First, let's go over what the IER found. A Powerwall system costs about $7,300 when installed by SolarCity (where Tesla CEO Elon Musk is also CEO). In the optimal case where utility systems have peak and off-peak rates that are often priced at about a 60-percent discount, the Powerwall system can save its user about $193 a year, meaning that it'd take about 38 years to pay for itself (a 2.6-percent annual return on investment). Install a solar-panel system, and the numbers get only a little better, as the costs savings rise to about $358 a year and the payoff period shrinks to about 31 years. For utilities that flatten their rate program so that the same rates are charged all day and night, there's a pretty good chance that the $7,300 Powerwall will never pay for itself.
Now, here's where Tesla responded. The automaker said a more accurate way to calculate Powerwall's payback period is to look at regions with renewable-energy policies like feed-in tariffs. That's where, "the consumer can utilize a Powerwall to consume more of their solar generation and the payback is less than 10 years, while providing the non-economic benefits as well," Tesla says. Tesla also sent Engadget this statement:
To which, of course, the IER responded as well.
"The report done by the Institute for Energy Research is elementary, at best, and completely misses the value of the Powerwall. For one, IER assumes a fabricated rate structure without citing any source. Transparently, if a consumer were to have a rate structure defined in the article, then the payback calculation is indeed correct. However, very few people with this sort of rate structure are interested in Powerwall for financial reasons. They are interested for energy independence, backup security, environmental reasons and tech early adoption, none of which are taken into account."