Solar grid tie inverter night time loss

[PaulMel]

I have three grid tie inverters connected to my grid thru standard AC plugs. The system pushes about 1kw to the grid during the day but during the night they draw a total of 45 watts from the grid. This has been confirmed by measuring the ac current in the ac cord with a multimeter and with a current transformer. I even placed a 5 watt resistor in series in the line to see if it would get hot - it did.
Now the strange part. I have two of those common plug in the wall "Energy monitors". One shows the approximate 45w, the other shows 0w.
I did a test with the mechanical kwh meter mounted on the house to see if it was registering the 45w.
I turned all loads off in the house thru the breaker panel except for the breaker with the inverters on it. Then watched the dial on the meter for 20 minutes. It did not move. So I connected a 60w bulb (on an extension cord ) and stood by the meter and it turned about one full turn in 10 minutes.
So,the inverters are drawing power from the grid at night but don't appear to be affecting the power companies kwh meter.
I work in electronics every day but can't understand this, can you explain what's happening?

 

[nsaspook]

I suspect most of the 'power' used by the idle inverters is reactive with a low power factor. Most house meters won't charge you for reactive power use.

http://www.nlcpr.com/Deceptions1.php

 

[Windadct]

If the GTI is "rectifying" the Utility AC - you may be loosing the 45W in the inverter, they have bleed down resistors. The current on the AC line may be very non-linear, basically just a little current pulse at the peak of the AC Waveform. This current pulse may be so fast that the electrical mechanical meter does not move at all. You could test this by disconnecting the panels at night. But solar panels will back radiate energy at night - but proper installs have a blocking diode - usually for each panel, to prevent this, so you can also try your resistor trick in the DC string to check - but my money is on the inverter.
Also note - if the house is already consuming power - and the disc is already rotating, it MAY be able to capture the pulse and the 45W - for example repeat your experiment WITH the 90W bullb AND the GTI breaker on - it may rotate 50% faster.

 

[PaulMel]

Thanks very much. I thought it may be the power factor but I expected to increase the power measured not be ignored.
My energy monitor does not measure the power factor.
That looks like a great informative link you added, I look forward to reading it, thanks.

 

[PaulMel]

Does anyone know how the new digital kwh meters react to reactive power?
It is nice to be able to turn my meter basckwards but with the new digital ones I don't know how the reverse current flow will be measured.

 

[Windadct]

Some meters can and some can not - reactive power will not turn it backwards, think of reactive power as current - but no real power delivered to you.
Since this is a solar application - do you have an agreement with your utility that they have to buy back your surplus solar energy? If so then their meter should run backwards. However - now that I think about it - since the rate they buy it back is different then the rate they sell it yo you - you would need a digital meter - so that it keeps 2 totals - for each direction of power flow.

 

[PaulMel]

I'm not producing an excess of power to benifit from a "Netmetering" arrangement. The meter does not turn backwards more than it turns forward. It turns backwards quickly during sunny days but then at night and dark cloudy days it turns more forward . In the summer I almost cut my power bill to zero but it's up during the winter. My goal is just to break even.

 

[nsaspook]

If this is a guerrilla install it better be to exact code if you have an electrical fire and expect your insurance company to pay (they still might not). Grid-tie converters connected to standard house AC outlets are very dangerous as they can easily overheat existing wiring when being feed from several sources. The only safe way to wire them (backfeed) is directly to the correctly sized main house panel buss bar with properly sized circuit breakers.

 

[PaulMel]

Yes. Thanks to the internet I've read a lot of details of how to "properly" install GTIs directly to the panel using the 220v connections. My three inverters only put out 120v @ 4.5A each for a total of 1620 max. I have only measured a max power of 1400w over a year period. They are connected to a dedicated AC receptical in the basement with a 15A breaker in the panel. The inverters are mounted outside in a building with the panels on the roof. To reduce losses the wire is double 12ga for the live and double 12ga for the neutral because it's a long run of about 100ft to the breaker panel. (this was cheaper than trying to use 10 or 8ga)
There were no inspectors/electricians involved. I kept the money instead of giving it to them.
It's a great hobby to DIY.