Dave you can clean them in place but I'd scrub and rinse a couple times on those. It's harder to get a good job on screens that are still in the frame due to flex. To get the best job I'd go inside first and dry brush that side to help loosen what you can.
For the others you need to scrub and rinse each side of the screens. Believe me the dirt isn't just on the one side.
Try scrubbing the screens with a soft bristle brush and a good soap then rinsing with soft wash nozzles from your pressure washer. They get really dirty and the soap will help loosen the dirt so you get a good clean.
Cleaned the solar screens today. Followed your tips. The only time consuming part was climbing up and down the ladder and unscrewing the second floor screens. They look good as new. Owner was so pleased, he's having me back to clean his driveway.