I have a garmin 800 as well. I really want a Wahoo Elemnt Bolt, but I can't justify it right now. The garmin flakes out a little too much for my taste at longer distances. Stopped giving me cues at the end of a recent 300km ride, for example, right when I needed them most. At least it didn't lock up. But watching the map was an unwelcome extra task after a difficult ride.
I prefer to charge mine using a battery pack. One 2600 mah battery pack keeps my 800 good for about 20 hours. So 4 or 5 should be good for a 1200km ride. Unless you are touring.