[rrd-users] How to graph slow changing counters

Thorsten Erdmann thorsten at trektech.de
Mon Mar 30 15:34:57 CEST 2020

Hi Simon,

you are right (and I know) that with such a low resolution it is difficult to draw sensefull data. But I want to understand what RRD does. I think the aggregation is wrong:

If I sample the data every 5 minutes (1/12h) and get a single 1kWh step in one of these 5 minute intervals then I get a peak about 0,3kW in the dayly graph. But why? I get headache with these calculation. I would say the formular is 1kWh / (1/12)h but that gives 12kW, which is definitely wrong.

Another weird thing is what happens on the weekly graph. I get some "nice" step lines up and down. Shouldn't it be simply a flattened out curve just as you said. I cannot give you a picture right now, because my server is down for implementing the S0 photosensor, just as you suggested. I will implent both. So I can adjust the S0 pulse counter once a day or so to the real counter value. 


> -----Ursprüngliche Nachricht-----
> Von: rrd-users [mailto:rrd-users-
> bounces+thorsten=trektech.de at lists.oetiker.ch] Im Auftrag von Simon
> Hobson
> Gesendet: Montag, 30. März 2020 14:09
> An: rrd-users at lists.oetiker.ch rrdtool
> Betreff: Re: [rrd-users] How to graph slow changing counters
> Thorsten Erdmann <thorsten at trektech.de> wrote:
> > I want to graph the data of my energy meter. But it is measuring only in
> 1kWh raster. So with normal power usage I have every 2..4 hours a step of
> 1kWh. The meter displays it’s data as kWh, so it gives e.g 600 for the absolute
> counter value of 600kWh.
> > I sample the absolute energy every 5 minutes. So I took this as step and
> 600s as heartbeat.
> >
> > What I get is short peaks with a height which is displayed as 330u over a
> period of 24h. With longer periods like a week the values get smaller and
> smaller and there are weird stepping curves. So how can I measure/graph
> such low resolution measures in a senseful way?
> Realistically, there's not a lot you can do with such low resolution data. You'll
> get a spike each time a unit clocks up, then as you zoom out for a longer scale
> view, it will flatten out to (as you've found) a rather low value. If you are
> using a unit every (say) 3 hours (taking a mean between your 2 and 4 hours),
> then that equates to an average of 333W - or if you are just storing kWH,
> 0.333kW. Again depending on how you store and calculate, this then equates
> to around 93µJ/s - at least, if my brain is working right this afternoon !
> You don't say how you are interrogating the meter - is it via a data source it
> provides ?
> If the meter is providing you with a counter value when asked, then this will
> give you the best long term accuracy - your counter will always match what is
> in the meter.
> As an alternative, many meters provide a pulsed output of some sort (even if
> it's only an LED flashing). A bit like the old Ferraris disk meters with their "X
> revolutions/kWH", the meter pulses will give you an indication when a much
> lower quantity of energy has been used - it should say on the face of the
> meter how many pulses/kWH. The downside to counting these pulses is that
> if your counter isn't running or otherwise misses them, then you lose sight of
> energy consumed and your RRD calculations will show a lower figure than the
> meter.
> You could, of course, collect both sources - use the pulses for short term
> indication (ie better graphing up to a few days duration) and the register
> count for longer term.
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