PV self-consumption rate and dependence on time step

Is the PV self-consumption rate (i.e. amount used in home) sensitive to the time step? SAM gives a self-consumption rate of 68% with a 5,200 kWh annual load but published data from the UK suggests this is closer to 52-55%. It was suggested that the difference is a result of the time step (hourly time step average overestimates the self-consumption rate since in reality the overlap functions are much spikier) than differences in load profile characteristics.
 
Would you mind checking the attached project file to see if the 68% self-consumption rate is properly determined? And is there a way to check if the hourly average is indeed leading to an overestimation? SAM calculation gives 2,101 kWh PV generation where 1,420 kWh is used to offset the load. 

Here is a summary of the data:

• SAM project file
• Representative load profile for Irish market from ESBN. For rural domestic customers, the most representative Load Profile is LP3 (Rural Domestic 24H) where the latest load profile coefficients are contained in the worksheet LP3 24H 24Hr Consumption within the file Derived-Load-Profiles-2022.xlsx
• The file "lp1_24h_2022_s2_5200" is my own conversion from 15 minute to 1 hr data (simple summation). 
• The TMY for Dublin was obtained from PVGIS

Hi Scott,

In SAM, the meaning of the kW of electricity generated by the system in each hour depends on the solar resource data. For hourly data from the NREL National Solar Radiation Database (NSRDB), the W/m2 of solar irradiance for each hour is an instantaneous value at the midpoint of the hour, so the kW of electricity generated by the system in a given hour is the power output at the midpoint of the hour. If the W/m2 data from PVGIS represents hourly averages, then the kW of electricity that SAM calculates for each hour would be the average over the hour.

The load data should match the weather data. If you are using weather data with instantaneous solar irradiance data like that from the NSRDB, the load data kW should be measured power values at the midpoint of the hour. If the solar irradiance data represents an average over each hour, the load data should also represent an average kW over the hour.

I think the method you use to calculate the self-consumption rate from the SAM results is correct. For the PVWatts / Residential model, The value of Electricity from System to Load in kW for each hour is calculated as follows:


The annual values for each of those variables is the sum of the 8760 hourly kW values.

This answer is for hourly simulations, but would be similar for smaller time steps. SAM's simulation time step depends on the weather data time step, and the load data time step must match the weather data time step. SAM can use weather data with a simulation time step as small as one minute, so if you had access to 15-minute weather data and 15-minute load data, you could do this analysis based on a 15-minute simulation over the year.

Best regards,
Paul.

Hi Paul,

I believe Scott has a point. The self-consumption ratio is given by (Energy produced - Energy to grid)/(Energy produced). With Net Billing one needs an UNCOMPENSATED ANNUAL "Energy to grid" quantity to determine self-consumption. The SAM data tables do not provide this, nor any other quantities from which it could be derived.

The only way to solve this, is to export "Electricity from grid" and "Electricity to grid" from the (sub)hourly level to a spreadsheet and sum over the year. This is very cumbersome. The utility company must provide these numbers on the invoice, so I do not understand why I can't just pick them from SAM.

This raises the question if the "Electricity bill with system" in the cash flow table is correct, because "Electricity from grid" and "Electricity to grid" are needed to calculate the amount in $. Unsurprisingly, it is correct if I use the sums from the spreadsheet. But also here there is no way to split the bill in bought and sold energy dollars.

Is there an easy way I have overlooked?

Hi Scott,

I also did it the wrong way first, and came to a self-consumption of 77% in a project of mine.
In the cumbersome way I found 30%, which is pretty normal according to literature.

Best regards,

John

For those referencing this using newer versions of SAM:

In SAM 2022.11.21, you can relatively easily calculate the self-consumption of the a PV system for whatever time-step your analysis is based on. SAM bases the time-step on the weather data, so the granularity of the calculation (and the entire SAM analysis for that matter) will rely on the time increments available in the weather file. For an hourly simulation, the output in "Data tables" shows a tab for "Hourly data" where you'll find a statistic for "Electricity from system to load (year 1 hourly, kWh)". You can export this to excel and then sum up the values for the entire year to get a total kWh exported from the PV system directly to customer load for year 1 of the simulation. Then simply divide this total by the AC energy generated (year 1) to get the % of PV generation used to serve customer load, or the self-consumption rate.