Monthly grid to load vs monthly use with system

I am running a PV+storage simulation but am having trouble interpreting the results. I have the following outputs:

-Monthly Load
-PV to Load
-Battery to Load
- PV to grid
- Grid to Load
- Year 1 monthly use

I would expect (monthly load)-(PV to load)-(battery to load)-(PV to grid)=(grid to load), but it doesn't. It does come close to year 1 monthly use, but not quite. There is also a major error in the calculation in September-December and I don't understand where it's coming from, as the solar generation graph appears normal as does the load without the system, so it's not a typo somewhere. 



However, it does work out this way: (monthly load)-(PV to load)-(battery to load)=grid to load. I'm having trouble understand what year 1 monthly use is supposed to be representing, and also not sure where the major error is coming from. Any insights would be appreciated. Thanks!   

Hi Nathan,

If your simulation uses the net energy metering option, the discrepancy you are seeing in September through December may be due to net metering credits.

SAM dispatches the battery in each time step (hourly or subhourly) of the simulation. Exactly how the battery charges and discharges depends on the dispatch options you choose, but in general, you can expect the PV array to charge the battery if needed when PV power exceeds the load, the battery to discharge to the load as it can when PV array power is less than the load, and PV power to go to the grid when the battery is fully charged and PV power exceeds the load. You can see this behavior by looking at time series data on the Results page on the Data Tables, Time Series and other tabs.

The electricity bill calculations are based on accumulated monthly totals like the ones you show below. I suspect that the discrepancy you note might be the effect of net metering credits, but can't tell for sure without seeing your file.

During any time steps when the battery is fully charged and the PV array is generating more power than the load, the PV array power will go to the grid. This would cause your first equation to not be valid.

The "electricity use with system" variable shows the monthly effective energy usage:

Electricity Use with System = Electricity Load - Energy to Load from System - Energy to Load from Battery - Energy to Grid from System

Best regards,
Paul.

Thanks Paul. Is there any way to turn off net metering carry over? I didn't see any option for it when creating a new battery simulation. The system I'm designing does not have net metering available. 

Here is my original simulation, with the errors in months 9-12



I changed the weather file just to see if that was a potential source for the error. Errors are now only in months 11-12.

Hi Nathan,

The billing options are on the Electricity Rates page, under "Metering and Billing." You can find descriptions of the options in the Help topic for that page. You can choose the option that best matches the method used to compensate the system owner for power the system generates in excess of the load.

If the owner does not receive any compensation for excess generation, you could choose the net billing or buy all / sell all option and set the sell rate to zero.

Best regards,
Paul.

Thanks Paul. That solves that problem.However things still aren't making sense based on the formula you provided above. Would you be willing to take a look at my file to see if you can find the source of the problem? It doesn't look like there is a way to attach files to a message, but I could send via email. 

Hi Nathan,

You should be able to attach a file to a reply using the Attachments button the text editor, but if you have trouble with that you're welcome to This email address is being protected from spambots. You need JavaScript enabled to view it. to me.

Best regards,
Paul.