I found this graph interesting. Comparing 2 strings. I just washed the blue string, 1 row at a time:
Nearly 10% improvement after removing a light layer of dust.
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What are the gaps in the blue, is that where you washed them? Can you give the output difference after an hour?
Looking at the graph, it looks like a 6% improvement towards the end of the graph, and I’m willing to bet most of it is temperature related - wait for the temperatures to equalize and report back.
If we are betting on temp related gains (can very well be) then I’d recommend not washing hot glass with cold water… 
10% would be less than a 30°C delta, I don’t think that’s much of a risk.
That is the panels’ temp delta. The water might come in at less than 10 degrees, no idea what temp the panels could reach baking in sun all day.
The gaps were as they were being washed.
Some of the delta likely was temp related - relative production up 150W on a clear day with no playing…
I heard that some PV brands require the panels to be washed at least yearly in order to not void the warranty.
My panels did not have this (except if I missed the very fine print).
Anyone seen something like this?
Quite possibly - but I have not seen one (yet). Canadian has the ‘constant shading’ limitation. One other manufacturer (I can’t recall which) has a ‘regular shading’ limitation. Can’t recall seeing a washing limitation though.