Been thinking about this I’m not sure this will work, I have a emonTH with the optical sensor attached as i dont have useable power by my meter. The emonTH stores the pulses and then send them every 60 seconds to save battery life so my condition will be potentially incorrect as when the pulses get sent the current grid import or export could have changed.
Ah, you didn’t mention the data wasn’t anywhere close to synchronous.
As you say, it’s the old way - a calendar, a pen and the individual registers on the meter.
You could tweak the emonTH sketch to send the pulse count much more frequently, but at the cost of battery life. If you say just under every 10 s, you should still get a few months from a set, which is probably enough to get a fairly close calibration on one set of batteries. (Then change it back to just under a minute.)