The daily heartbeat
She runs on one lithium cell and a solar panel the size of a postcard, sealed in for good, with no way to plug her in. So every day she breathes: down through the night, back up with the sun. The lovely part is how steady that breath has become.
The number that actually matters
You'd think the interesting figure is how full she gets. It isn't. A solar buoy's whole survival hangs on the opposite end: the dawn floor, how empty she is at first light, just before the sun can start helping. If that floor holds level day after day, she's in balance. If it creeps downward, the panel is quietly losing the race, and trouble is a couple of weeks out.
So here is each day since she went back in the water: the tall bar is the daytime peak, and the amber tick is where she woke at dawn.
Reading the last couple of weeks off the buoy…
Boring, in the best way
She has settled into an equilibrium. She wakes at almost exactly the same charge every single morning, the sun refills her by lunch, and a small top-up at dusk banks a little extra for the night. The overnight breath is shallow and repeatable. There is no downward drift, which is the whole point: it means the little panel is comfortably keeping pace, and the cell is not ageing out from under us.
It took some work to get here. Early on she was finishing charging by late morning and then throwing away the whole sunny afternoon, going into every night at about two-thirds full, and one run of cloudy days walked her down to an emergency rescue. The fuel-gauge story and the dusk top-up that came out of it are what turned a nervy power budget into this steady heartbeat.
It's the least dramatic chart on the whole site. For an instrument that has to look after itself at sea for years, that's exactly what you want to see.