How to measure watt draw of DC components

mjp24coho
mjp24coho Solar Expert Posts: 104 ✭✭✭
in Off Grid Solar & Battery Systems #1
I'm getting ready to wire my satellite internet system to draw off the DC side of my system (31 V). I have a device to measure the wattage draw of my AC components, but how can I measure the wattage draw of my DC devices?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: How to measure watt draw of DC components

    There are quite a few small Doc Watson type DC AH/WH meters. Good for upwards of 20 amps (is rated for more current--But I would watch the wiring and internal shunt resistor that they don't overheat).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #3
    Re: How to measure watt draw of DC components
    mjp24coho wrote: »
    ... how can I measure the wattage draw of my DC devices?
    A lot more easily than you can for AC devices, since you do not need to take into account any phase shift between current and voltage waveforms (power factor) which can cause the product of the RMS voltage and the RMS current to give you an incorrect answer. (Non-linear loads can also cause a low power factor, just not as easily described and modeled.)
    If the voltage is reasonably constant, you can just display or record the current and multiply by the nominal voltage to get a good enough idea of the power consumption.

    However, FWIW, it is more expensive to measure the current in a DC circuit without using a shunt, since a DC clamp-on ammeter has to use Hall effect technology instead of the simple transformer technology usable for AC.
    SMA SB 3000, old BP panels.
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  • Plowman
    Plowman Solar Expert Posts: 203 ✭✭✭✭✭
    #4
    Re: How to measure watt draw of DC components
    BB. wrote: »
    There are quite a few small Doc Watson type DC AH/WH meters. Good for upwards of 20 amps (is rated for more current--But I would watch the wiring and internal shunt resistor that they don't overheat).

    -Bill
    I used a Watts Up meter between my charge controller and battery bank for a while, but took it out a couple weeks ago when I noticed it was heating up pretty good. I was definitely maxing it out. I plan on replacing it with a Trimetric, but the little Watts Up will still be good to measure lower amp stuff.

    The Watts Up appears to be quite accurate, gives almost exactly the same reading as my Extech clamp meter. I put Powerpole connectors on it to make it easy to move around. It's been really handy.
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  • solorone
    solorone Solar Expert Posts: 257 ✭✭✭
    #5
    Re: How to measure watt draw of DC components

    I could not function without a decent clamp meter. I know Flukes are $ but you can find them on EBay. I have a couple of 36s, a must have, darn good diagnostic tool. It is 600 A, and I happen to see it spike at max one time on a 1986 Trace 1500 W @ 12v. The reading was only there for a nano sec.
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