by Bill Howard
The 'Tesla Powerwall' battery module is a small, light, maintenance-free system that is guaranteed for 10 years. It uses the same lithium-ion cells as in a Tesla vehicle.
The Powerwall system runs $3,000 to $3,500 per module, and you'll spend several thousand more for an inverter and installation.
Awesome as Tesla's devices may be, they're good for hours, maybe a day - not as replacement power when the grid goes down for days at a time.
Deliveries start later this year and the prices will be softened by federal tax credits of 30% of the battery price. California has a 60% be-a-fool-to-not-try-this rebate.
Tesla's quantities of scale in manufacturing will
also drive the price down.
The upshot - more battery-powered
Tesla is a company like Apple:
A basic Powerwall module is dazzlingly small at 51.2 x 33.9 x 7.1 inches (HWD) and 220 pounds.
It's small enough to be wall-mounted in your garage wall, even outside, as long as temperatures don't go beyond -4 to +110 degrees F (-20 to +43 degrees C).
The flooded batteries most commonly used today are insanely heavy, need to be checked weekly, and sit on the floor in leak-proof plastic cases about the size of a thoroughbred jockey's coffin.
As I mentioned above, the 'Tesla Powerwall' is
maintenance-free and carries a 10-year warranty with an optional
buy-in for a second 10 years; it will be interesting to see if
that's a full replacement or pro-rated.
But these credits may serve a valuable purpose:
It's all based on the underlying idea that the majority of Americans have concerns about climate change, and solar puts a dent in the use of fossil fuels.
Elon Musk in Thursday's press conference reiterated that climate change is a real issue.
For both, that's before installation (definitely not a DIY project for most) and the AC-DC inverter needed to convert 350-400 volts DC power to 120 or 240 volts AC.
There may also be a charge converter needed to
interface to solar panels, to manage power going into the Powerwalls
if they're not charged by energy grid power. Each runs in the low
thousands of dollars.
A kilowatt is 1,000 watts and a single 15-amp household AC circuit delivers about 1,800 watts and a 20-amp circuit delivers about , so you're getting about one circuit worth of continuous power with the ability for extended periods, and the ability to draw more than 25 amps when the hair dryer or toaster oven kicks in. That's instantaneous power.
The Tesla Powerwall page notes that the lights in one room, or a flat-screen TV, each use 0.1 k kilowatt hour (100 watt hours), which could be one 10 watt LED bulb or TV for 10 hours a day. It rates a clothes washer at 2.3 kWh per use and a dryer at 3.3 kWh per use.
In other words, a single load of wash draws down the
majority of a single Powerwall unit.
These are broad calculations that leave out transformer losses, the differences between DC and AC (AC watts are calculated as volts times amps times a fudge factor of about one-third, which your physics teacher may have blown right by when he saw the class already nodding off), and a system may show less longevity when dealing with massive drains of battery power.
This also assumes all 10 kWh in the specs are available for use. So take this as a start and feel free to add detailed scenarios in the comments.
For longevity (power delivered over time), look to the kilowatt-hours figure:
In other words, your $3,500 investment (plus inverter, plus electrician, plus solar panels) gives you one 15-amp circuit running draw for a quarter of a day.
Time to think about chaining together two or three
Powerwalls, and doing high-drain applications (laundry, dishwasher)
early in the day on sunny days.
With that, you could throttle back daily demand -
maybe no air conditioning and line-dry the laundry - and get a week
of runtime in an extended power failure. Or you could have more than
15 amps of time-shifted power draw in the evening.
A small, permanently mounted 7-10 kilowatt generator ($3,000) gets off-grid homes through multiple cloudy days and the shortest winter days.
Musk could say "to infinity and beyond" if Buzz Lightyear didn't get there first.
As Musk has said,
Powerpack has already been in low-key testing in businesses across the country including Wal-Mart.
The biggest installations could be at electric generating plants, especially those with the lowest-cost or lowest-pollution electricity. They'd store energy created at off-peak hours and deliver it during late afternoon and early evening at peak demand periods.
When utility officials or government planners talk about a shortage of electric generation capability, they mean "in the afternoon on a hot, humid day."
Even California has enough generating capacity from
midnight to 6 a.m. Duke Energy and others are installing battery
systems adjacent to wind farms and hydro power sites.
The convenience factor compared with flooded batteries, the wet-cell lead acid batteries much like in your car, are immense. They have to be checked weekly, the fumes can be problematic, and there's even a slight risk of sparks causing a fire.
There are already sealed battery packs for backup and
storage, but Tesla's lithium ion technology and longevity could
prove preferable to consumers.
Should you buy?
You'll buy Powerwall from a Tesla partner that can handle the entire installation. They include Treehouse, SolarEdge, and Green Mountain Power.
Tesla on its site says the prices of Powerwall
($3,500 for 10 kWh, $3,000 for 7 kWh) are the selling price to
installers. Since the price will be known to customers, it's not
clear if the batteries will be marked up, or if the installer will
recoup its costs and profit from the installation service.
An installed generator runs $4,000 to $5,000 (including installation) for a 7-kilowatt unit that powers the essentials of your house to 20 kilowatts ($7,500 to $10,000) for a whole house generator that runs for weeks.
In most areas, municipal gas delivery does not go out
during a power failure. In earthquake-prone areas where gas lines
can rupture, you may need a propane tank, and that may be regulated
in urban areas.
If that means the price a consumer pays, the initial
Tesla solution is $350 a kilowatt hour.