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I think that depends on how often you want to update the data. Like - how long is a piece of string? If the data does not change rapidly, then you do not need to update it very frequently. Giving a flippant example, if you are recording barometric pressure, every 5 mins would surely be good enough. If you are measuring power (I think you might be!) then work it out from the accuracy you want when you measure something like an electric shower. If it's on for 5 minutes and you want it to within 1%, you need to sample roughly every 0.05 minutes, or 3 seconds. The rate that the emontx uses, about every 5s, looks sensible.