Newbie has an idea: A watt meter for many uses

I am new to Arduino and electronics in general.
I found Arduino to be a good starting point for projects.

I was recently faced with the task of measuring DC power in a variety of applications.
So I thought I would combine my need with my desire to build something on my own as a hobby.
So I wanted to ask if it is possible to build a simple customisable watt-metter, that can be used for a variety of DC applications, with customisable units, for example: W, W/h, average Watts per device (having some sort of input for the number of devices on the "software" that can make a simple division), etc.

My idea was to be hand-held, so I was actually thinking of a very simple device maybe like a simplified handheld multimeter.
So necessary functional parts are I guess the Arduino, a screen and some buttons!!! Components, I hope you will help me form some basic idea, about what is needed!!!

Sorry if I posted in the wrong place, and sorry for my poor knowledge in electronics!!!

If you have seen or built anything similar, any help or ideas will be much appreciated!

Yes there are many such projects, do a search of the forum.

There are lots of projects like this:-
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1275670716/7

or this
http://community.pachube.com/node/395

The most difficult part is the current sensing especially if high voltages are involved. This can involve breaking the circuit so that current flows through a shunt, or if the current is high a hall probe will allow a non intrusive measurement.

I built a simple voltmeter that worked really well using a voltage divider but as for amperage that's not so easy I tried a high side low ohm resister in series and a op-amp to measure the current but didn't have much luck with it and ended up moving house and never returned to the project but please let me know how you get on as I would like to build it into my bench power supply it would be so cool.

My research so far:

  1. Measuring the voltage by driving through a simple voltage divider directly on the Arduino, like in the lipo monitor project:
    http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1275670716/7

  2. Measuring the current using a Hall effect transducer on the cables coming from the source

I am getting my study face on, since I am reading a lot of stuff online.
Sources:
http://wattsview.com/
http://scienceshareware.com/bg-current-monitoring.htm

I also researched using a Shunt resistor, but I am not fully understanding that, so more study will follow.
Source: How to measure current and power using a current sense resistor

Hall effect seems to be a good option, but I like to explore all options.
Also I am not sure how to interface the Hall thingy!!!

I have found some very usefull components on sparkfun:

Sorry for my ignorance, but what does "breakout" mean?
I mean, is the board complete and ready or not?

Are the components I found suitable for my application?
I guess measuring voltage through a simple divider and current through a hall transducer, will give me the ability to write software that will make V*I and show Watts.

What are your suggestions?

When I reach the point of having chosen components, I guess I will bother you again asking about how to "write" the code!!! :-/

P.S.: One very good page with several application on measurements:
http://scienceshareware.com/how-to-measure-AC-DC-current-with-a-hall-effect-clamp-.htm
only problem is they use some ready board to read hall sensor. i prefer building something myself if necessary.

but what does "breakout" mean?

It is generally used to indicate that a component with a small surface mounting foot print has been mounted on a very small PCB with large solder pads so you can wire it into your own application without having to cope with the surface mount aspects. Sometimes it includes a few extra components like a voltage regulator, capacitors and such but mainly it is just easy access to the component's electrical connections.

Also I am not sure how to interface the Hall thingy!

Normally they have a voltage output which you measure with the analogue input of the arduino. However some have digital interfaces normally SPI so you write code to get at the reading.