[rrd-users] Handling solar inverter data in rrdtool ?

Simon Hobson 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.

Simon Hobson

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