watt meter for DC

Particularly if the 12 vdc solar system you are planing to create is charging a battery bank off grid? A wise investment for the "here and now" as well as the "long haul" is a meter such as the Bogart Trimetric.
It does not display wattage but it does display amp hours in to and out of the battery on a daily basis as well as an accumulative amp hours out. Minimum and maximum voltages. A little math and you can have your wattages.
I have been utilizing one for more than eight years.
If you consider this, don't just look at the "brochure" and price, download the manual and read through it. It will give you more detailed info about what you can accomplish.

http://www.bogartengineering.com/

You'll need a shunt and a digital voltmeter.  A shunt is simply a precision, low-resistance, high power resistor. You'll use ohms law to determine current and power.  Let's say that you have a 0.01 ohm resistor rated at 100 watts. If you want to know how much power your batteries are delivering, put the resistor in series with your batteries.  Let's say, that with a load attached, you measure 0.3 volts across the resistor.  Apply ohms law:  I = e/r.  I = 0.3/0.01 = 30 amps.  Next measure the voltage across the load.  It will be about 12 volts.  Since current flow is the same in all parts of a series circuit, you have 30 amps at 12 volts across the load.  Apply ohms law again.  This time:  P = EI.  P = 12 times 30 = 360 watts. 

You can put the shunt resistor in series with the PV panels to determine their output in the same way.  So there you go, a low-cost substitute for a TriMetric.  You should be able to buy a shunt from the alt energy store, or else an electronics outlet such as www.digikey.com.