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

[Joel HATSCH]

Hello *,

I'm trying to feed data from my solar inverters into rrdtool and plot some 
nice pics out of it. Using google, I have found several websites explaining 
how to generate nice looking pics, but I didn't found a description of the 
database schema and data feeding.

My inverter (SMA) records every 5 minutes the instantaneous power and the 
overall yield (both in kW (resp kWh), float with 3 decimal places).

If I store the values as GAUGE, I can draw daily production curves out of the 
instantaneous data, but I can't put them in a RRA - AVERAGE'ing or MAX'ing or 
LAST'ing instantaneous production makes no sense, only the sum

Regarding the overall yield, if I declare it as COUNTER (always increasing 
value) and use the LAST function to compress a whole day of data, I can plot 
the data, but as it is absolute, I get e.g. 40KWh increases from 3500 to 3540, 
however I'd like to see a graph where 40KWh are plotted for that day (i.e. 
data always ranging between 0 and 50kW for each day, not continuously 
increasing).

Moreover, COUNTER data seems to be divided by 300 (5 minutes) to get a 
frequency. I worked around this by multiplying the data by 300 in a VDEF 
statement.

As far as I understand, I'll have to do either some pre-processing (before 
feeding into rrdtool), or some post-processing on the data (when plotting the 
pics). To use COUNTER I have anyway to multiply the data by 1000 to convert it 
to integer.

My question : what's your advice on the data structure (DS, RRA), and on 
how/when to do the data conversion

Thanks

Joel