The left half of the Duet is concerned with electricity. I want to focus on the "Speed Dial" on the top and the "Daily Counter" in the middle. The units can be £, kWh or Kg of CO2 - a click on the fascia buttons switches between the three modes.

Cold Hard Cash

As you will see in the first picture, the speed dial shows how "fast" you are using electricity. The more you are using, the more the dial lights up. A perfect metaphor for usage. There's an indicator in the middle of the dial which gives you a numeric representation of your speed

In the middle of the unit is a daily counter.  This shows how much you have used since midnight.  The above image shows monetary usage, this next image shows kWh usage - the actual amount of electricity.

kWh Used

Immediately two things are obvious - kWh are more precise and simultaneously harder to understand.

Perhaps the least useful of the three modes shows Kg of CO2 released due to energy usage.

Kg of Carbon Dioxide

Given that the duet doesn't know whether my electricity is being generated by coal burning or wind turbines, it's a slightly spurious figure.

As Joe Baguley points out in his article about the Humm Trio, understanding the read-out makes it easier to understand what's going on.

I now know my dishwasher costs 16p/cycle, compared to my washing machine at 7p, and that running it on the thermonuclear remove the spot-welded lasagne from the dish mode only takes it to 17p.

Money is highly conceptual.  You can't hold a kWh in your hand and most of us have never seen a Kg of CO2 - but we can all visualise the pennies trickling from our wallets.

The problem is that the monetary graph doesn't have enough precision in it; pennies are counted - not tenths of pennies.  Switching your oven on may generate a leap from 8p an hour to 50p an hour - but switching off your lights may only drop you from 8.8p to 8.1p - a change that you won't see on the dial.

I would make the following suggestions

• Where possible include as much prescision as possible.  There's space on the readout for fractions of pennies - make use of it.
• Allow the different readings to be displayed in separate units.  For example, the Speed Dial could be in kWh while the daily total could be in £.
• The Duet allows you to set your unit cost of electricity - perhaps it should allow you to set how green your tariff is to ensure the Kg CO2 count is correct?
• It's fascinating to watch the speedo jump when the oven switches on, or drop when the TV goes off - perhaps large events should be recorded on the SD card.  Currently it only has 30 minute precision.  Given a whole day of 30 minute readings takes ~50KB of a 2GB card - I wouldn't think it unreasonable to record either twich the amount of data or only where there have been significant changes in usage.

Overall, the speedo idea is neat - you don't need to understand everything that goes on to get a quick visualisation of how much energy you are using.