by Bill Howard
May 01, 2015

from ExtremeTech Website
 

The 'Tesla Powerwall' home battery system puts a higher-tech face on technology you can already buy:

backup batteries that store the sun's energy for use in peak evening hours, or that protect your house in a power failure.

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.

• For home users, Tesla will sell two Powerwall modules optimized either for backup (power failures) or load-shifting from afternoon to evening.

• For business, Tesla will sell massive battery packs that can load-shift power or provide short-term UPS-like coverage until the backup diesel generator kicks in.

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 homes
 

Powerwall unit dimensioned

Tesla is a company like Apple:

It has the ability to create or legitimize nascent markets in tablets, smartphones, smartwatches (with the jury still out), and now home battery power.

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.

Because Tesla knows how to generate media coverage, homeowners will start thinking more about battery backup. The federal and state tax credits lets homeowners line up at the government trough alongside defense contractors and transfer payment recipients (the poor, seniors).

But these credits may serve a valuable purpose:

To spur early adopters to invest in battery backup, which generates more battery and inverter production, which drives down prices.

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.

How Powerwall works

There are two Powerwall modules.

• There's a 10 kWh unit optimized for backup or power-failure applications, at $3,500.

• There's a 6 kWh unit for daily cycle applications for $3,000.

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.

The Powerwall delivers 5 amps (at 350-400 VDC), 8.5 amps at peak. If you recall high school physics, volts times amps equals watts. For a single Powerwall, Tesla cites 2.0 kW continuous, 3.3 Kw peak.

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.
 

Caution:

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.

You may need more than one Powerwall
 

For longevity (power delivered over time), look to the kilowatt-hours figure:

• 10 kWh means 10 kilowatts delivered for an hour

• or 1 kilowatt for 10 hours

• or 1.8 kilowatts (a 15 amp household circuit) just under six hours

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.

The average American house uses 30 kilowatt hours a day, according to the US Department of Energy. A Japanese home, smaller and more efficient, uses about a third as much power. This suggests a typical US home might need 2-3 Powerwall units.

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.

People who've gone off the grid, or who want to take advantage of solar, shift their dryer, oven, and stovetop from electric to gas or propane. They run their dishwashers and washing machines in the morning to draw directly from the solar panels. LED bulbs are a must.

A small, permanently mounted 7-10 kilowatt generator ($3,000) gets off-grid homes through multiple cloudy days and the shortest winter days.

Powerpack for business

Powerwall can be joined in microgrids. For business, the upsized version is called Powerpack, and Tesla CEO Elon Musk says it's an "infinitely scalable system." The basic module is a 100 kWh block at $250 per kilowatt-hour that can scale up from 500 kWh to 10 mWh. Or higher.

Musk could say "to infinity and beyond" if Buzz Lightyear didn't get there first.

As Musk has said,

"Our goal here is to change the way the world uses energy at an extreme scale."

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 demand for the batteries in Powerpack and Powerwall will lead to more gigafactories than Tesla's first in Nevada. Tesla says the initial economies of scale will drive down battery costs by 30%.

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?

The Tesla site already has a signup page for Powerwall-intenders and says first deliveries begin this summer.

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.

If you're concerned about a long-term power failure, such as from a hurricane or tornado, you need to look beyond battery backup to a portable generator.

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.

Hopefully, the cost will come down over time. The holy grail is to get battery power under $100 per kilowatt hour.

If that means the price a consumer pays, the initial Tesla solution is $350 a kilowatt hour.