This should have been automatic.
For agreements structured like Sunrun's BrightBuy with Protection Plus, the servicer's own contract language says a performance-guarantee shortfall refund is issued automatically, by mail, on each in-service anniversary when your actual output falls short of the guarantee. If that didn't happen for you, you're not asking Sunrun for anything new, you're asking them to do what their own agreement already says they should have done.
An automatic refund, confirmed in a public filing, but not always automatic in practice.
How to follow up with Sunrun
There's no dedicated claims-request process published by Sunrun for this, and per the agreement language above, none should be needed. When you call, reference the automatic-refund provision directly rather than framing this as a new claim you're originating.
A letter that says what it actually is: a missed automatic refund.
GuaranteeCheck's Sunrun letter is written differently from its other servicer letters on purpose. It states the automatic-refund provision, confirms your shortfall against your real production data, and asks Sunrun to issue what their own agreement already promised, rather than reading like a fresh claim you're filing from scratch.