Understanding "average power"


#1

Hi,
Apologies in advance if this is a dumb question. It is all very new to me.
I am trying to understand the "average power - yellow line on the graph. I see the occasional downward spike and cannot fathom what that means.
Any help would be appreciated.
Thanks
James
PS I have attached an example.


#2

See https://pvoutput.org/help.html#intraday-defs


#3

thanks mate, i had already read that.
It means nothing to me
I cannot translate that to helping me understand the downward spikes


#4

There is missing data where the dip is from 4:05 to 4:30, an average reading over a longer period of time will usually differ from the instantaneous reading at the end.


#5

Thanks for that. I get it now.
So with the data being automatically fed into PVOutput it will be some sort of unknown abberation.


#6

PVO’s power output line is really just a series of single samples of power output at that moment in time, each taken at regular intervals (which might be every 5 or 10 minutes say).

It represents a significant downsampling of your actual production.

I’ll use a camera analogy to help explain.

Think of it like taking a still photo of your PV system’s power output once every 5 or 10 minutes. It only tells you what the output was at the time of taking the photos but tells you nothing about all the moments in between the photos.

Drawing a line between such data points is misleading, really it should only plot such data as dots since this data does not tell you what happened in between the data samples.

To fill in the gaps it would need to sample your PV output on a much more frequent basis, e.g. every second.

The camera analogy would be using a video camera instead of taking still photos with large time gaps in between. Or taking still photos every second.

So it’s possible for the power output to be say 5kW at the time a sample is taken, and 5-minutes later to also be 5kW when the next sample is taken but the actual power output in between those two samples is unknown, e.g. say a cloud passed overhead after the first sample was recorded but was gone before the second sample was recorded.

The two end points will show 5kW but if you had sampled all the points in between you would see the effect of the cloud on power output. In this case drawing a line between those two data points would give the false impression that power output was maintained at 5kW the whole time.

Average power accounts for the impact of the cloud since it is based on the difference in cumulative energy production at the two data samples and cumulative energy production is totalled with high frequency (it’s akin to the video camera).

Now this still doesn’t tell you exactly what happened between the two samples at each end of the interval, however it will tell you the average power output for that interval and it will be different value if the power in between each sample deviated significantly.

Personally I’d like to turn off the power output line and only show the average power line since it more accurately represents the system’s performance.


#7

Another way to think of the average power value is it represents the slope (first time derivative) of the cumulative energy between each data sample. IOW it tells you how quickly the cumulative energy line is going up.

e.g. at 11:00am let’s say the cumulative energy output is 8.00kWh, and at 11:05 it is 8.60kWh.

So over that 5-min interval, the energy production was 0.60kWh.
5 minutes is 5/60ths of an hour, so
the average power for that 5-minutes was 0.60kWh / (5/60) hours = 7.2kW


#8

thanks for all the help