Question About SAM outputs

Hi, I'm using the attached SAM file to calculate pre-solar residential utility bills for a given load and rate structure. Using the Monthly data output section, I create this output:

Electricity load | (kWh/mo)
Jan 1555.81
Feb 1133.7
Mar 1065.35
Apr 959.81
May 1105.34
Jun 1306.35
Jul 1550
Aug 1383.13
Sep 1194.78
Oct 1163.2
Nov 953.133
Dec 1425.53

Electricity energy charge without system | ($/mo)
Jan 162.324
Feb 108.299
Mar 99.6602
Apr 86.963
May 104.715
Jun 155.443
Jul 191.82
Aug 166.608
Sep 139.221
Oct 112.028
Nov 86.2259
Dec 145.187

Electricity sales/purchases without system | ($/mo)
Jan -122.64
Feb -92.0796
Mar -87.1313
Apr -79.4902
May -90.0264
Jun -124.175
Jul -145.47
Aug -130.886
Sep -114.424
Oct -94.2155
Nov -79.0069
Dec -113.208


Will you please help me understand what the difference is between (the absolute value of) 'Electricity energy charge without system' and 'Electricity sales/purchases without system?' Based on the rate data entered into the inputs in SAM, and analysis of other SAM models, I would expect the difference to be any fixed monthly charges that might apply. But something else appears to be impacting the difference here.

In the attached SAM file, I have the enable net metering input set to off. If I turn Net Metering on, and set the year end sell rate to zero, I get these different results.

Electricity sales/purchases without system | ($/mo)
Jan -172.324
Feb -118.299
Mar -109.66
Apr -96.963
May -114.715
Jun -165.443
Jul -201.82
Aug -176.608
Sep -149.221
Oct -122.028
Nov -96.2259
Dec -155.187

I can corroborate these results in an excel rate model, so I think these are the right ones. Why does SAM change the Electricity sales/purchases amounts prior to solar based on the net metering input?

Any insight you are able to provide would be much appreciated.

Thanks,

Tom

Hi Tom,

When you run the case in the file you attached with net metering turned off, SAM applies the Tier 1 rate in each Period because it compares the "Max Usage kWh" value to the net load in each hour. So, for January, the "without system" monthly electricity purchase is 1,555.81 kWh * $0.0724/kWh + $10 = $122.64.

With net metering turned on, SAM calculates the tiers on a monthly basis, so that in your case, the $0.0724 rate for Period 1 applies to the first 500 kWh over each month in that period, and then the 0.1104 rate applies up to 1,000 kWh, and so on as you would expect.

I'm not exactly sure what the energy charge amount is representing here: It should be the value of electricity purchased at the rates you specify under "Energy Charges" on the Electricity Rates page, but I agree that it seems to be something else. The sales/purchases value is the one that goes into the financial model, so you should make sure that that value makes sense.

We're working on improvements to these various rate structure combinations for the next version of SAM due for release at the end of this week.

Best regards,
Paul.