Update: Absorbed Glass Mat Batteries vs. The Tesla Power Wall

I’m updating this blog post based on recent revelations about the Tesla Power Wall. The 10kWh Power Wall was abandoned and never produced. The 7kWh Power Wall has been down graded to 6.4 kWh and has a different cycle life than what we were lead to believe. See this article at InvestorIntel.

http://investorintel.com/cleantech-intel/tesla-energys-incredible-shrinking-powerwall-warranty/

There are a few things to consider here with comparing the capabilities of stored energy. First, the best way to compare apples with apples is to compare the total kWh life vs cost. Total kWh life is the usable energy storage multiplied by the cycle life at that usable storage level. This even allows comparison of the same battery using different depths of discharge.

Storing Energy

This “cost” is the battery portion of electric cost in storing and then using the power, only. You still have production costs associated with your solar array! Here are the facts:

  • PW7 has a cycle life of “3000 cycles”, 6.4 kWh usable storage and costs $3000.
  • 3000 / (6.4kwh * 3000) = $0.143/kWh – Update, the PW7 actually is 6.4 kWh. So the real value is $0.156/kWh
    • BUT – Based on the above article, the battery will degrade quickly, losing 20% of it’s capacity in under 1000 cycles.
    • It will LAST for many years, but by end of life at 3000 cycles, it has lost 40% of its capacity.
    • This will increase the effective cost of the power pack to around
    • Total Capacity over 3000 cycles will be 14,720 kWh. –> $3000/14720 kWh = $0.204/kWh
    • This is type of degradation is TYPICAL of all Lithium-Ion batteries
  • High-quality AGM batteries at a 50% DOD have a life of 1800 cycles. A 2.4 kWh battery costs around $350.
  • $350 / (1.2 * 1800) = $0.162 / kWh – still less expensive than Power Wall
  • Nano Carbon AGM batteries at a 50% DOD have a life of 3000 cycles. A 2.4 kWh battery has a cost of $400.
  • $400 / (1.2 * 3000) = $0.111 / kWh

In conclusion, the PW7 has a higher total life cost compared to a standard AGM battery, and is 50% more expensive than a Nano Carbon AGM.

Mi-Grid vs. The Tesla Power Wall

Now let’s compare a PW7 system to a Mi-Grid 2000. The Mi-Grid 2000 is designed to connect to 8-10 kW of Solar, produce 8kW of inverter power, has 38.4 kWh of battery storage. It also includes a 12 kW Natural Gas Generator. The total cost to install a Mi-Grid 2000 with 8kW of solar panels is around $45K.

The Power Wall needs a Grid Tied inverter that is designed to work with it. Fronius makes one that is 8kW. It ties to the battery & solar panels to your utility power. It needs a separate automatic disconnect to separate it from the utility when the utility power fails.

The PW7 is limited to a maximum discharge of 2kW. To match the Fronius Inverter you will need to supply four PW7s. The PW7 system is not integrated. It must be assembled on site, using an electrician. Total cost to install 8kW of solar panels, PW7, Fronius inverter, control system, safety equipment and a backup generator will add up to $60K.

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