[ale] Monitoring Total Household Electricity Consumption

[Alex Carver]

On 2016-02-09 13:21, Greg Clifton wrote:
> GA Power has several rate plans available these days. The newest they are
> promoting is the so called "Smart Usage" rate plan. The off peak rates are
> even cheaper than the plug in electric vehicle plan, but the gotcha is that
> they charge $6.53/kWh for the peak 30 minutes of consumption  each month,
> both peak (June-Sept) and off peak.
> 
> The GA Power web site allows me to monitor daily usage, but does not
> provide the 30 minute granularity necessary to calculate if this rate would
> save or cost me $. Which prompted this email to see if anybody on the list
> has a suggestion of an affordable device or Rpi project that could log my
> power consumption on a minute by minute basis.
> 
> Thanks in advance,
> Greg Clifton


If they're monitoring during peak hours then they're likely going to
replace your meter with a smart meter in order to get that data
(otherwise it's all guessing).  Some of the available smart meters use
the Zigbee protocol and can be paired with in-house displays.

Otherwise there are plenty of RPi projects available to perform live
measurements.  I've looked at some of them but I would recommend, if you
really want good data, to get the proper transducers for measuring
voltage and current safely.  Many of the projects cobble together
various sense elements that aren't fully accurate or don't offer proper
galvanic isolation.  At minimum you want a current transformer for each
hot line and neutral[1] (ideally not a clamp type but one that would
require you to remove and thread the wire through) and a voltage
transformer for each hot-neutral pair.

Proper transducers would be like this:
http://www.omega.com/pptst/MFO_RCT.html
http://www.omega.com/pptst/OM9.html

Then use a few ADCs like this one:
https://www.adafruit.com/products/1083

The ADCs would read the data from the current and voltage transducers
(make sure all transducers put out a DC voltage).  The RPi can read this
ADC using the I2C/SPI interface which reduces noise since the ADC can be
placed close to the transducer and only a serial data bus  goes back to
the RPi.

You have to measure voltage and current separately and then measure the
phase angle between them to get Watts and VArs (Volt-Amp reactive).
Utility meters only measure Watts, VArs are ignored for billing purposes
but it's a good thing to know because it can indicate some bad
electronics plugged into your system.

[1] You want to measure neutral in order to detect a current imbalance.
 For a residential split-phase (240VAC) system, the vector sum of the
two hots should equal the vector neutral (vector meaning direction
matters).  If not, some current is leaking straight to ground bypassing
the neutral and you should go find it.