Battery storage for the home. Extremely expensive, but available. Update: merely double my costs.

Tesla announced a new home battery product called Powerwall. A very cool idea. In spite of what you might think from reading my blog, I think this is a good idea.

5/1 – AP at Daily Bulletin – Tesla charges into home battery market despite challenges

It is merely the next step in a very long journey, but I can grasp the great idea. Someday storing electricity at home may be viable.

On the other hand, it is astoundingly expensive at the moment. On the other, other hand, that’s what happens at the front-end of breakthrough technology.

You can find more info on the battery at Tesla’s Powerwall site.

I’m going to stretch my knowledge, probably beyond the breaking point. Let me know if I slip a digit or miss a step.

The nifty graph on the Powerwall site shows maximum solar would be available during the lowest point in the day for usage. There is a high demand in the morning and evening which is not when most of the solar energy is available. If my little brain understands this graph correctly that means that the batteries would have to absorb and store essentially a full day’s worth of electricity so it will be available in the evening and the next morning before the solar power goes way up the next day.

The specs at the site say there are two models.

The first is has 10 kWh capacity for $3,500 which has a weekly capacity for backups. This would be $350,000 per megawatt hour. The second model for daily cycle applications is 7 kWh for $3,000. This would be $429,000 per mWh.

The power output is 2.0 kW continuous or 3.3 kW peak.

Possible use in my home

I took a look at my electric bill last month. If I understand the numbers correctly, we would need three of the 7 kWh batteries.

During the hottest summer months it looks like we would need to have about six batteries to make sure we have enough juice to run air conditioning all evening (ave about 39 kWh per day / 7 kWh = 5.6, rounded to whole units of 6).

That means we would have to buy $18,000 of batteries and find the space to store them in our home. By the way, that would be equal to about 29 years worth of electricity at the amount we paid last month for generation charges. (That is assuming I understand I am reading my electricity bill correctly, which is a dangerous assumption since I merely have the math and analytical skills of the CPA, not an engineer.)

Better cost than backup battery I use for my computer.

I am a believer in backup batteries. I have an uninterruptible power supply in my office. It has a very small capacity but is sufficient for my office needs.

It is also far more expensive on a kWh basis than the new Powerwall batteries. In that respect, the batteries are cool.

Here’s my comparison:

Powerwall has 2.0 kW continuous output. Cost is $3000 for 7 kWh. That is $1500 for 1 kW continuous output or $428 per kWh storage.

A mid capacity UPS is available for $57.95. It has 330 watt output. Capacity at half load is 160 watts for 13.5 minutes.

Let me try to adjust that for comparability.

The UPS has .33 kW continuous output, which would be $175 for 1 kW continuous output. I think that storage capacity is 160 watts, or 0.16 kW, for 13.5 minutes or 0.225 hours. If my non-engineer brain is working correctly that would be 0.036 kWh. At cost of $58, that would be $1,583 for 1 kWh storage.

I will ignore the Powerwall having a 10 year warranty, which implies a roughly 10 year or more life and a three-year warranty for the UPS, which suggests it has a three or four life.

If my calculations are vaguely close to accurate, that gives the storage capacity cost of $428 per kWh for the Powerwall and $1,583 for a UPS.

That is very good. Storage cost of the Powerwall is 27% of a small backup.

The cost for capacity is huge

I think the cost will need to drop by factor of 10 or 20 to be a breakout product.

Spending $18,000 for battery storage for a modest sized house is not going to work.

Look at it another way.

The $18,000 cost to buy 6 Powerwall batteries when spread over the implied life of 10 years (warranty term) would be $1,800 a year. That is $150 a month. Generation charge last month (April) at my house was $51. So the battery storage cost per month would be three times as much as the cost to purchase the electricity last month.

Tripling the cost of electricity is not going to work.

Yet it is very cool progress. All we need is a radical breakout in battery storage technology, sort of like what fracking has already done for world oil production.

I will leave out of the discussion how much and how many rare earth minerals go into the battery and how all that stuff is disposed at the end of its 10 or 12 year life.

What do you think? Did I miss any major steps in my calculations?

Update:  Diogenes, @WallStCynic tweets:

$TSLA all-in battery cost is $5,000.That’s $500/kWh,or $0.11/kWh.$SCTY charges $0.15/kWh on new leases. Current CA avg grid cost is $0.18.(1)


So $SCTY panels, combined with a $TSLA home battery will provide electricity at $0.26/kWh, or 50% ABOVE the California grid cost! (2)

Increase all my above calculations by two-thirds, from 3,000 to 5,000.


Cost is $5k instead of $3000 for 7 kWh. That is $714 instead of $428 per kWh storage. That is $714,000 per megawatt hour. We are approaching the range of building a new natural gas plant, and all we have is battery storage.

For 6 batteries at my home, that would be an upfront cost of $30,000. Amortized over 10 years that would be $3,000 a year or $250 a month. That would be merely five times the cost for generating electricity that I used last month.

Update: 5/1 – Wall Street Journal – Will Tesla’s Newest Battery Pan Out? – Article reinforces my understanding above that one battery won’t be enough to actually power a home. What is missing from the following characterization of a battery’s capacity?

Since the average house creates an electrical load of 2 to 5 kilowatts, the packs could power an entire house for a couple of hours. Or they could run essential household circuits—a refrigerator, microwave, lights and fans—throughout the evening when solar panels don’t produce, provided there was enough sun during the day to charge up the batteries.

Where is air conditioning mentioned? Or heating for those homes with electric heat?

Running just the ‘frig and lights will leave me a tad warm for the entire summer.

Also, the article points out that the $3,500 for a battery is just the start of your costs. You’ll also need the solar panels, an inverter, building permits, smart controls, and labor. That can run $20,000 or more, according to the article. Oh, also consider that putting the batteries on the wall of your garage will expose them to extra heat, which shortens their expected life.

Let’s assume only $20,000. Let’s assume that includes a $3,000 battery. That means the other stuff would be around $17,000 (or more) on top of the batteries.

So to have a self-sufficient home, I’d need to put in another $17,000 along with $18,000 for batteries.

I’ll pull a wild assumption out of the air that the whole thing would last 10 years. That would spread the $35,000 over a decade for cost of $3,500 a year.

That is twice what I paid last year for electricity. Twice.

I’d still need to pay something for capacity charge and buy extra electricity when it is really hot.

Or raining.

Or when it is cloudy.

Let’s round that doubling up to around three. Going fully solar would only double or triple my electricity costs.

What do you think? Did I mess up my calculations?

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