[rrd-users] Handling solar inverter data in rrdtool ?
linux at thehobsons.co.uk
Mon Jul 2 09:20:16 CEST 2012
Joel HATSCH wrote:
>My inverter (SMA) records every 5 minutes the instantaneous power and the
>overall yield (both in kW (resp kWh), float with 3 decimal places).
I'd say use the counter. I assume the instantaneous reading is
actually for the instant read rather than an average over the
previous 5 minutes and so may well be significantly different.Long
term it will probably average out and be fairly accurate, but short
term may be way out.
Eg, if a cloud passes over the panels just as you take a reading,
then you may get a low figure when output is actually high.
Similarly, if a break in the clouds passes just as you take a
reading, you'd get an artificially high figure.
Since you aren't likely to want anything other than kWH in your
output, I'd suggest just storing that. Internally it will be stored
as KWH/s (all things are stored as rates) and so on output you'll
need to multiply by your time span to get a total amount of energy.
Eg, if doing daily figures, multiply by 86400 and you'll kWH/day.
Also, since your inverter is keeping the counter, you could afford to
increase the heartbeat considerably. If you fail to collect data for
a while, then your next collection will result in a big chunk of
lecky being counted but it will be spread evenly through the
intervening data slots. You'll lose the detail of when it was
collected (eg a couple of days would show as a flat line at whatever
the average rate was) but your totals would be correct and after
consolidation you'd probably not notice it on the "one period/day"
long term graphs.
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