My meter and Inverter don't agree

heynow999 · February 2009

I have a 2.5Kw grid-tie system on my house and the inverter shows more power than is produced than than the hydro meter shows? I sell everything to the grid, and I have a Fronios 3000 inverter. It then goes about 30 feet to a hydro meter than records what I sell to the LDC. It seems to be about 10% off. Right now, my froniuos shows it has produced 140 KW, but the hydro meter shows less than 130Kw. The hydo meter is a 10X's meter, so it doesn't show the last digit. It could be from 120 to 129, but either way it is at least 11 kw behind.

When I explained this to my wife she said synically, "this surprises you?"

solartek · February 2009

Re: My meter and Inverter don't agree

Is this a new system? I know that on the last 2 Xantrex inverters I installed the lifetime totals already showed 6kW and 7kW hours when the systems were first turned on. My assumption has been that the non-zero lifetime values come from manufacturer testing of the units before they are shipped. Could this explain your discrepancy?

BB. · February 2009

Re: My meter and Inverter don't agree

Probably, you are lucky if your various meters are accurate within +/- 2%... If one meter is +5% and the other is out -5%, that could account for the 10% error.

Also, it would not be the first time that a manufacturer "errors" on display readings--and the error is +10% in their "favor".

Remember everyone is "arguing" whether their inverter is 92.5% or 95.6% efficient. Making the customer "happier" (or at least not calling up with their AC/DC readings showing 92.1% efficiency) when they run the "numbers" through their calculator.

Add losses due to wiring voltage drop, meter tare, inverter tare, and differences between the way the revenue meter vs inverter meter calculate power, harmonics, and phases--...

Also, because your meter is a 10x meter (is this a big installation with a shunt in the box--or this is just what is done your your area with even relatively small 100-200 amp services?), the accuracy it is capable of displaying:

10/120=1/12=0.083= ~8% displayable accuracy for your reading

If there are current shunts (typically used for large services), that can also add inaccuracies too.

I would tend to believe the billing meter (within its displayable accuracy). However, it would not be the first time the billing meter was wrong. Most utility companies will confirm the accuracy of their meter if you have questions.

Sorry, lots of questions and hand waving--little in the way of answers.

-Bill

heynow999 · February 2009

Re: My meter and Inverter don't agree

I can say that I did not check when it was first turned on whether it was at zero or not, at least I don't remember checking it. It may not have been at zero. That may be one cause.

I do know that the descrepincey has been getting bigger. Last month, the differance was about 7 Kw, now the differance is more than 14 Kw. As of today, the Fronious shows 144Kw, while my meter has yet to turn over to 130Kw.

This is a small installation on my house (2.5Kw) with two meters. One measures my use, the other meter (the one in question) measures my production.

I think BB is right in that the inverter is being a bit optimistic, and there are some losses in the wires and connections between the meter and the inverter.

What I am going to try and do is reset the inverter to zero next time my "Sell" meter clicks over. Should be today as we have a lot of sunshine.

BB. · February 2009

Re: My meter and Inverter don't agree

You can measure the AC voltage at the inverter output and at the meter bus (when the system is under load). That would give you a good first order estimate of wiring losses.

Assuming you have a 7kW per month error:

7,000 watt*hours / (31 days * 24 hours per day) = 9 watt average "load" 24*7*30 days per month

The utility meter may have a 3 watt tare load.

Fronius shows a 0.15 watt night time load and a 7 watt "day time" load for one product line... My GT system "runs" around 10-13 hours per day.

So 3 watt meter + 7 watt*12/24 = 6.5 watts of "tare losses" average

Only have to find that other 2.5 watts of loss--wiring (assuming high output currents 6 hours per day, that would give 2.5*24/6=10 watts to wiring losses)? (assuming above losses are not registered by the inverter but registered by the utility meter).

Does this make sense?

-Bill

Solar Guppy · February 2009

Re: My meter and Inverter don't agree

Bill has a good summary ...

You have a Tar loss of the inverter .. This is subtracted by the utility meter
You have lines losses interconnecting from the inverter to the utility meter

And lastly, the Fronius reported AC watts is will have tolerance deviation. A few years ago I tested all major brands of GT inverters, Xantrex, SMA, PVPowered and Fronius.. on the three units I tested from Fronius, all three reported >+5% in power generation. The contacts at Fronious were "surprised" and claimed the manufacturing process would be checked and corrected if necessary.

As long as I've been testing RE products, I have found more manufactures units report "optimistic" values than accurate values

The one exception is Xantrex, they are better than 1% on both the GT and XW lines including the Charge Controller.

As an obersvation, customers love bragging out how great product "xyz" performs, of course using the units reported data. The more in error in the positive, the happier the customer is with the product, unless of course your being metered else where and they show the difference!

heynow999 · February 2009

Re: My meter and Inverter don't agree

That makes a lot of sence.

I was thinking, it is 30 ft from the inverter to the meter, how can it lose %10 in that short of a run, but I never thougth about ALL the loses.

Actually, when I do the math, it seems like I lose about %12 between the inverter and the meter! Today the meter rolled over to 130kw when the inverter was at about 147kw. 130 divide by 147 = .88

I will have to add another panel to cover the loses!

My meter and Inverter don't agree

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