Problems with Electric Load

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Problems with Electric Load

Dear Paul,

I am trying to simulate a PV plant production, I use the photovoltaic (detailed)- commercial model and also an specific weather file. My doubts are related to Electricity Rates and Electric Load sections. At the first one I select "All generation sold at sell rate(s) and all load purchased at buy rate(s)" and "Use hourly (subhourly) sell rates instead of TOU" because I have the input hourly electricity prices array(8760x1).
Otherwise my plant won't load any energy from the grid, so I change the default "Energy Usage" array to zeros-array on Energy Load section. This last change does not produce variations on some SAM final results, my question is how can I get the same Net Present Value for the project if I don't take any energy from grid and if I take yearly 8000000kWh during 25 years?
Another question, where are the loaded electricity rates specified? Is the model using the same sell rates for the loaded energy?


Paul Gilman


I'm not sure I completely understand your question. Would you mind attaching your .sam file along with the weather file (.csv) you are using for your analysis to your post above (click Edit to add an attachment)? Otherwise, you can email it to me.

Best regards,


Hello Paul,

I'm sorry I didi'nt explain very well my problem. I have attached 3 files:
* "model with default electric load array" is the .sam file of the model I am using where the electric load array is the default SAM uses for photovoltaic-commercial model.
* "model with electric load = 0" is the .sam file of the I am using where electric load array is a 8670x1 zeros array, so the system has no consumption from the grid.
* the third file, .csv, is the weather file

My question is why both models calculate the same Net Present Value?
In one case the system buys 7,646,295.50 kWh yearly from the grid and the other one doesn't need any energy from the grid. So I don't understand why both have the same net present value.
And another question is which are the electric buy rates the model is using for the case that it has consumption from the grid ("model with default electric load array")? Are the electric buy rates equal to sell rates that I give as an input to the model at Electricity Rates section?

Paul Gilman

Thank you for sending the files.

The only difference between the two scenarios is that one has a load, and the other does not have a load.

SAM calculates the NPV based on the annual electricity bill savings, which represent the value of the renewable energy system. The bill savings is calculated as the annual "bill with system" minus the "bill without system."

In the scenario with no load, the annual bill without system is the sum of the fixed monthly charges (12 months * 89.72 $/month = $476.64). The bill with system is the sum of the fixed charges, energy charges, and demand charges, and comes to $-23,414. Note that the demand charge applies to the 0.0606 kW consumed by the inverter at night. It is a negative value because SAM assumes the electricity customer receives a payment at the hourly sell rates for the electricity generated by the system. Therefore, the bill savings is $477 - $-23,414 = $-23,891.

In the scenario with a load, the annual bill without system is now the sum of fixed, energy, and demand charges and is $826,028. The bill with system is now positive at $802,136 because the system output is small compared to the load (7.6 million kWh annual load compared to 0.62 million kWh power generation). The net savings are $826,028 - $802,136 = $23,892.

Because you chose the "all generation sold at sell rate(s) and all load purchased at buy rate(s)", the difference between the "with system" and "without system" bill is the same in the no load and with load scenario: In both cases, all of the power generated by the system is sold at the sell rates. In the no load scenario, the value of the system is value of the total electricity sold. In the with load scenario, the value of the system is the difference between the with system and without system electricity bill. But, with the addition of a load, both of those values increased by the same amount because the electricity purchases are not affected by the system, so the net value of the system stays the same.

That is a little tricky to explain, but I hope it makes sense.

Also, I noticed that the capacity factor and energy yield for your system seems low, which I think may be due to the data in your weather file. I compared it to results from the hourly weather file for Madrid in SAM's default weather file for reference.

Best regards,


Thank you very much Paul

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