Power factors

JoshKJoshK Solar Expert Posts: 232 ✭✭
So I used a Kill-A-Watt meter to record the Watts my stuff uses, but it has occured to me since then I should have recorded VA because that would include the power factor loss. Is that correct that the most important reading is the VA, not the Watts, when sizing a battery supply?


  • BB.BB. Super Moderators, Administrators Posts: 30,206 admin
    Interestingly-- VA (AC RMS voltage and current measurements) is important for sizing wiring, transformers, AC Inverter, Generator "Head" (the alternator), and circuit breakers+fuses.

    VA is not really important for sizing the battery bank. The actual power draw from the DC Battery bus really is driven (for the most part) by AC Watts (VA*PF=Watts).

    VA combines both "True Power" (Watts) and purely "Reactive" Power (VA-Reactive) into something called Apparent Power (VA).

    • REVIEW:
    • Power dissipated by a load is referred to as true power. True power is symbolized by the letter P and is measured in the unit of Watts (W).
    • Power merely absorbed and returned in load due to its reactive properties is referred to as reactive power. Reactive power is symbolized by the letter Q and is measured in the unit of Volt-Amps-Reactive (VAR).
    • Total power in an AC circuit, both dissipated and absorbed/returned is referred to as apparent power. Apparent power is symbolized by the letter S and is measured in the unit of Volt-Amps (VA).
    • These three types of power are trigonometrically related to one another. In a right triangle, P = adjacent length, Q = opposite length, and S = hypotenuse length. The opposite angle is equal to the circuit's impedance (Z) phase angle.

    All of the "math" associated with AC Power is actually a very complex subject. It turns out that the "Reactive Power" does no "work", therefore does not draw any energy from the battery bank (other than various "losses" in the circuitry). Sort of like a swinging pendulum, you see it moving back and forth, but it does not take any energy to make the motion once you get it started (other than friction losses).

    Connect the pendulum to a spoon to mix some liquid--Then you have to add energy to the pendulum to mix the liquid--Or it will quickly stop. That energy to mix the liquid is the "power" from the battery bank.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MarkCMarkC Solar Expert Posts: 188 ✭✭✭
    Thanks again BB - this somewhat confusing aspect of AC circuitry is of much interest to me as a user of UPS type equipment for off-grid use. My "older" UPS (an APC SURT model) has quite poor power factor ratings, but OK efficiency, so It works well as a Priups generator. My newer APC SMX UPS has high power factor and very high efficiency (98% at loads) -so I'm hoping it will work well for "off-grid" service (actually emergency hurricane service). I have had the same question about the Power factor - does it actually affect the DC battery draw - appears efficiency is the key to the battery calcs. I realize that using these UPSs in continuous service is not their intent, but I'm not using more than 60% loads for any continuous service.
    Thanks again for an understandable explanation.
    3850 watts - 14 - 275SW SolarWorld Panels, 4000 TL-US SMA Sunny Boy Grid tied inverter.  2760 Watts - 8 - 345XL Solar World Panels, 3000 TL-US SMA Sunny Boy GT inverter.   3000 watts SMA/SPS power.  PV "switchable" to MidNite Classic 250ks based charging of Golf cart + spare battery array of 8 - 155 AH 12V Trojans with an  APC SMT3000 - 48 volt DC=>120 Volt AC inverter for emergency off-grid.   Also, "PriUPS" backup generator with APC SURT6000/SURT003  => 192 volt DC/240 volt split phase AC inverter.  
  • JoshKJoshK Solar Expert Posts: 232 ✭✭
    Thanks for clearing up the battery confusion Bill... You said it matters for sizing a generator though, so does it matter when sizing a Solar array? Sorry... the answer created a new question :)
  • BB.BB. Super Moderators, Administrators Posts: 30,206 admin
    For sizing a genset... It affects the wiring and the alternator head, but not the fuel powered motor... I.e., larger currents (VA vs VA*PF) because of poor power factor of loads needs heavier copper wiring and iron/steel pole pieces in the alternator to avoid overheating/saturation (too strong of magnetic fields) of the alternator components.

    Remember that one of major source of losses is I2R (=Watts) heating--Since the Amperage is higher in poor PF loads, the I2 heating is higher too (2x more current is 4x more waste heat).

    The Alternator provides the "isolation" of high VA loads between the electrical side of the alternator and the "power" (Watts) side of the engine power needed to turn the alternator shaft.

    The AC inverter provides the "isolation" between the AC and DC power systems... Since the solar panels are on the "DC" side of the inverter, they do not have to be sized for power VA (Power Factor) loads.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JoshKJoshK Solar Expert Posts: 232 ✭✭
    Thanks Bill.
  • VicVic Solar Expert Posts: 2,987 ✭✭✭✭
    And, as an aside, my old Kill-a- Watt device has "PF" as an alternate push of the "HZ / PF" button ... ie, it will directly measure PF, with unknown accuracy. FWIW, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • JoshKJoshK Solar Expert Posts: 232 ✭✭
    Yep, I have been using that. Thanks for sharing for others though.
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