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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

Nic
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nelsonic
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  • 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 07:04
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    You should also factor the expected rise in energy costs. – Erik Nov 30 '18 at 08:09
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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 03 '18 at 01:30
  • I have removed your call for discussion. As THelper commented, this is a Q&A site. –  Dec 03 '18 at 15:11
  • If you expect to use 24 kWh each night, then you might have to look into your electricity consumption. – gerrit Jul 14 '23 at 06:35
  • @gerrit I agree that 24 kWh can seem like a lot if you live alone. But it's a "normal" when running a 3.5 kW air conditioning unit for 8 hours. Thanks. – nelsonic Jul 15 '23 at 04:14
  • @nelsonic Heating or cooling needs no electricity storage. You can simply store hot or cold water when it's sunny in a well-insulated tank, then use that after dark. That even works on seasonal scales if you have enough water and bury it. Storing heat (or lack thereof) is easy and relatively cheap. Storing electricity is expensive. Storing electricity when you use it only for heating or cooling is a poor choice. – gerrit Jul 15 '23 at 20:14
  • @gerrit we're well aware of the physics of using water for heat/cold storage. But when we procured a quote for doing this in the centre of a major city it was 3x the price of a PowerWall setup and involved major building works. Thanks. – nelsonic Jul 20 '23 at 08:11

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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.

Aron
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  • 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 01 '18 at 08:24