[RE-wrenches] power factor

[Brian Teitelbaum]

boB,

Not only does that make sense, it's the best explanation of the effect of power factor on an off-grid system that I've read.

I would than assume that this means that Drake's refrigerator is actually consuming more power from his battery system than is just measured in AC watts at the fridge. Some energy is lost as heat in the inverter system while the fridge is running, at least more than would be if the fridge had a higher power factor. I would also assume that measuring the DC amperage in the battery cables, and subtracting the "normal" inverter losses, would give a more accurate reading of the Fridge's true running watts.

There are a number of plug-in products on the market that are supposed to improve power factor, apparently through added capacitance. How would they work here, or would they?

Brian Teitelbaum
AEE Solar



-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of boB Gudgel
Sent: Wednesday, August 19, 2009 9:28 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] power factor

Drake Chamberlin wrote:
> One question about power factor:
>
> We know it is the ratio of the real power to the apparent power, and
> that most utility meters only register real power (watts).  Reactive
> power doesn't dissipate as heat like power loss through a resistor.
>
> The question is:  What is the quantitative effect that reactive power
> has on a battery bank that is feeding loads through an inverter?  If
> the power factor is 56% (like a refrigerator that I want to replace)
> does that mean that only 56% of the power being drawn from the
> batteries is being used to power the refrigerator?.  If so, where does
> the rest of the power go?

If you just take your meter and multiply Volts X Amps, that's VA, or
apparent power.   If it's all real, then it's like the resistor and gets
counted by
the utility meter.  Some turns into heat (resistor) and some gets some
real work done.

The rest, is VA but is Reactive and is  VA Reactive or VARs.

So, the reactive current that comes out of the batteries, into the
inverter out the AC output, comes BACK into the battery.  (this is for
an inverter
that has the ripple reflecting back into the battery)

So, your batter kind of discharges and then recharges 120 times per
second for a 60 Hz system.
So, where does the power go ?   The real power gets used of course and
most of the rest, the VARs gets put back into
the battery.   HOWEVER, since the transformer and wire and FETs and
transormer and stuff have resistance, some of those VARS get turned
into heat by that inefficiency, or I-squared-R losses.  That's one
reason why low power factor isn't so good.

Does that kind of make sense ?

boB

>
> Thanks,
>
> Drake Chamberlin
> Athens Electric
> OH License 44810
> CO License 3773
> NABCEP TM  Certified PV Installer
> Office - 740-448-7328
> Mobile - 740-856-9648
>
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