Tiered Energy Rates -- SAM treats daily as monthly?

When using SoCal Edison Tiered electricity rates -- downloaded in the "Electricity Rates" tab in Photovoltaic Solar -- SAM seems to treat the daily rate tiers as monthly. Is there a fix for this?

Example:
Tier 1 Max Usage = 11.1 kWh
Tier 2 Max Usage = 14.43 kWh
Tier 3 Max Usage = 22.2 kWh
Tier 4 Max Usage = 1e+038 = infinity, basically.

So each day, the first 11.1 kWh are tier 1, the next 3.33 kWh are tier two, the next 7.77 kWh are tier 3 and the rest are tier 4. PER DAY.

But SAM does this PER MONTH. So in a month with 900 kWh use, the first 22.2 kWh goes to tier 1-3, then the remaining 877.8 kWh goes to Tier 4.

What is the fix?
I could just multiply the tiers by 30 days (or better 365/12 = 30.417 ) but then that means the model assumes that every day in the month has reached the Tier 1 limit, when in reality some days do and some don't.

It seems like SAM generates a varying daily energy use in the model. Am I missing something... is there some box I am supposed to check to use Edison's DAILY TIER LIMITS correctly in SAM?

Thanks,
Tim

Hi Tim,

SAM treats the tiers as monthly tiers (or as a period within a given month). There is not an option to treat them as daily tiers.

I'm not sure what you mean by SAM generating "a varying daily energy use." You provide the load data on the Electric load data. By default, that data is hourly data so the daily load profile varies over the one year simulation period. (You can use sub-hourly data, such as 15-minute, 30-minute, etc., if it is available.)

Would you mind telling me what specific SCE rate structure you are using?

We are making improvements to SAM's rate modeling for the next version of the software, so perhaps we could work this in.

Best regards,
Paul.

Paul,

First off -- thanks for your reply and for your help. SAM is a great tool!

Second Question First:
2) "What rate structure?": I am using "Domestic Service: D - Baseline Region 13" from Southern California Edison Co ( www.sce.com/NR/sc3/tm2/pdf/ce12-12.pdf ). I am inputting Monthly Load Data into the "Calculate Load Data" on the "Electric Load" tab. (Photovoltaic Solar)

What is loaded into SAM when I select this rate structure is a set of DAILY TIERS (11.1kWh, 14.43 kWh, 22.2 kWh, etc). So then when SAM does the calculation of electric cost without system, it puts like 97% of energy use into the highest tier for the month. Example: If I use more than 22.2kWh on the first day of the month, then SAM calculates the rest of the month's energy use at the highest rate, rather than "resetting" on the next day to start with the lowest tier.

An approximate solution is to multiply the daily tiers by 30 to give a "monthly tier" -- but Edison calculates the use costs daily, not monthly, I believe. So it isn't perfect, but it gets closer.

1) "Varying Daily Energy Usage": By that I mean that SAM seems to use a model that simulates energy use that is varying from day to day WITHIN the Electric Load Data that I provide.

Example: Lets say that in April I input 600 kWh of use. SAM does not assume that on every single day I am using 20 kWh... rather, SAM assumes that some days I use more, some days I use less. SAM generates an electric use profile for the month (Simulate > Time Series > Hourly Data > Electricity Load (Year 1) (kW)). That profile has a bit of randomness to it.

Thanks again for your help. Please let me know if any of this is unclear or if I can provide any additional info.
I appreciate your time and assistance,
Tim

Hi Tim,
Thank you for the additional information. For now, SAM only models monthly tiers, so I think your imperfect workaround is the best you can do for tiered rate structures that use daily tiers instead of monthly.
As for the load data, SAM's residential financial model offers two options for entering monthly load data. The first is the Input Time Series Load Data option, which is designed for you to provide hourly or sub-hourly load data. However, because we recognize that such data is hard to find, we added the Normalize supplied load data to monthly utility bill data check box that makes it possible for you to enter a set of monthly electricity consumption values instead. That works by taking a set of hourly load data for each month and scaling it up or down so that the monthly total equals the value you enter. There's a more detailed description of how that works in the Help topic for the Electric Load page. So, given your example for April with 600 kWh, SAM takes the default hourly data for April (total 369.36 kWh) and adjusts it so that the total is 600 kWh by multiplying each houlry value by 600 / 369.36 = 1.62. You can see the original "default" monthly total by clearing the Normalize check box, and you can see the underlying hourly data by clicking View load data.
The other option is Calculate Load Data. That allows you to enter some energy parameters for the residential building along with the total monthly electricity consumption values and uses an algorithm to create a set of hourly load data.
The important thing to remember is that SAM's simulation uses an hourly (or subhourly) time step and it requires load data that matches the simulation time step.
Best regards,

Paul.