I recently got several proposals for (non-Tesla) solar installation, with a Tesla PowerWall added for critical systems backup. The best price was from a local firm that has gotten good reviews from customers, but is fairly new. The second-place finisher was a Tesla-certified PowerWall Installer, and they cautioned me that the first firm was not certified. How much should this worry me? Though not certified, the first firm has done PW work, and gave me references for it. Is certification something I should insist on?