I'm trying to determine if investing in a couple of Tesla Powerwall units to store solar energy is a wise financial decision.

My energy cost is $0.1649/kWh. The installed price of the two Powerwall units is $15k. I expect fully charge and discharge the Powerwall 300 days per year (based on historical average sunshine hours per month) and use 24kWh each night.

Based on these assumptions I calculate that: 300 days x 24 kWh x $0.1649 = $1187.28 per year so (15000/1187.28 =) 13 Years "pay back period".

The Warranty on the product is 10 Years and the useful life is claimed to be 15 years. However many people claim that the performance (storage capacity) will degrade down to 70% of original capacity after 10 years so this calculation should factor that in too ...

Is it economical to buy a Tesla Powerwall or simply pay "the grid" (our major energy supplier) for (renewable) power at night?

Are you a Powerwall owner? Have you done a similar calculation? What is the conclusion?

Note: I'm a huge fan of what Elon is doing and have much love for Tesla as a company, but I want to know from a purely economic perspective if it makes sense to invest in a Powerwall?

For reference: I've expanded on this calculation and included all the supporting research and references in: https://github.com/dwyl/home/issues/25

  • I think you've asked a good question, but please be aware that answers and comments aren't very suitable for discussions. It's best to do that in chat. – THelper Nov 30 '18 at 7:04
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    You should also factor the expected rise in energy costs. – Erik Nov 30 '18 at 8:09
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    Powerwall only becomes economical viable if your locale does not support feed in, at all. Otherwise, you can use the grid as your battery. – Aron Dec 3 '18 at 1:30
  • I have removed your call for discussion. As THelper commented, this is a Q&A site. – Jan Doggen Dec 3 '18 at 15:11

No it is not. Your calculations of 13year ROI is based on the assumption that charging the power wall is free during the day time. It is not.

Depending on your locale, either you need to factor in the cost of additional solar capacity or lost feed in revenue.

This will easily push your ROI into the 20 year territory.

Assuming that your locale supports feed in, you would invariably reach break even with heavier investment on the solar front.

  • great points. I expect to have enough "spare" solar energy to fully charge the Powerwall each day and the remaining solar will be fed into the grid for a kWh lower price (the feed-in tariff is no longer subsidised where we live in Portugal), So I have discounted the cost of solar panels form my decision. But you are correct, most other people doing this calculation will need to factor buying the solar capacity to charge the Powerwall into the up-front cost of the investment. 👍 – nelsonic Nov 30 '18 at 20:17
  • @nelsonic you still have yet to discounted the potential lost income from selling to the grid... – Aron Dec 1 '18 at 8:24

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