The real answer is it doesn't. There are other threads going into more detail, but the short of it is the IRS, in a private letter ruling - https://www.irs.gov/pub/irs-wd/201809003.pdf (which officially means it does not establish precedent,) stated that PWs could claim the credit, but only because the taxpayers stated that 100% of stored energy came from the sun. The IRS went into some detail to note that Congress had used a 50% rule elsewhere but omitted it here, so 100% was the rule. This means that potentially any use of storm watch could actually invalidate the credit. I think the assumption is that because it is generally expected to be a very minimal amount (though folks in CA fire country may not feel that way after this year) and is intended to assist with emergency conditions, it is probably something that can slide (and the customer is largely not in control of when it happens.)@jjrandorin: Interesting. Do you know why being able to switch from solar to grid for 20 or 30 minutes each day to charge the powerwall would affect one's ability to get a income tax credit? If so, how does Stormwatch get around this restriction? There is probably a thread on this somewhere here, but I haven't found it. Thanks!