Tesla Energy: Powerwall Home Battery

The internet (and public radio, for that matter) is abuzz with the launch of the Powerwall Home Battery, by Tesla Energy. 

Link through to read the details, but here are some highlights:

• Applications include load shifting, decreasing need to sell solar power back to utility, and back-up power. 
• Two primary capacity ratings, 7 kWh (daily use applications, $3000) and 10 kWh (back-up applications, $3500)
• Does not include inverter. 
• Ten year warranty.

Sorry, Elon Musk, but I need to rain on your parade. As the owner of a small condo (1200 square feet, including the basement, which is finished as my office) that has electric everything (heat, hot water, dryer, stove and oven), and as an electrical engineer concerned with energy efficiency, I happen to monitor my home energy use. I'm not a spendthrift - I've converted all the thermostats to digital / timer, all my lights are CF or LED, I keep the heat at 65F when I'm around and 58F overnight or when I'm out.

In February this year (admittedly brutally cold), I used an average of 175 kW-Hr per day.

 April (not quite warmed up, but more typical of a regular month), it fell to 60 kW-Hr per day.

So tell me just how useful your 7 or 10 kW-Hr worth of battery is going to be for me?

Yes, if one is burning fossil fuel for heat, cooking, clothes drying, hot water, has a modern energy efficient home, then I imagine 10kW-Hr could give you close to a days worth of power, if you don't dry your hair, turn on the A/C, or switch on a space heater. And it seems to be that if the bulk of home energy use is still coming from fossil fuels (to run a home for 12 hours on 10 kW-Hr, that seems to be a prerequisite) we're not really that revolutionary.

Edit: And because this has been sticking in my craw for a day or so, I did some additional digging.  According to the US Energy Information Administration:

In 2013, the average annual electricity consumption for a U.S. residential utility customer was 10,908 kilowatthours (kWh), an average of 909 kWh per month. Louisiana had the highest annual consumption at 15,270 kWh, and Hawaii had the lowest at 6,176 kWh.

909 kW / month = 30 kWh / day. And with most of the USA averaging 4-5 hours of peak solar production, (let's call it 8 hours of solar power), our "back-up" is going to need to supply at least 16 hours, or 20 kWh of demand. Twice the Tesla device capacity.

This is not rocket science (oops, bad analogy, I'm a big fan of Space-X). Folks have been using battery back-up in UPS (Uninterruptible Power Supplies) for decades for critical applications (computers, networks, patient care, etc.) - in much higher power ratings. But truth be told, it's really not all that cost-effective, most applications switch to some other form of energy storage for extended outages. Perhaps there's a mechanical solution (magnetic bearing flywheel? compressed fluid?), a thermal one (some form of heat engine), a chemical one (fuel cell), or something else. 

I like what you are trying to do. But it's sad that in all the interviews I've heard and stories I've read today, nobody has really done the math. 10 kW-Hr is not nearly enough capacity to take a typical home off the grid, even on a sunny day.