More followup on the Tesla Powerwall home battery

More comments on the nifty new battery for home use. Mentioned this earlier.

5/3 – Seeking Alpha – The Silliness of Tesla’s 10kWh Back-up Battery – Just a few silliness tidbits from the author’s long list of silliness factors:

  • Cost of $3,500 doubles to $7,140 with Solarcity’s installation package
  • Output capacity of the battery is 2 kw
  • Average modest home can easily draw 3 kw (i.e. would need 2 Powerwall to run an average home on an average day without a/c)
  • Central air conditioning for a medium-sized house draws 5kw
  • Even at 2kw, the battery would be dead in 5 hours – not much backup


Would need two just to power an average home on a mild day. Toss in any a/c, let alone a hot day, and you would require 4 batteries (5kw plus 3kw divided by 2kw). That assumes you don’t need a/c longer than about 5 hours and then can do without refrigeration and lights overnight. And I’m assuming you could put 4 batteries in series (or is it parallel?) so they are all online simultaneously.

Also read some discussion today that the battery is designed for 1,000 cycles at 70% drawdown.

I hope that doesn’t mean what I think it means. Will it even last 10 years if you can only draw it down to 70% up to 1,000 times? Only have 100 days of heavy use a year or the life is shortened? What am I missing? (Yes, yes, I know I’m showing my skills are in accounting.)

5/1 – Forbes – Why Tesla’s Powerwall Is Just Another Toy for Rich Green People – In an analysis that is way over my head, the installed cost of storage would be $0.15 kWh. Add that to what it costs to generate, which on a Solarcity lease starts at $0.15 and can increase 2.9% per year for the 20 year lease (uh, that means you are committed to paying Solarcity for 20 years? Wow.)

That brings the starting cost down to $0.30 kWh, which is um, astronomical.

What happens when technological revolutions drop the cost of installed panels by, oh, say, 75%? You could by paying nearly 20 cents a kWh when your neighbor installs a new system that only costs a nickel or three cents per kWh. Until the 20th year.

Article said it might take 8 Powerwall units to store enough electricity for a home and then struck out the comment. Don’t know how many the author thinks it would take to run a home, but the point is it would be multiple units. Three? Five?

5/4 – The Register – Tesla Powerwall: not much cheaper and also a bit wimpier than existing batteries – Ponder this as you consider how many Powerwalls you want to put on your Christmas list. One unit will provide 2 kW power.

That’s 2,000 watts at any moment. Here’s the consumption of a few home uses, per the article:

  • 1,500 watt – electric kettle
  • 950 watt – toaster
  • 1,200 watts – air conditioner
  • 2,400 watts – ironing for your wrinkled shirts

Unless you buy two units, you can do one of those things at a time. Do any two and you pop the circuit breaker. Actually, you can’t iron your shirts again. Well, if you want pressed shirts, I guess you could buy another $5,000 unit.

Author thinks a typical home would need 3 units.

Also, you can’t draw a battery to zero or it causes damage. So that means only around 70% of the 7 kWh battery would be available on a regular basis, or about 4.9 kWh.

That means adjusting all my previous calcs by only having 4.9 kWh available from each of the $3.5K list price / $7K installed batteries.

3 thoughts on “More followup on the Tesla Powerwall home battery”

  1. The Powerwall was originally announced with 2 kW power output with 3.3 kW peak, but Musk announced at the June 2015 Tesla shareholders meeting that this would be dramatically increased to 5 kW power output with 7 kW peak, with no increase in price. He also announced that Powerwall deliveries would be prioritized to partners who minimize the cost to the end user, with a total purchase and installation price of $4000.

    1. Hi Phillip:

      Thanks for the info. That would supercede the comments both articles. Seems that would shift the analysis in each article. I think the underlying point that multiple batteries would be needed to power a home would still stand.

      Thanks for taking the time to comment.


Leave a Reply

Your email address will not be published. Required fields are marked *