---
date: 2016-05-02
strapline: Fun with SOAP and rrdtool
thumbnail: /res/images/wemo/switch.thumb.jpg
title: Monitoring power draw with WeMo Insight Switches
url: /2016/05/02/monitoring-power-with-wemo/
aliases: ["/2016/05/02/monitoring-power-with-wemo.html"]
image: /res/images/wemo/desk-1d.png
description: Using SOAP requests to get raw data, and rrdtool to produce graphs.
---

<div class="image right">
 <img src="/res/images/wemo/switch.jpg" alt="WeMo Insight Switch">
</div>

I recently picked up a couple of <a href="http://www.belkin.com/uk/p/P-F7C029/">Belkin's WeMo
Insight Switches</a> to monitor power usage for my PC and networking equipment. WeMo is Belkin's
home automation brand, and the switches allow you to toggle power on and off with an app, and
monitor power usage.

The WeMo Android app is pretty dismal. It's slow, doesn't look great, and crashed about a dozen
times during the setup process for each of my two switches. It also doesn't provide much
information at all about power: you can see average power draw and current power draw, and that's
basically it.

Belkin has provided an option to e-mail yourself a spreadsheet with historical power data, and can
even do it on a regularly scheduled basis, but that's not really a nice solution if you want
up-to-date power stats. Even if you were happy with data arriving in batch, having to get hold
of an e-mail attachment and parse out a weirdly formatted spreadsheet doesn't make for easy
automation. It also relies on Belkin supporting the service indefinitely, which isn't necessarily
going to happen.

<!--more-->

### Enter the SOAP API

Fortunately, the devices themselves expose an API to get the data. Once a switch is setup with the
WeMo app, it connects to a WiFi network. You can discover WeMo devices on the network using a UPnP
broadcast, and then from there a list of supported services. Each service has a number of SOAP
actions, with nice obvious names, and parameters documented in the XML service description.

For the insight switches, there's a service at `/upnp/control/insight1` which has a number of
actions including `GetPower` and `GetTodayKWH`. Sending a SOAP request isn't too difficult
(albeit nowhere near as nice as a REST JSON API) &mdash; here's a sample request I made using
the <a href="https://chrome.google.com/webstore/detail/dhc-rest-client/aejoelaoggembcahagimdiliamlcdmfm/">DHC chrome extension</a>:

{{< highlight http >}}
POST /upnp/control/insight1 HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Accept-Language: en-GB,en;q=0.8
Content-Type: text/xml
Origin: chrome-extension://aejoelaoggembcahagimdiliamlcdmfm
SOAPACTION: "urn:Belkin:service:insight:1#GetPower"
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.29 Safari/537.36

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/" s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
    <s:Body>
        <u:GetPower xmlns:u="urn:Belkin:service:insight:1"/>
    </s:Body>
</s:Envelope>
{{< / highlight >}}

The name of the action (in the SOAPACTION header) and the XML namespace in the body are both
constructed from information in the service definition. The result that comes back is:

{{< highlight xml >}}
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/" s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
    <s:Body>
        <u:GetPowerResponse xmlns:u="urn:Belkin:service:insight:1">
            <InstantPower>92170</InstantPower>
        </u:GetPowerResponse>
    </s:Body>
</s:Envelope>
{{< / highlight >}}

The current power draw in milliwatts is returned in the `<InstantPower>` argument.

### Putting the data to work

So now we have a way to pull the current power draw, a nice thing to do would be to plot a graph
of it to see the variation over time. This is the short of thing you'd expect to see in the Belkin
app, but it's sadly missing. After a bit of research I settled on the tried and true
<a href="http://oss.oetiker.ch/rrdtool/">rrdtool</a> to store data and generate graphs. I created a
new database to store the values from the two switches:

{{< highlight bash >}}
rrdtool create power.rrd \
    --start now \
    --step 60 \
    DS:wemoComputer:GAUGE:120:U:U \
    DS:wemoNetworking:GAUGE:120:U:U \
    RRA:AVERAGE:0.5:1:1440 \
    RRA:AVERAGE:0.5:10:1008 \
    RRA:AVERAGE:0.5:30:1488 \
    RRA:AVERAGE:0.5:120:1488 \
    RRA:AVERAGE:0.5:360:1488 \
    RRA:AVERAGE:0.5:1440:36500
{{< / highlight >}}

This creates a database file which expects values to be given every 60 seconds for two data series:
'wemoComputer' and 'wemoNetworking'. These are gauge types (i.e., a value we read off a gauge,
rather than a counter that keeps going up) with no minimum and maximum ('U').

It then defines a series of round-robin archives. Each of these stores a fixed number of entries,
and the oldest is overwritten when they become full. The interesting arguments are the step count
(second to last) and number of samples (last argument). The first one takes every sample and keeps
1,440 of them (i.e., a full day at 1-minute resolution); the last one has a step of 1,440 and keeps
36500 of them (i.e., 100 years at 1-day resolution). The use of RRAs ensures that rrdtool can retain
enough data to create historical graphs, while providing a fixed, finite maximum file size. With
sufficient RRA coverage you can graph most timescales and have data available at a reasonable
resolution.

Next, I created a small python script to retrieve the power using SOAP:

{{< highlight python >}}
#!/usr/bin/python3

import requests
from xml.etree import ElementTree

def get_power(ip_and_port):
    headers = {'Content-type': 'text/xml', 'SOAPACTION': '"urn:Belkin:service:insight:1#GetPower"'}
    payload = '''<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"
                             s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
                  <s:Body>
                   <u:GetPower xmlns:u="urn:Belkin:service:insight:1"/>
                  </s:Body>
                 </s:Envelope>'''
    r = requests.post("http://%s/upnp/control/insight1" % ip_and_port, headers=headers, data=payload)
    et = ElementTree.fromstring(r.text)
    return et.find('.//InstantPower').text
{{< / highlight >}}

I then have a dictionary of IP addresses to data series names, and the script polls each one in
turn and then executes an `rrdtool update` query to add the items to the database. I have this
script running every minute via cron.

{{% update 2016-05-06 %}}
After leaving my script running for a few days, it suddenly
stopped getting any data. It turns out the WeMo switches spontaneously change the ports they listen
on every now and then. To reliably query them, you need to perform an SSDP search on the network,
and get the correct address from the switches' response.
{{% / update %}}

After leaving the script to run for a bit and gather data, it's time to make some graphs. I use
the following to create a graph with a background gradient:

{{< highlight bash >}}
rrdtool graph desk-1d.png
        -o -X0 -w800 -h500 \
        -u 2000 -l 20 -r \
        DEF:raw=power.rrd:wemoComputer:AVERAGE \
        CDEF:power=raw,1000,/ \
        CDEF:powerz=power,2000,LT,power,2000,IF CDEF:powerzNoUnk=power,UN,0,powerz,IF AREA:powerzNoUnk#ff0000 \
        CDEF:powery=power,1500,LT,power,1500,IF CDEF:poweryNoUnk=power,UN,0,powery,IF AREA:poweryNoUnk#ff0000 \
        CDEF:powerx=power,1000,LT,power,1000,IF CDEF:powerxNoUnk=power,UN,0,powerx,IF AREA:powerxNoUnk#ff0000 \
        CDEF:powerw=power,900,LT,power,900,IF CDEF:powerwNoUnk=power,UN,0,powerw,IF AREA:powerwNoUnk#ff0000 \
        CDEF:powerv=power,800,LT,power,800,IF CDEF:powervNoUnk=power,UN,0,powerv,IF AREA:powervNoUnk#ff1b00 \
        CDEF:poweru=power,700,LT,power,700,IF CDEF:poweruNoUnk=power,UN,0,poweru,IF AREA:poweruNoUnk#ff4100 \
        CDEF:powert=power,600,LT,power,600,IF CDEF:powertNoUnk=power,UN,0,powert,IF AREA:powertNoUnk#ff6600 \
        CDEF:powers=power,400,LT,power,400,IF CDEF:powersNoUnk=power,UN,0,powers,IF AREA:powersNoUnk#ff8e00 \
        CDEF:powerr=power,200,LT,power,200,IF CDEF:powerrNoUnk=power,UN,0,powerr,IF AREA:powerrNoUnk#ffb500 \
        CDEF:powerq=power,180,LT,power,180,IF CDEF:powerqNoUnk=power,UN,0,powerq,IF AREA:powerqNoUnk#ffdb00 \
        CDEF:powerp=power,160,LT,power,160,IF CDEF:powerpNoUnk=power,UN,0,powerp,IF AREA:powerpNoUnk#fdff00 \
        CDEF:powero=power,140,LT,power,140,IF CDEF:poweroNoUnk=power,UN,0,powero,IF AREA:poweroNoUnk#d7ff00 \
        CDEF:powern=power,120,LT,power,120,IF CDEF:powernNoUnk=power,UN,0,powern,IF AREA:powernNoUnk#b0ff00 \
        CDEF:powerm=power,100,LT,power,100,IF CDEF:powermNoUnk=power,UN,0,powerm,IF AREA:powermNoUnk#8aff00 \
        CDEF:powerl=power,90,LT,power,90,IF CDEF:powerlNoUnk=power,UN,0,powerl,IF AREA:powerlNoUnk#65ff00 \
        CDEF:powerk=power,80,LT,power,80,IF CDEF:powerkNoUnk=power,UN,0,powerk,IF AREA:powerkNoUnk#3eff00 \
        CDEF:powerj=power,70,LT,power,70,IF CDEF:powerjNoUnk=power,UN,0,powerj,IF AREA:powerjNoUnk#17ff00 \
        CDEF:poweri=power,60,LT,power,60,IF CDEF:poweriNoUnk=power,UN,0,poweri,IF AREA:poweriNoUnk#00ff10 \
        CDEF:powerh=power,50,LT,power,50,IF CDEF:powerhNoUnk=power,UN,0,powerh,IF AREA:powerhNoUnk#00ff36 \
        CDEF:powerg=power,40,LT,power,40,IF CDEF:powergNoUnk=power,UN,0,powerg,IF AREA:powergNoUnk#00ff5c \
        CDEF:powerf=power,30,LT,power,30,IF CDEF:powerfNoUnk=power,UN,0,powerf,IF AREA:powerfNoUnk#00ff83 \
        CDEF:powere=power,20,LT,power,20,IF CDEF:powereNoUnk=power,UN,0,powere,IF AREA:powereNoUnk#00ffa8 \
        CDEF:powerd=power,0,LT,power,0,IF CDEF:powerdNoUnk=power,UN,0,powerd,IF AREA:powerdNoUnk#00ffd0 \
        LINE:power#080
{{< / highlight >}}

This seems a bit unweildy, but it's fairly straight forward. The options tell rrdtool to create
a graph with a canvas size of 800x500 pixels, a lower limit of 20W, upper limit of 2kW, and a
log scale. The first `CDEF` divides our raw value (which was in millwatts) by 1000 to get a value
in watts. rrdtool uses reverse polish notation like some calculators, so you provide the arguments
before the operator.

The big block of `CDEF`/`CDEF`/`AREA` parameters creates a series of area plots in different colours
according to the power level. They're in descending order so the smaller areas are drawn on top of
the larger layers. This results in a graph that looks like this:

<img src="/res/images/wemo/desk-1d.png" alt="Graph of power usage over a day">

This graph shows the total power for all the things plugged in at my desk. You can see the idle
power draw is around 60W. When I'm using the computer it jumps up to around 130W, and when the
computer is under heavy load (playing games, for example) it goes up even further to the 200W mark.
With a couple of small tweaks to the rrdtool command, I also have a graph showing the entire week:

<img src="/res/images/wemo/desk-1w.png" alt="Graph of power usage over a week">

The two huge spikes near the start of the data are caused by a heater under my desk. They're also
one of the main reasons I chose to plot the graphs with a logarithmic scale. With a linear scale
between 20W and 2kW, the graph would be completely dominated by these large spikes &mdash; in fact
the difference in height between the two spikes would take almost half the graph! The
relative difference between the normal values of 60-200W and the heater's value of 1.8kW isn't
actually that interesting, and certainly not worth using about 80% of the graph to demonstrate. The
log scale helps to compress this, and emphasises the difference in the smaller values more.
