Renewable Energy Questions/Discussion > Solar (heating or electric)

Need some math help on system production..

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tomw:
Ok, folks.

We installed a grid tie 7,700 watt SMA SunnyBoy system.

The smart meter was installed as part of that interconnect agreement.

I gather 4 figures from my system:

Inverter data:

Total KWH produced (daily, monthly, yearly)

Meter data:

Delivered -Pushed out to the grid.

Received -drawn from the grid.

Net -Like it says, different between the above 2 figures.

Trying to sort out the percentage of our use we produce with the system.

I a just having  brain farts on figuring this out, not seeing the way. When I graded papers for a professor in college we used the formula what you have divided by what you should have times 100 for  test scores. This works great but it doesn't seem to work in this situation?

The fly in the buttermilk is that I have no way to know how much we use that doesn't go through the meter. The inverter connects to the breaker panel through a switch and breakers and excess flows to the grid and if needed power  flows in from the grid when the system is not producing enough to service loads. All thru one bidirectional "smart" meter.

For instance, I have these lifetime values:

Inverter total production:
2,374.8 kWh

Meter data:
438.6 NET   1606.85 OUT   2045.51 IN

Now, the part that stumps me.

What percent of my total consumption is from the SMA?

Careful, it is not as simple as it seems. At first I thought I could just use meter NET divided by inverter total but that completely misses the instantaneous usage that often is a large part of the inverter output.

I know that we "bought" 438.6 KWH since Nov 19, 2016 (day of meter swap and commissioning)

Late winter was very overcast / cloudy and short days but last month we produced about 300 KWH more than we used that month.

OK any ideas?

Thanks.

Tom

rossw:
Well, I might have this wrong, but my take on it is that:

You produced 2374.8
You imported 2045.51 and exported 1606.85 (making a nett of 438.6 used from grid)
So your total consumption was (2374.8 produced + 2045.51 imported - 1606.85 exported)  2813.46 consumed.

You produced 2374.8 locally of the 2813.46 total you used: (2374.8/2813.46)*100/1 = 84.4%
Or, the other way, you produced 2374.8 but consumed 2813.46, so you CONSUMED 118.47% of your production.

Anyone else want a go? :)

oztules:
Same thing I came up with... different path perhaps.
ie...total production was 2374kwh... of which you used directly only 2374-1606 exported not used by you at the time = 768kwh out of the 2374kwh produced.

So you used 768kwh of your own electrons, but had to then import some 2045kwh from  the man... making total usage by you  768+2045=2813kwh

That makes total usage............2813kwh
and total produced...................2374kwh

And so I get the same thing with (2374/2813)x 100 = 84%... so 84% of your electrical needs came from home grown electrons, while 16% was imported from the powers that be.

Interesting that most of your generated power was exported ( 1606kwh as against 768kwh used at the time of production )...... grid makes a good battery :)


..........oztules
EDIT: It is interesting  that it will depend on the billing system, as to whether you need to reschedule the house power usage profile. If you get net metering, and net billing then who cares, but if you get charged at an import rate that is materially different to the export rate, then it may not look too pleasant.

Over in Aust, some providers do net billing ( Flinders Island is one  :)), but some give you only 5-8 cents/kwh for your export, and charge you up to 25c/kwh for your imported power. In this latter case, you need to get the hot water running during the day only, and any other loads you can schedule in the daylight hours.... particularly at peak solar times.

Tom needs to be on net billing, as his usage does not mirror his production timing.

rossw:
Your calculations demonstrate how important "time of use" is.
The question not asked, was how well was he using the power he produced. This is largely a financial question (where you have 100% efficient storage, as the grid appears to be), but the sting in the tail is the cost per kWh.

If he buys power off the grid at 12c, and sells power TO the grid at 4c, the inequity is obvious:

Sold     1606 @  4c =   $64.24
Bought 2045 @ 12c = $245.40

Managing your usage to minimise what you export (and therefore what you import at a different time) may not result in any difference in your percentage of used power generated, but could hugely impact your power bill at the end of the day.

oztules:
You wrote that whilst I was editing mine.... agreed.... timing is everything.

And if we hear a massive explosion from the direction of Toms place... I'll deduce he just found out his bill was not net billing.... but the other one.....

.........oztules

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