Yes it worked well, it took about 70 minutes charging my battery to full up from 3am. So I started out this morning with a full charge and now there is just enough solar coming through the cloud to keep it topped up. On a sunny day I would be shipping the surplus solar. The plan now to set discharge at 50% rate from 4.30-6.30pm, but to stop at 20% SOC to leave enough to get through the rest of the evening.
An email from growatt service to say they updated my inverter firmware remotely and reset my clock, so maybe that was the problem rather my persistence trying different things. Anyway, things now seem to be in control and behaving correctly.
I also discovered this resource put together by some bright sparks at something called energy-stats.uk (bravo 10/10) which tells you neatly, one day ahead of time, what the next day Agile unit price profile is going to be. It confirmed my instinct of a very cheap import last night and still full export price this evening.
AgileUnitPrices
The trick is to keep a broad pattern of overnight import and evening discharge, but to watch out for events to skew things in case it is worth adjusting the schedule.
No idea what all this effort is worth, I figure I might net around £1-2 per day. Nor do I know if this is better than all the other options. Not enough to persuade me to spend £1,800 on a second battery but I will be watching for a drop in price.
It feels about right though, harvesting surplus wind power overnight which could otherwise be curtailed and supporting the grid through the evening. I suspect as more offshore wind gets rolled out we will increasingly see very low or negative overnight prices. So arbitrage is good for the pocket and good for the planet.
