Howdy.

I've had my system up and running for a month and a half now. I have a utility-grade meter installed to measure the electricity generation from my STXR 2500 per requirements of the company that I'm selling my certificates to.

It seems as though the "life time" reading on the inverter is running faster than the utility grade meter. I'm wonder if this is due to the 5w tare loss? Does the inverter meter include the power to run the fan? Or is this simply a matter of running within the tolerances of the meters? Does the STXR meter round or truncate (to the nearest 0.1 kwh)?

Here are some comparisons over time, STXR on the left, Schlumberger on the right:

63.7 61
98.9 96
112.2 109
128.5 125
182.6 176

Bill

Hi Bill,

Two things ...

The AC Fan is a load not seen by the inverter AC power chip , so yes , this will skew things high a bit ... the fan uses 5-15 watts , depending on the speed it is running at

the Tare losses are about 5 watts ... So you'll lose 60-70 wHr day on the external meter do to this ...

Let me see if I have the numbers right then.

My system has been running for 46 days. The STXR shows 183.9 kwh while the Schlumber meter shows 178 kwh. The latter could be as low as 178, or as high as 178.999 because of digit truncation. Let's call it 178.5.

My digital Schluverger meter is not a net meter (unfortunately) so it advances regardless of the direction in which the electricity flows. Since the STXR uses 4 to 5 watts while the array is in the dark, assuming that there is 12 hours of darkness per day, the Schlumberger meter is reading about 54 wh / day on the high side or about 2.4 kwh over the 46 days. That means that the Schlumber meter should probably be reading about 176.1 kwh.

The difference is

183.9 kwh - 176.1 kwh = 7.8 kwh

Splitting this out on a daily basis gives us:

7.8 kwh / 46 days = 170 kwh / day.

Your saying that about 60 wh /day of this difference is due to tare. 170 - 60 leaves 110, which is probably lost to powering the fan.

That sounds about right.

Interesting.
Thanks again,
Bill

I dunno - 110 wh/day sounds a little high to me. We're talking about April weather, and the fan running on hi-speed over 7hrs to consume that much.

I've a friend who has found a moderate measuring discrepancy between the standard utility net KWH meter and the ones he uses to monitor his multiple inverter outputs. He even went to the extent of buying more expensive, supposedly accurate ones and found more discrepancies between the three. Seems that most meters have more built in inaccuracies than we're led to believe.

-- Larry Hale