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Free Power Wall from Free Power? Math.. didn't work for me :(


Active Member
Dec 5, 2014
So... NRG Home

$0.00 per kWh from 9PM to 6 AM. If I use 1500 kWh/month

How long of using this + a power wall, until the power wall pays for it's self?!

It's math time people! you know, that thing your teacher in 9th grade said would happen, and for the past 30 years... hasn't?! Well it's here now!

1500 kWh / 30 days = I need to suck down 50 kWh/day.
So we will need a power wall of at least 50 kWh. Let's assume a 80% in efficiency, and 80% out? is that a good approx?

So I need 63 kWhs of Power wall at 7kWh each = 9 power walls.

9 power walls at $3,000 each = $27,000

Hookup cost: $5,000 (estimate?)

Total install cost = $32,000

Power cost now: $0.11 kWh

Power costs at $0.00 (let's just pretend I can pull this off) and ignore power IN (charging) efficiency, since the cost is $0

I would need to consume 290,909 kWh for this system to pay for it's self.

Or, roughly pay for it's self in: 193 months / 16 years.

Or way after it doesn't work any more because it's been used daily for 16 years.

So... yea, Math means... that ain't gonna happen.


Burrito Founder
Nov 10, 2011
Your math confuses me. From your last power bill, do you have data on your daily avg, daily max, and daily min consumption for the month?


Jun 21, 2014
One problem with your math is that it assumes you currently use all of your power between 6AM and 9PM. Whatever power is currently used at night does not need Powerwall capacity. The one obvious thing you could do is only charge your car off peak, so you don't need to "Powerwall" that capacity.

I bet you could get to 50% off peak use, so you just removed $16,000 right there.

Another thought is that Powerwalls are not a very good value. There is a 60kwh battery for $8,000 for sale right now, you can buy a 24kwh Leaf for $9,000. I know those are not exact surrogates for a Powerwall, but if you could figure something out along those lines......maybe read WK057's stuff.

If I lived in PA and had free power, I'd look at off peak heat and ceramic bricks.


Nov 28, 2015
United States
Yeah your calculations are good. In fact, you give the Powerwall a little too much credit. As Evbwcaer said, it's not clear if the 1500 kWh/month number is the total electricity consumed, or just the electricity you pay for (the stuff during the day). Also, you're gonna need a rectifier as well as an inverter to charge and discharge the Powerwall, so I think your installation costs are a little low.

Here's another useful way to look at it:

Assume the installation, inverter, and rectifier are free, and that you would always use the output of at least one Powerwall, and then suppose you just get one Powerwall. This is the fairest possible light to cast the Powerwalls in for your use case.

So in this scenario, you would have one 7 kWh Powerwall for $3,000. Every night, it would charge up 7 kWh for free, and every day, it would discharge that for your use, saving you $0.11/kWh * 7 kWh = $0.77 of electricity. To pay for just itself would take 10.66 years, or just past the warranty period.

Even in that extreme scenario, it probably wouldn't be worth it. If you add the cost of installation, an inverter, and a rectifier, even if you spread that cost out over several Powerwalls, it's even worse.

So yeah, absolutely, your math is right and a Powerwall would not be economical for you for that use case.


Active Member
Feb 27, 2013
East Hampton
Just as a point of information: do the referrals have to lead to sales, or is it just good enough to have people referred and test drive?


Well-Known Member
Feb 17, 2013
United States
So... yea, Math means... that ain't gonna happen.

Yeah.... at >$400/kWh and a $0.10/kWh arbitrage... the math ain't gonna work... you need to think of it more in terms of backup power that also provides a dividend. The Powerwall is only the beginning... once storage hits ~$100/kWh the math will start to work out.

Even the power pack marketed to utilities at ~$250/kWh is probably more valuable for it's kWs than it's kWhs...

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