Comment by philipkglass

You'd be able to generate 1.14 kWh at the panel level if you kept the panel pointed directly at the sun throughout the day [1]. This is called "2 axis tracking" and it was sometimes used for solar farms when solar panels were much more expensive. Now that panels are much cheaper, 2 axis tracking has practically vanished from the market. The added expense and mechanical complexity isn't worth it. Single axis tracking, where the panels just rotate to track the sun from east to west, is still popular in large solar farms. It captures more sun than leaving the panels stationary but has less complexity than 2 axis tracking.

For a rooftop solar panel, you're not going to have any sort of sun tracking. The lack of tracking will reduce your output at the panel level. You will also lose more output if dust, debris, and bird droppings don't get cleaned away regularly.

You also lose some energy when the direct current electricity from your panels gets converted to alternating current in the inverter. How much loss depends on the inverter and how heavily loaded it is.

The NREL tool you linked says it's designed for "homeowners, small building owners, installers and manufacturers", which implies that it's for rooftop systems. It includes estimates for those loss factors I mentioned above, which is why I expect that it falls short of the number you calculated.

[1] EDIT: I forgot another significant factor: temperature coefficient of performance. A panel gets its efficiency measured at "standard test conditions" which include a moderate (near room temperature) panel temperature. Panels lose some efficiency as they heat up, which means that they don't perform as well as you might naively expect in the middle of the summer. The loss varies by panel technology. The very best conditions for panel output -- where they actually surpass reported efficiency -- is "bright sun but cold air," like noon on a freezing cold day with clear skies.