watt meter for DC

4 Posts
May 6, 2008 03:05 am
watt meter for DC

i'm starting a full 12V DC solar system and looking for a 12V DC power meter.  seems the kill-a-watt, only works with 120AC.  any ideas?
                    thanks
« Last Edit: May 6, 2008 03:10 am by ryan libre »
 
May 6, 2008 04:59 am
Re: 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/
 
4 Posts
May 6, 2008 05:22 am
Re: watt meter for DC

i'll keep that in mind for the future,  right now my panel and battery cost only 400$. i just run my lights, laptop, fan, and battery charger now.  (off grid) so i think i'll hold off on a 200$ meter for it.    do you or anybody know of a simpler/cheaper device? 
                    thanks
 
184 Posts
May 6, 2008 09:36 am
Re: watt meter for DC

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
 
 
4 Posts
May 6, 2008 10:11 am
Re: watt meter for DC

thanks, 
     i'll have to play around with that till i fully understand it,  but i think that was the answer i was looking for. 
    but does that mean a simple DC watt meter does not exist?
 
18 Posts
May 6, 2008 11:08 am
Re: watt meter for DC

I know what you need.  Get a Doc Wattson at www.powerwerx.com for about $60.  This meter measures volts, amps, Watts, and total kilo-Watts, highest Wattage, total amp hours, the minimum voltage.  You can read the doc's but it will keep track of your total Wattage by tracking voltage and amps.  I use 2 to keep track of power coming in through wind and solar.  I wanted to know how much from each.  They also have a Watts-Up meter but it has lower Total Watt before it rolls over.  Careful if you use one between battery and inverter as Doc-Wattson will max out at 100 amps.
 
184 Posts
May 6, 2008 02:49 pm
Re: watt meter for DC

thanks, 
     i'll have to play around with that till i fully understand it,  but i think that was the answer i was looking for. 
    but does that mean a simple DC watt meter does not exist?

Measuring current is not as easy as measuring voltage because current flows through a circuit, while voltage is measured across a voltage source or a component.  If you get a good quality digital voltmeter, and a precision resistor, you'll get fairly accurate results.  I'm not sure about the accuracy of other types of devices mentioned here.  But on the other hand, being able to measure watt-hours is a good feature, and something you'll probably want eventually.
 
May 7, 2008 04:53 am
Re: watt meter for DC

Thats the "rub" isn't it?
I mean we ether pay a lot for a lot or a little for a little. Ether way, we get what we pay for. Still, there are those rare occasions when we pay a little and get a lot.
When I started out it was with a relatively small system. 154 watts of PV, Trace C-30-A charge controller, and three 8-D "18 wheeler cranking batteries."
Dim lights - batteries low.
C-30-A clicking rapidly - batteries charged.

When the "future" arrived I was astonished by the TriMetric and wondered why I didn't get it sooner. The enchantment has worn off some but its still right there, in the wall by the front door under the indoor/outdoor digital thermometer telling what I need to know.

Its a wild feeling, when the TriMetric is set to display amps in/out and there are clouds coming and going. If I watch the amps in, going up and down, my peripheral vision is picking up the changing light levels from outside and it feels like I am in motion.

I am not trying to sell you a TriMetric, but if I had it to do all over again, I would have made some kind of monitor as much of a priority as the PV modules. Those first three batteries would have lasted longer.

Trojan T-105's are expected to last 5 to 7 seven years, (if sized and treated right) mine are still working on their eighth year. But I will never buy anymore T-105's. They have been good batteries but for one thing, the terminal posts break off way to easy. Sorry Trojan, but for that reason alone I am going to try Surrettes next.

God its great living in a country founded on Christianity and Democracy.
When asked, "Whose name is on that coin?" the reply was, "Caesar's"
"Then you should give to Caesar that which is Ceaser's and give to God that which is rightfully Gods, your eternal soul."
These words, or something to the effect of, were spoken nearly 2,000 years ago. During a time when mankids biggest worries wasnt energy, but what happens to our eternal soul. There is no reason why they shouldn't still be true today. Of course it would be Washington in place of Caesar. Washington being synonymous with the nations capital. There is a reason why we do not see lugage racks on hearse's.
 
May 7, 2008 11:32 pm
Re: watt meter for DC

I know what you need.  Get a Doc Wattson at www.powerwerx.com for about $60.

Here is the link to the manufacturer's website:
www.rc-electronics-usa.com

They have two models, you can compare the specs here:
Model R102 - "Doc Wattson":
http://www.rc-electronics-usa.com/ammeters/r102-amp-hour-specs.html

Model WU100 - "Watt's Up":
http://www.rc-electronics-usa.com/ammeters/amp-meter-specs.html

If your battery capacity is less than 65Ah, you will probably be fine with the WU100.  I'm considering getting one for my small 12V stuff.  I think the TriMetric is much better off for keeping close track of your batteries though.  I don't believe these units above will give you the "Net" usage on your batteries, but they will give you the basic stats like watts, volts, amps etc of your DC loads.

 
18 Posts
May 12, 2008 11:13 am
Re: watt meter for DC

A Trimeteric IS for keeping track of the "state of charge" of your batteries.  You kind of need one if you have a bigger system.  The cost is justified (for a Link or Trimetric) for a larger system (with more then one battery).  My larger system has 8 Trojan L16H batteries (cost over $2000) so I have a Link 10 watching them.  A smaller system I just completed (portable - (hand cart) with 2 T-125's = $360) only has the Watt's Up meter.  The Watt hour function is accurate for all rates because it multiplies voltage by amperage for a moment by moment tally (100 Watts @ 12v = 100 Watts @ 120v.  I put my meter between the charge controller and the battery to keep track of charging current and total charge.  At the other end I plugged in a Watts Up KWH meter to keep track of inverter usage.  Last thing if you use 100 Watt hours put 120-130 Watt hours back in (80% batt efficiency).
 
4 Posts
May 13, 2008 04:55 am
Re: watt meter for DC

thanks everyone,  that was a good range of answers.
 
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