Having the phrase "solar panel" in a property description may correlate with a 40% increase in "expected sale price" (the blog post doesn't say what they mean by this), but having solar panels in your home correlates with all sorts of things that haven't been controlled for in the analysis (for instance how warm the local climate is).

Much more useful would be an estimate of how much a home's value would increase if solar panels were installed. That figure is certainly much less than 40%.

Reference: I built Opendoor's home valuation model.