# [rrd-users] Question about averaging daily ; but with 5 min. steps

Alex van den Bogaerdt alex at vandenbogaerdt.nl
Mon Dec 14 12:37:00 CET 2009

I haven't read the entire thread, not even this entire message, so forgive
me if I'm stating something obvious or something already covered. Just
ignore me in such a case.

----- Original Message -----
From: "Jean-Yves Avenard" <jyavenard at gmail.com>
To: <rrd-users at lists.oetiker.ch>
Sent: Monday, December 14, 2009 12:11 PM
Subject: [rrd-users] Question about averaging daily ; but with 5 min. steps

> the aim of my calculations is to monitor how much energy I output to
> the electricity grid via solar panels.

Energy is measured in Joules.  1 kWh is just 3600000 Joules.

> I have two RRD files ; one containing my power usage ; stored as GAUGE
> ; as watts . As we get charge in Watt/Hour (Wh) I perform the
> calculation using CDEF to transform watts over time into Wh.

Wh is not watt/hour. Wh is watt*hour.

kWh = 1000 Wh = 1000 J/s * 3600s = 3600000J.

> The 2nd one is for the solar output, this one is a DERIVE counter of a
> Wh value reported by the solar inverter that converts DC to AC.

So basically it is a joules counter. You can use derive if you want although
counter should also work (unless your solar panels actually use electricity
from time to time).

Make sure you understand W=J/s and kWh = 1000*3600*1J. Inside rrdtool
everything is <something> per second, so inside rrdtool you should store
Joules (or, if you want, kWh) which will show up as Joules per second aka
Watt.

Everything follows from that and you should have no difficulties processing

HTH
Alex