On Sun, 15 Nov 2020 12:35:06 +0000, Roger Mills wrote:

You can buy energy monitors which either use a clamp sensor round one of
the meter tails or count flashes on new-type meters. But they only
appear to give instantaneous readings and daily totals.

The CurrentCost one http://www.currentcost.com/products.html has a
TTL serial output from the display unit. You can either interface to
a serial port (be that RS232, 5 V, 3.3 V, etc) yourself or get a TTL
to USB serial convertor. The sensor is a clip on current transformer
to a 3 x D cell battery powered unit that links to the display. Needs
a bit of care fitting around one tail but it's capable or being
pretty accurate. The data stream is XML and easy to decode,
containing instantaneiouos power readings and history.

I have the TTL to USB convertor plugged into a HP Microserver and a
Perl script that logs the instananeous min/max and calculates the
average power level over 5 minute intervals. Bit of PHP and GNUPlot
produces a web page for any given day.

The 5 min plots can spot things like SHMBO'd forgetting to switch the
iron off or energy wasting gadgets like the "keep warm" kettle or
filter coffee maker stewing the coffee...

Does anyone know of anything more granular - preferable with the ability
to capture the data and insert it into a spreadsheet? Sounds maybe like
a Raspberry Pi application?

A Pi Zero is perfectly capable of the data logging and web page stuff
using the XML. Would need to convert the TTL from a CurrentCost to
3.3 V Pi GPIO levels. It would be trickier to to interface directly
with a suitable current transformer as Pi's don't have ADC's.
Aurdino's do, so a bit of signal conditioning from the CT and away
you go.

--
Cheers
Dave.