Volts X Amps = Whats?

Desert Rat
Desert Rat Solar Expert Posts: 147 ✭✭✭
in Off Grid Solar & Battery Systems #1
That's a joke, not a misspelling.;)
I was checking a TV/DVD player for a phantom load with my Kill A Watt meter and was surprised by the readings. One button toggles between Watts and VA which I thought should be the same, but are not. I checked several other loads, and some showed the Watts and VA to be the same, while with others the two readings were quite different. The Kill A Watt manual says that Watts is active power and VA is the apparent power. What does this mean?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Volts X Amps = Whats?

    Look up "Power Factor".
    That's the difference. Induction loads (things with transformers/windings) can have very poor PF whereas resistance loads don't.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #3
    Re: Volts X Amps = Whats?

    What you're seeing is the effects of PF (Power Factor). Some loads, mainly inductive such as electric motors, can load a power supply quite differently than an old fashioned light bulb, or a resistance electric heater. With the latter two, the max current flow will be at the point of max voltage. Not so with uncorrected inductive or capacitive loads. With the inductive load, the max current flow lags the peak voltage flow. Thus, although the amperage flow may be large, it occurs after the peak voltage, thus at a lower voltage, thus lower wattage per amp, than with the electric heater. With a capacitive load, the max current flow LEADS the max voltage, so again, since the two peaks do not line up, the actual wattage is not what you might think. PF correction can be accomplished by adding for example, a proper sized capacitor across the terminals of an induction motor. One leads, the other lags, and properly combined they cancel each other out, greatly improving the PF of the load.
    Hope this helps. :D
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #4
    Re: Volts X Amps = Whats?
    Desert Rat wrote: »
    That's a joke, not a misspelling.;)
    I was checking a TV/DVD player for a phantom load with my Kill A Watt meter and was surprised by the readings. One button toggles between Watts and VA which I thought should be the same, but are not. I checked several other loads, and some showed the Watts and VA to be the same, while with others the two readings were quite different. The Kill A Watt manual says that Watts is active power and VA is the apparent power. What does this mean?

    In addition to motors and other inductive loads, low power factor can result from non-linear loads (like power supplies using DC-to-DC conversion, like the one in your TV, computer, etc.) These tend to draw current only when the AC waveform is very close to the positive and negative peaks, with no current the rest of the time.

    Any time the current is not directly proportional to voltage on an instantaneous basis, the calculated or measured power factor will be less than one. In the case of a non-linear load, the inverter will have to deliver a peak current which is much higher than the peak current to get the same power from a linear load.
    SMA SB 3000, old BP panels.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Volts X Amps = Whats?

    Very simply--For the AC (120/240/230 VAC side), use V*A (VA or volt amps) for your planning (wiring size, inverter sizing, transformer sizing, load sizing--For inverter, most assume maximum rating is Watts=VA).

    For the DC side (including inverter input) think Watts:
    • V*A*Power Factor*1/inverter efficiency = Vdc-in*Idc-in = inverter DC input power requirements

    More or less. The battery will "see Watts" and the Inverter will "see VA".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #6
    Re: Volts X Amps = Whats?

    Let's get practical. Two examples:

    Your Kill A Watt tells you that your loads are 300 watts. You buy a 300 watt inverter. You find that your inverter cannot supply the loads because the VA of those loads is 400.

    The PF of battery chargers (or the charger mode of an inverter/charger) has been a vexing problem for many of our forum members with small generators. You may find that your 1600 watt Honda generator cannot push 1000 watts into your battery because the battery charger has a low PF. If you put your Kill a Watt on the generator it shows that the charger is drawing 1600 VA, but only 1000 watts.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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