Low Cost Digital Watt/Joule Meter Design for 20V AC/DC, 20A, 1mJ–999999J, 1mW–400W

we will low cost design digital watt/joule meter with 20Volt AC/DC and 20AMP current input

Also added some specification in below
- meter is capable of measuring energies from 1mJ to 999999J or power from 1mW to 400W in a.c. or d.c. circuits.
-Two ranges selected by a switch labelled mJ/mW and J/W are available.
The auto-ranging six digit display will give direct readings of energy or power.
-The maximum current input is 20A.

Please let me know if any more idea.

Ho many of these are you going to sell?

Do we get a cut?

I you use a PIC, ARM or even a PIC basic controller and a display and current resistor and a time base.
And you program that Joule=Watt * seconds and Watt=Voltage * current.

It will take a 30 minutes max to program that incl. all display error and keyboard control.
The problem is that the hardware will cost you in single build more than 45USD and you can find these units build in China (by the large numbers) for 20USD.

If you want to build it for a study project then make the project interesting.
Add a graphic display and include a recording function in a way that you can show the change of voltage, current and watt in a analog time frame.
Add a user adjustable or event (current , voltage ) triggered timer.

I built a smart "Watt/Amp Hour Meter (Watt Meter)":http://arduinotronics.blogspot.com/2012/05/amp-hour-watt-hour-meter-working.html that can be used for ac or dc (depending on sensor). It's programmable, and can be used for load shedding, safety shutdowns, solar/wind charge control, and much more.