Watts vs Volt-Amps - huh ??

**inetdog** · 07-12-2013, 03:30 PM

Originally posted by Sunking

Fluke is good but I suspect your clamp-on is a 600 Amp Version? They are not accurate at low current readings. Best accuracy if you want to spend a few coins is a SHUNT, then use th eFluke volt meter to read the voltage drop and convert to amps.

Ohm's Lab makes really good ones and there are dozens of other less expensive ones out there. Just make sure not to buy one any larger than you anticipate needing. Best accuracy is at or near full scale.

For very temporary use, just for testing, you could also increase the precision and the accuracy of a clamp-on ammeter by running several (N) turns of the DC wire through the clamp. This will make the meter reading N times the actual current.

Now I do not expect you to be doing this with your 1/0 or other large actual DC wiring, but just with temporary wiring that will support the inverter drain when powering just the loads you are testing. If you are going to modify the wiring though, just putting in the shunt temporarily will give you better accuracy.

**Robert1234** · 07-12-2013, 05:51 PM

Clamp on meter has dual scale, 400A and 40A. I was at the 40A level. Will look for the other meter as it is also dual scale. I have another in-line DC amp meter as well, but I'm not going to disconnect wires at this point. Had a wide open day.

Only got 108 ah thru the CC today - another rainy, overcast day in the southeastern US. VA calcs say system should be at 3% below target cutoff levels at 8 am. Watts calc say batts will only be half of target discharge. Setting the plant beds for constant flooded instead of drain-fill just in case the pumps go off.

---------------------------------

Just took another DC side reading for grins & giggles with pumps and lights on.

6.83 amp @ 26.9 v = 183.7 Watts

Previous reading was 183.9 Watts
(28.6v @ 6.43amp)

Like I say, precision is impressive. Question remains if it is accurate.

Used my other amp clamp as a back up. It's lowest scale is 200a. It read 6.7 as compared to the 6.83. I don't even know what brand that clamp is.

AC Volt amps are still ~190. AC Watts are 134.

**inetdog** · 07-12-2013, 07:06 PM

Originally posted by Robert1234

Previous reading was 183.9 Watts
(28.6v @ 6.43amp)

Like I say, precision is impressive. Question remains if it is accurate.

Used my other amp clamp as a back up. It's lowest scale is 200a. It read 6.7 as compared to the 6.83. I don't even know what brand that clamp is.

AC Volt amps are still ~190. AC Watts are 134.

Just as a VFD for a motor may be able to send excess power fed back by the motor during braking to the AC supply (or it may be forced to waste it as heat in an internal resistor), an inverter may be able to store internally the reactive power component of the output demanded of it. But it is not guaranteed. Some or all of the reactive power may be wasted as heat inside the inverter rather than being efficiently reused for the next part of the voltage cycle.

You are probably not looking at 100% efficiency converting 1834 watts to 190VA, nor are you looking just as a very low efficiency at converting 184 watts to 134 watts, but rather something in between.
(Although the latter is quite possible, since you do not know what the inverter drain is for zero (or very low) output. It is not unheard of for a 1000W inverter to have a no-load power drain of 50 watts.)

The efficiency of the inverter at any particular power output usually depends, to first approximation, on two terms. The first a constant no-load drain and the second a loss roughly proportional to the output power.

**bcroe** · 07-13-2013, 12:00 AM

Power Factor

I don't think anyone mentioned it yet, but PF for sine waves is just the simplest case. For those you
could add the real currents & reactive currents, and from the totals calculate a combined PF. But with
electronic stuff (switchers, or even rectifiers) the current waveform could be about anything. In theory
you could have one load with some current spikes & gaps, another with reversed pattern so the spikes
& gaps cancel each other, and get a PF of 1 (don't expect it to happen!). And caps aren't going to help
as much for non sine current waveforms.

The good news is, some equipment now uses switchers to get a much better power factor. Bruce Roe

**SunEagle** · 07-13-2013, 10:45 AM

kill a watt test

I did my own testing with a kill a watt P3 P4460 meter which I just purchased.

I used a 13 watt CLF, a desk top fan and a 20 watt LED lamp. All values were given by the kill a watt meter.

CLF - .59 PF, 0.18 A, 13.4 W, 122.4 V
Fan - .99 PF, 0.27 A, 34.1 W, 122.4 V
LED - .90 PF, 0.16 A, 18.6 W, 122.4 V
All three added together - 0.61 A, 66.1 W

All three measured together - .94 PF, 0.56 A, 65.1 W, 122.4 V

There was a very small difference between the added and measured values for all three items so it seem even though the CLF and LED lamps may have some type of electronic power supply it is not affecting the amperage vectors that much. Even the PF seems to be in line considering the CLF has the lowest but is also the smallest W load.

I need to find me a small 120v pump or inductive load to see how that affects the measurements. Interesting experiment.

**Naptown** · 07-13-2013, 11:04 AM

Look at the amps and watts on the CFL vs the LED
Amps were higher on the cfl but wattage was lower than the LED

**SunEagle** · 07-13-2013, 11:39 AM

Originally posted by Naptown

Look at the amps and watts on the CFL vs the LED
Amps were higher on the cfl but wattage was lower than the LED

I'm not sure how the kill a watt makes these measurements unless it takes into account the PF and then calculates the amperage. You would expect a higher va due to a lower PF.

**inetdog** · 07-13-2013, 02:47 PM

Originally posted by SunEagle

I'm not sure how the kill a watt makes these measurements unless it takes into account the PF and then calculates the amperage. You would expect a higher va due to a lower PF.

i believe that the amperage numbers from the KAW reflect magnitude only, so amps times volts == VA.
The watt figure is obtained from an actual wattage measurement (integrating instantaneous volts times amps), and the PF is derived from the ratio of the two. But based on that the PF for the fan should have been .97.
So something in the way the KAW gets the numbers is not quite right, at least at the 2% level. Now the amp value was only given to two significant digits, so the "real" value could be off by as much as +/-3%.

I agree that to see any effects of capacitive versus inductive reactance cancellation you will need to find a good inductive load.
Also, though, the CFL and LED will have significant distortion PF included in their measurement.

**SunEagle** · 07-13-2013, 03:04 PM

Originally posted by inetdog

i believe that the amperage numbers from the KAW reflect magnitude only, so amps times volts == VA.
The watt figure is obtained from an actual wattage measurement (integrating instantaneous volts times amps), and the PF is derived from the ratio of the two. But based on that the PF for the fan should have been .97.
So something in the way the KAW gets the numbers is not quite right, at least at the 2% level. Now the amp value was only given to two significant digits, so the "real" value could be off by as much as +/-3%.

I agree that to see any effects of capacitive versus inductive reactance cancellation you will need to find a good inductive load.
Also, though, the CFL and LED will have significant distortion PF included in their measurement.

I should have recorded the VA measurements as well to see how they compared to the other metrics. I will go back and check tomorrow but my voltage is a little high today so the next time all of my measurements may be different. I was surprised with the fan PF being 0.99. It did bounce a little between 0.98 and 1.0 but settled at 0.99.

The KAW is pretty neat but I am still learning about it. The listed accuracies run between 0.2 to 2% depending on which function it is measuring.

I am currently getting a profile of my small frig so I really know what it will take to keep it running on a battery system. It is drawing about 81 watts now but the compressor is not running so I want to get a 24 hour run on it. I am also checking on some 12v Pure Sine wave inverters since my cobra is a Modified Sine wave and will not really run the frig.

**inetdog** · 07-13-2013, 03:17 PM

Originally posted by SunEagle

I should have recorded the VA measurements as well to see how they compared to the other metrics. I will go back and check tomorrow but my voltage is a little high today so the next time all of my measurements may be different. I was surprised with the fan PF being 0.99. It did bounce a little between 0.98 and 1.0 but settled at 0.99.

The KAW is pretty neat but I am still learning about it. The listed accuracies run between 0.2 to 2% depending on which function it is measuring.

I am currently getting a profile of my small frig so I really know what it will take to keep it running on a battery system. It is drawing about 81 watts now but the compressor is not running so I want to get a 24 hour run on it. I am also checking on some 12v Pure Sine wave inverters since my cobra is a Modified Sine wave and will not really run the frig.

If it is drawing 81 watts without the compressor running, you are looking at an air circulating fan continuing to run or more likely door edge heaters that should be turned off. There may also be a heater that comes on only during the defrost cycle, which you should not try to alter.
Or maybe the light really does stay on when you close the door.

**SunEagle** · 07-13-2013, 03:27 PM

Originally posted by inetdog

If it is drawing 81 watts without the compressor running, you are looking at an air circulating fan continuing to run or more likely door edge heaters that should be turned off. There may also be a heater that comes on only during the defrost cycle, which you should not try to alter.
Or maybe the light really does stay on when you close the door.

Well I tried to climb in and close the door but I couldn't fit.

Actually the compressor is running when drawing the 81.7 watts adn 1.0 amps but it is very quite. I needed to put my hand on it to feel the vibration. It is one of those Majic Chef probably 4 cu foot. It has a nameplate rating of 120 watts & 1.5 amp at 120 V.

**inetdog** · 07-13-2013, 03:35 PM

Originally posted by SunEagle

Well I tried to climb in and close the door but I couldn't fit.

Either find a small kid

or put a small PV panel inside and run the wires out.

**Robert1234** · 07-13-2013, 09:55 PM

Well, inverter wasn't off this morning but it should have been. Battery bank was at 19.4 volts. I shut it down and put the tanks on "life support". System definitely is burning amp-hr based on volt amps. Now to adjust and see what effect it has.

**YYP123456** · 08-30-2013, 02:48 AM

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**Naptown** · 11-01-2013, 05:00 PM

I am brain dead today
I need to convert 14.1 amps at 208v three phase to watt hours.
Anybody know the formula?

Watts vs Volt-Amps - huh ??

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