The math just doesn't add up

[lhpdiver]

We had our 10 panel PV system installed just about 2 months ago. Our Inverter is a Fronius 3 (I believe).

For the first month or so I recorded the numbers on the power company's bidirectional meter. We are producing more than we are consuming and that is a good thing. Today we received our first electric bill.

From the bill : Production (324) - Consumption (233) - residual of old meter (22) = roughly the 70 kwh credit we will carry forward.
Today the meter reads Production (360) and Consumption (275)

BUT - when I look at the inverter it says we have produced 597 kwh. That is a difference of 237 kwh !!

Before installing the system we received 4 quotes. 2 of the 4 people were concerned we would lose kwh if we used the existing wiring between the area of the inverter (the pool room) and the meter (perhaps as much as 100 feet) SO we had new wiring (and conduit) installed.

From the manufacturers spec sheet the inverter itself uses 8 watts/hr when operational and <1 otherwise.

I'd appreciate any ideas on why the numbers on the inverter don't match up with the meter. I'm not electrically inclined. Could there be a short ? Could the gauge of the wire used be insufficient ? Is either the inverter or meter broken ?

Thanks

[Ian S]

Your solar is set up so that anything you produce goes first to your own usage then any excess is sent to the grid and any deficit is taken from the grid. The "missing" 237 kWh is actually energy your solar produced but that was immediately consumed by your household: it was never seen by the utility. What the utility calls your "Production" is really excess production over and above what you used and that is fed back to the grid. What the utility calls "Consumption" is what you took from the grid not your total consumption. There is nothing wrong with your system.

[lhpdiver]

Your solar is set up so that anything you produce goes first to your own usage then any excess is sent to the grid and any deficit is taken from the grid. The "missing" 237 kWh is actually energy your solar produced but that was immediately consumed by your household: it was never seen by the utility. What the utility calls your "Production" is really excess production over and above what you used and that is fed back to the grid. What the utility calls "Consumption" is what you took from the grid not your total consumption. There is nothing wrong with your system.

Thanks Ian for your input.

But - for the period:
may/14 - jul/15 we used 324 kwh (this is before install and usage was probably comparable to current billing cycle)

Given that - can you please help me with the math to reconcile the numbers I gave earlier for the inverter/meter ? Just get me in the ballpark.

Thanks

If our actual consumption was roughly 325 kwh and our missing 237 kwh went towards that, that leaves roughly 100 kwh.

[Ian S]

You have a dynamic system so all your readings have to be obtained at the exact same time. If you can get all those numbers, you can make a calculation of total consumption as follows:

Total Consumption = Consumption from Grid + Total Production - Production Sent to Grid

However, if your meter is like many, it's cumulative so that your bill is calculated by subtracting the previous period's reading from the new reading to get the new numbers for amounts consumed and sent to the grid in the latest period. So in that equation above, if you want the total consumption for the latest period, you'd have to get the numbers from your last bill for Consumption from Grid and Production Sent to Grid as well as the Total Production from the inverter as of the date and time your meter was last read by the utility and subtract each from the appropriate term on the right side of the equation. The Total Production number may be the trickier one to get unless you have a good history of your production by date and time and can determine its value as of the exact same time as the utility numbers. If not, then since your system has only been running for a few months, you could just calculate the Total Consumption since the solar was turned on - not just for the most recent period - and see if that makes sense for the same timeframe before solar. Hope this helps.

[lhpdiver]

You have a dynamic system so all your readings have to be obtained at the exact same time. If you can get all those numbers, you can make a calculation of total consumption as follows:

Total Consumption = Consumption from Grid + Total Production - Production Sent to Grid

However, if your meter is like many, it's cumulative so that your bill is calculated by subtracting the previous period's reading from the new reading to get the new numbers for amounts consumed and sent to the grid in the latest period. So in that equation above, if you want the total consumption for the latest period, you'd have to get the numbers from your last bill for Consumption from Grid and Production Sent to Grid as well as the Total Production from the inverter as of the date and time your meter was last read by the utility and subtract each from the appropriate term on the right side of the equation. The Total Production number may be the trickier one to get unless you have a good history of your production by date and time and can determine its value as of the exact same time as the utility numbers. If not, then since your system has only been running for a few months, you could just calculate the Total Consumption since the solar was turned on - not just for the most recent period - and see if that makes sense for the same timeframe before solar. Hope this helps.

I wish I had been recording the inverter's production numbers over time - but your equation makes sense. The inverter was 'online' a few days before the bidirectional meter was installed (it had a head start). Also - one of the reasons we really needed our PV system is because we live in an area where we are supposed to keep our consumption below 250 kwh/month or we get sent to the dog house and our bill will double (we lose the government subsidy). We had an energy survey done earlier this year and they found that our old meters (with the spinning dials) were 'slow' - by 15% ! Recently they swapped out ALL the old meters in our community. This is the first billing cycle with the new meters. Some folks are going to be in for a surprise when they open their electrical bills...

Thanks again.