasional power outage, although these are rare in Tucson and occur mostly during summer evening thunderstorms when the Powerwalls are likely to be discharged, or close to it, to begin with (see Figure 8).

One final question. How many Powerwalls would it take to allow the Bynum household to go completely off-grid? According to my calculations, approximately eighty.