Question: Tesla Power Wall

[NeoGreen]

http://www.treehugger.com/clean-technology/tesla-powerwall-game-changing-batteries-homes-and-businesses-starting-3k.html

and

http://www.teslamotors.com/powerwall

What is the significance to the "5amp" limitation in the specs?



What does that relate to day to day / hour by hour usage?

The graphic on appliance usage lists each by typical KWH, but what would be the amp load requirement from an electric dryer verses a fridge versus a laptop?

(electricity is not my strong suit)

I have a few acres and would love to install my own solar farm and NOT be hooked to the grid.

If I use an average of 14KWHs per day (~600 watts average per hour, discounting peak vs off-peak) how many units would I need?

I presume, at a minimum, 2 of the 7KWH rated units (which I understand would provide up to 4KW per hour)?

Reasonable?
(I know, better to go with 2 10KWH units, just to be safe)

Thanks in advance
NG

[enlightenment]

1. I know next to nothing about this,

except that the 10KWH units are optimized as back-up systems and the 7KWH units as daily systems, so your initial idea may be the right one.

Also, the $3000 (for the 7KWH) cost is to installers (and I guess the cost of an inverter is around $4000).

Exciting stuff, though. Do keep us informed on your progress!

[NeoGreen]

3. Wow...

...had no idea an inverter would run as much as a single unit (ignoring rounding errors).

Typical, the real expense is all in the "hidden costs".

No definitive plans yet, but including a battery in whatever system I eventually install has always been part of the plan.

[Erich Bloodaxe BSN]

2. I haven't done anything with electricty in forever, but...

I do have a fuse panel still, and most of my fuses are 15 or 20 amp. And I've blown a few over the years, suggesting that running several large items on a circuit runs to more than 15 or 20 amps. And I think the oven gets a 40 amp circuit.

Here's a little blurb I stole from some website

Electricity is measured in terms of amperage, voltage, and wattage. Amperage (amps for short) is a measure of the AMOUNT of electricity used. Voltage (volts) measures the pressure, or FORCE, of electricity. The amps multiplied by the volts gives you the wattage (watts), a measure of the WORK that electricity does per second.

Think of it this way: Electricity flowing through a wire is like water flowing through a garden hose. The amount of water that can fit through the hose depends on the diameter of the hose (amps). The pressure of the water depends on how far open the faucet is (volts). The amount of work that can be done (watts) depends on both the amount and the pressure of the water (volts x amps = watts).

[NeoGreen]

4. I guess I have always framed it it in opposite terms...

...when using the water analogy:

Volts = Volume (i.e. diameter of the pipe) You can have a huge volume/diameter flowing but it wont harm you when it hits you at very low pressures (i.e. amps).

Amps = Pressure (i.e. PSI in pipe) even a small volume (low volts) could harm you at very high pressures (i.e. Amps) as in those high pressure industrial devices that use water to cut stone.

Watts is just how much water you used (Volume (dia) * Pressure (psi) * Time)

Maybe I'm mixed up in how I framed the analogy.

[bobalew]

7. Yeah, Volts is analagous to Pressure as it is call electromotive FORCE, Current is the Volume

or how much can be transported at any given"Pressure"...

[bobalew]

5. That's not a "Limitation", but is the nominal continuous maximum output.

It can put out on an occasional surge, up to 60% more, if needed.
The Output is a continuous 2 KW per unit. If your house uses greater than 2KW, at any moment in time, double up. The 7 / 10 KW is the total CAPACITY of the Battery, or how much power it can supply over time. If you're are using , say 1.5 KW ( Roughly double your estimate) per hour in off peak (Night-time), then a single 10 KW capacity unit would yield you 10 KW/ 1.5 Kw per hour=6.66 hours of power. Better get 2, to get a 13 hour back-up when thee is no solar or wind (& I suggest investing in both, for Off Grid use) Being in the boonies myself, you never know when you might really need that extra wattage to run a pump from your well or stream water supply in an emergency.
The UL listing, BTW, at least portends that the units have been through some fairly rigorous safety compliance testing, but do note, Lithium batteries are inherently capable of some really spectacular failures. Keep them outside, away from flammable locations. Also, make sure you've located the inverters very close to the battery, to minimize Long runs of DC cables, & use the largest gauge of wire that you can. DC power over distance causes losses in the wiring & HEAT. AC voltage transportation is much more efficient. Do remember that when inverted to AC power, there will be some efficiency loss, and there will be more amperage when converted to AC 240/ 120Volts. This is why we work in watts as a metric. That is a constant.
I hope this helps answer some of your questions.

[Demeter]

6. Look at the voltage 350-450V

5 amps at that voltage is 2000Watts of energy, available for 5 hours at maximum draw, longer for lesser draws, of course.

If you are running a DC house (using appliances from Recreational Vehicle designs), you'll need to step it down. Less voltage, more amperage. P=VI

If you are running AC (typical consumer products) you have to step it down AND invert it. Very lossy.

Thanks for posting this. I want to look into it further...

[bobalew]

9. MOST iNVERTERS THESE DAYS, WHETHER DC-DC CONVERTERS OR DC-AC CONVERTERS ARE A BIT MORE EFFICIENT

these days, thanks to High frequency Charge pump oscillator/ conversion circuits & High frequency conversion Inverter AC generators than yester-year. All step-down functions are accomplished vis these Nice new switching power supply designs. The loss in efficiency, depending on the design has been pared down to less than 8%, so now, not quite as lossy. Just keep the battery to converter/ Inverter leads Thick 8 short.

[FBaggins]

8. Don't consider the 7kwh model for your case

When you consider installation costs (plus inverter/etc)... the difference in price between the two models is negligible.

If you're really considering being entirely off-grid, you'll need much more than this.

[Demeter]

10. HELPFUL TUTORIAL ARTICLE: Should You Get a Tesla Home Battery? Let Physics Explain

http://www.wired.com/2015/05/get-tesla-home-battery-let-physics-explain/

[mackdaddy]

11. Actual usage technical information still lacking.

I have been trying to find some actual hookup data, but that is not published on any of their websites yet.

I have designed and installed several PV solar installations, but some critical data is not in the "technical specs" you have above which is all they have out yet.

The fairly high dc rated voltage is typical of what comes out of a string of solar panels. But it is not clear what is going to be used to supply the charging voltage to this box. Will it connect directly to the solar panels and take care of the MPPT energy optimizing from the panels, or will there need to be a separate "charge controller" that interfaces the solar panels to the battery box.

Also it is not obvious how the box is to connect to an inverter to make the power useful for the AC loads in your house, or if the batteries can also be charged from the grid power in low sunlight times or for use as a backup emergency power system. That is a high operating voltage for most battery based off grid or hybrid inverters.

Another major point is that most batteries should NOT be discharged fully, but only to about half of their rated capacity. This is called the "depth of discharge" and is a very important in actually sizing the battery bank needed. Most batteries if discharged beyond about half their capacity have vastly shortened operating life cycles, ie from thousands of cycles to a few hundred. (including lithium-ion types) This recommended depth of discharge, and expected charge cycles lifetime are also critical information.

There also may be a difference in the7kwh and 10 kwh in their configuration, so which to choose may be a little more complicated. One is targeted a daily cycling, and the others at more infrequent cycling. There may be a larger difference between these two models than just their capacity. I am thinking of the difference between a deep discharge cycle battery as used in a golf cart, and the starter battery used in you car which is high current, low discharge. Again this is not clear yet.

This does look like a great prepackaged solution, and I look forward to getting the real installers specs.