Yeah, you should add in the Costco rebate in conjunction with Sunrun to make the math kind of work out between the two providers.
I'll post an image here of the 15%; since some people may think I'm blowing smoke on the 15%...
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Also, keep in mind that Sunrun will usually do a new main service panel "for free" with their bid, while Tesla will up-charge like $4,000 for this if your home requires it. Sunrun will also do the more robust racking method while Tesla will tell you to pound sand if you don't want them to hastily slap panels on your roof. Sunrun will also use microinverters instead of large-inverters which may help you buffer against equipment failures that have a long turnaround time for warranty repair..
But Tesla will do the fancy skirts and dark-panels which looks nicer. And Tesla tends to paint the conduit runs while Sunrun leaves them as aluminum. Tesla is also not hated by PG&E, so if you go with Tesla you won't be part of the on-going war between Sunrun and the PoCo.
So it's worth making sure you have all the nickels and dimes in the pool before you outright discount one or the other as "too expensive."
Personally, if I had to do my system again, I'd go with a good local shop (admittedly it's very difficult to find these)... it's worth a bit of an over-pay to have the systems go in smoothly.
There are two additional discount from Costco Sunrun, one is the 15% equipment rewards, another is 2% executive membership rewards ( another 2% if have costco citi credit card). The two discount together will give only $1400.