Normal inverter power consumption

mshahmshah Registered Users Posts: 2
I just finish setup a small offgrid solar system. I connect 12v battery to china brand inverter(it says 1500w but i believe its a low end inverter). In between, i put dc voltmeter to monitor the power consumed from the battery. When I turn on the inverter without load, the voltmeter shows 10-12w consumed. But when i connect 20w load(lamp) the meter reading hike to 53w before slowly down and stop at 45w.
As for me the beginner, i not sure if this is normal. Because in my calculation it should be 10+20=30w. Is the inverter consume more power when load connected on top of the initial(10w) power it consumed? Any idea and helps very much apreciated. Thanks

Comments

  • BB.BB. Super Moderators, Administrators Posts: 28,467 admin
    Can you tell us a bit about what you meter(s) are using to measure the AC and DC power/watts/VA/amps/etc.?

    In general, if you measurements are accurate to within 5%, you are doing pretty good. Even to within 10% accuracy is not bad...

    And to measure True "Power" (Watts), you need a "power meter" (that measures both RMS Volts and RMS Amps, and the phase between the two) at the same time.

    Accurately measuring AC and Pulsing DC current (and Watts) is not easy (or cheap).

    To get better, you pretty much need lab grade instrumentation.

    Add the issues between TSW and MSW (true/pure sine wave vs modified square/sine wave) inverters, and using "simple/cheap" peak reading meters and true RMS reading meters--Can make it difficult to equate readings of power between the AC and DC sides of your inverter. 

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novakbill von novak Solar Expert Posts: 822 ✭✭✭✭
    mshah said:
    I just finish setup a small offgrid solar system. I connect 12v battery to china brand inverter(it says 1500w but i believe its a low end inverter). In between, i put dc voltmeter to monitor the power consumed from the battery. When I turn on the inverter without load, the voltmeter shows 10-12w consumed. But when i connect 20w load(lamp) the meter reading hike to 53w before slowly down and stop at 45w.
    As for me the beginner, i not sure if this is normal. Because in my calculation it should be 10+20=30w. Is the inverter consume more power when load connected on top of the initial(10w) power it consumed? Any idea and helps very much apreciated. Thanks
    This is normal.  High power inverters are very inefficient when run at low powers.

    How are you measuring current?
  • ThomThom Solar Expert Posts: 185 ✭✭✭
    Love my 300w inverter. 
    Off grid since 1984. 430w of panel, 300w suresine , 4 gc batteries 12v system, Rogue mpt3024 charge controller , air breeze windmill, Mikita 2400w generator
  • stmoloudstmoloud Registered Users Posts: 86 ✭✭
    edited December 2018 #5
    The inverter will have a self-consumption of watts. A 1500w will consume more than say, a 300w inverter. So that difference of 23 watt you are measuring could be the intermittent expense of what it will cost you to power a 1500w inverter. 
    760W panel array, 4 x 6v 220 ah Crown batteries, Tristar TS-45 PWM controller,  no name 600 PSW inverter. 
  • mshahmshah Registered Users Posts: 2
    edited December 2018 #6
    Thanks for the reply. I am using this watt meter to measure the current

    https://www.ebay.com.my/itm/LCD-DC-Combo-Meter-12V-24V-36V-48V-Voltage-Current-KWh-Watt-Car-Battery-Monitor/252795007261?hash=item3adbc1b11d:m:mS_b9zui8fMxoo-3l_EWh5g:rk:5:pf:0

    And I am planning to measure the ac current coming out from the inverter also so I can know the power consumed by the load

  • BB.BB. Super Moderators, Administrators Posts: 28,467 admin
    edited December 2018 #7
    Meters and Power Meters are a tricky piece of gear... There are two major ways to measure "variable" waveforms (sine wave, square wave, pulse wave, arbitrary wave, etc.).

    I believe that this guy is probably a "peak reading" meter... It measures the peak voltage/current/etc., and either displays that, or for AC readings, uses the RMS Average constant for a sine wave (basically divide by the sqrt(2)).

    Which leads us to "true RMS" reading meters (root mean square). Basically, these meters take something like 50,000 samples per second, and "run the math" to figure out the root mean square value of the waveform:

    https://en.wikipedia.org/wiki/Multimeter
    https://en.wikipedia.org/wiki/Root_mean_square

    For example, the DC input current for a single phase AC inverter (true sine wave) is (roughly) a Sine Squared wave form (looks like a 120 Hz rounded pulse train).

    And power power measurements, an AC power meter needs to measure the phase angle between Voltage and Current (where Voltage and Current are RMS measured values). Power = Voltage * Current * Cosine (phase angle between voltage and current) = Voltage * Current * Power_Factor (PF is 0.0 to 1.0 value similar to Cos(x) but for non-sine wave shapes).

    More or less, Vrms = Vdc (i.e., the RMS value is the same as the pure DC version of the value--I.e., 120 Vrms = 120 VDC).

    While Vrms*sqr(2) = Vpeak = 120 V * srt(2) = 169.7 Volts Peak of the sign wave (really +/- 169.7 volts peak around zero volts).

    https://en.wikipedia.org/wiki/AC_power

    Anyway, just to give you an idea of the difficulties in doing accurate voltage/current/power measurements. And to not get too wrapped up into trying to make very accurate measurements (i.e., accept ~10% accuracy).

    At this level of need--Just accept the meter (AC, DC, etc.) as roughly accurate and move on. You may end up with some inconsistent results (i.e., specs say Tare losses for inverter are 15 volts, and you may read 16-20 Watts tare losses with your panel meter, or you may measure 15 Watts or even 13 Watts... Just cannot tell without comparing with known standards/meters).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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