Measuring Solar Generation on a Fronius Inverter

Currently, I have four ways to monitor the total power generated by my solar panels.

1. Check the meter. This is the most accurate way - but it is located inside a cupboard.
2. Read the total from the Fronius API. Unfortunately, this rounds off to the nearest kWh, so isn't brilliantly accurate.
3. Use the Fronius API to check the power being generated every minute, then convert to kWh. That seems accurate, but show discrepancies when compared with...
4. Solar.web takes a reading every five minutes from the solar panels, and then graphs them.

Obviously, the meter in the cupboard is the one that counts. Those are the numbers which I submit back to the electricity company in order to get a payment. What I'm curious about is which method for accessing the API is the most accurate and whether I need to modify my open source code.

Let's take a look at how each one performs.

The Meter

I took a photo before sunrise, and another before sunset. 869.81 - 853.36 = 16.45kWh.

So, that's what I'm aiming at.

Fronius Total API

The Fronius API is really easy to access - here are the (truncated) results.

{
    "Body" : {
	"Data" : {
	    "DAY_ENERGY" : {
		"Value" : 17000,
		"Unit" : "Wh"
	    },
	    "TOTAL_ENERGY" : {
		"Value" : 880000,
		"Unit" : "Wh"
	    },
	    "YEAR_ENERGY" : {
		"Value" : 812000,
		"Unit" : "Wh"
	    }
	}
    }
}

The "Day Energy" has been rounded to the nearest kWh. It's broadly accurate, but not spectacular.

(The "Total Energy" is a little higher than the current meter reading - that's fine, it was installed shortly before the meter so it's quite usual to have a discrepancy.)

Minute by Minute Readings

I wrote some code to query the API and store the current value. It does this every minute and then produces a graph at sunset.

The calculation is pretty simple

// Calculate the total kWh generated.
// Sum the readings and divide by 60. (Because we read every minute & there are 60 minutes in an hour).
// Divide by 1,000 to get kWh
// Only need 2 decimal places of precision
$kWh = round( (array_sum($currentPowerArray) / 60 / 1000), 2);

In this case, the reading of 16.45kWh was exactly accurate.

Solar.Web

Fronius offer the Solar.Web system for a pretty nifty remote view of your systems.

Yesterday's reading was 16.89kWh and looked like this:

Taking a look on my system, it appears that the logging is done every 5 minutes.

I don't know if it's reading an average every five minutes, or it's an instantaneous read which just happens to be at the high point. Every reading from Solar.Web was a few hundred Wh higher.

Conclusions

Source	Reading	Deviation from meter
Meter	16.45	-
API Total	17	+3.3%
Minute-by-Minute	16.45	0%
Solar.web	16.89	+2.4%

Long story short? The most accurate programmatic reading is from my minute-by-minute script - but there's not much in it. Interestingly, the Fronius API is the most inaccurate. There's a firmware update available, so I'll see if that fixes things.