Today (Friday 2016 Oct 28) will be an announcement from Elon Musk about the Tesla Powerwall. This is a very interesting concept of storing power in your home and trying to lower the total cost of power. In our area, we are billed with time-of-use, which ranges from offpeak @18 cents, midpeak @26 cents and onpeak @36cents per kwh.

One obvious use of the powerwall is to charge it up during the offpeak (it will hold up to 6.4kwh) and use that power during the onpeak and midpeak times. How much would it cost and how much would it save for an average 30kwh/day use?

tesla powerwall 1.0 \$3500 US for 6.4KwH = \$4700Can plus installation
Our 30day average use is

100%= 74%*18+15%*26+11%*36=21 cents kwh average
=13.3+3.9+4.0=21cents
\$0.21 * 30 kwh/day = \$6.30/day

assume 100% offpeak = 21-18= 3 cents saving per khw * 30kwh/day minimum? = \$1/day
The powerwall has 5000 charge cycles or 10 year warranty.
365*10=3650 days * \$1 = \$3650
This does not even cover the cost of the battery, much less installation.

What happens if we conserve more and try 24kwh/day = \$0.75/day = \$2740.
OK, how about if we start using more power? and try 40kwh/day = \$1.20/day * 3650= \$4380
This is almost the cost of the battery. so still no real savings.

The other issue is that is the 6.4kwh capacity of the battery enough to run all use during mid and onpeak?
At our 30kwh/day use, 74% is at offpeak, leaving 26% at mid or onpeak, which is 30kwh/day*.26=7.8kwh, or not enough capacity. So the cost saving numbers come out even worse. We would have to reduce our daily average down to 25kwh/day with the same percentage breakdown to achieve the 6.4kwh. And the powerwall capacity would also reduce over time with age.

What about adding in solar panels to help charge? Assume that the going price is \$3/watt. Also assume only 5 hours/day production of power.

Add in one 100w solar panel for \$300. 100W*5 hours/day = 0.5kw/day from default 30kwh/day = 29.5kwh/day or 1.7% saving for about \$300 cost – and we come up even.

add in 1kw of panels for \$3000 *5 hours/day = 5kwh/day from default 30kwh/day = 25kwh/day or 17% saving for about \$3000 cost plus install

add in 2kw of panelsfor \$6000 *5 hours/day = 10kwh/day from default 30kwh/day = 20kwh/day or 35% saving for about \$6000 cost plus install.
So it seems that solar panels may be able to pay for themselves alone in 10 years, with no financial surplus to help cover the battery.

Things are not looking good for this system.
If you are on the grid now, there is no business case for this. If you have a lot of power outages, it would make a great backup system.
If the price of power doubles, so would our savings (\$3650*2=7300) and that would pay for the cost of the battery plus installation (estimated \$7000).

One can only hope the price of the powerwall drops, the Canadian Dollar rises, the price of solar panels drop and the price of power rises (only that last is a bad thing!).