# [rrd-users] how to calculate the sum of a DS

stefan Stefan.Bruder at gmx.net
Fri Jul 5 09:31:52 CEST 2013

```Hi Alex,
thank you for your explanation, but now I am little more confused.
my PV controller gives me the power created ( example 400Watt) each 10
seconds,
so I have 400W/10s.
For my understanding I should take the sum of all value (where watt >0)
divide it by the number of values multiply by
the timedifference from FIRST to LAST divide by 36000 (to get kWh)

DEF:Watt=PV.rrd:PVWatt:MAX
CDEF:Watt_times_86400=Watt,86400,*
GPRINT:Watt_times_86400:AVERAGE:%6.2lf
but it does not bring me the correct result.

Why do I need to have the RRA: defined as AVERAGE?
I also do not understand you last section, why is it bad to have such a
resolution?
Do I need to define a second RRA with different interval?

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```