This is probably the most difficult case to understand. There are two reasons it is difficult.
In theory, we have to subtract the solar kWh produced from the post-retrofit data. We will do this by leaving it in the post-retrofit data and adding a baseline modification to the baseline. It is the same thing, either subtract from one side of the equation, or add to the other side.
Two rates are used, one for which the utility is purchasing kWh and one for which the utility sells kWh. But because we are not counting savings for the solar, we need not be bothered by solar to grid kWh or costs or consequently, the utility buy back rate.
Starting with Baseline. Remember, Baseline represents how much energy “would have been used” if there was no ESP. I understand solar PV was part of the ESP, but for our calculations here, the ESCO did not want to include solar PV production. So, we need to pretend that solar PV was installed but was not part of the ESP. So, that would be just like if the client had installed the solar during the performance period, which we cover in Case 8—solar as a baseline modification.
Because we are tracking this as if no solar system was installed, we don’t need to separate out this baseline into Grid kWh and Solar to Grid kWh.
We only need to track Grid kWh. For the Grid kWh, we want to remove the effect of solar from the Baseline, and we do that by removing the Solar Produced kWh with a baseline modification:
Baseline Grid kWh = Fit line of Bill kWh – Solar Produced kWh
Where the Solar Produced kWh is for the performance period, and the Bill kWh is from the base year. Solar production reduces energy drawn from the grid, therefore it should be negative in this equation and in the modification.
The Actual data is also tricky. We want usage minus the Solar Produced, which is Bill kWh. Remember:
Bill kWh = Grid kWh – Solar to Grid kWh
Actual Grid kWh = Bill kWh
Now to calculate savings:
Grid kWh Savings = Baseline kWh – Actual kWh
Grid kWh Savings = Fit line of Bill kWh – Solar Produced kWh– Bill kWh
Where the Solar Produced kWh, and Bill kWh are for the performance period, and the Bill kWh for the fit line was during the base year.
Because we don’t have to fool with the different costs for buying and selling kWh, this ends up identical to Case 3.
This is also exactly like Case 8, which follows. It is just like the client installed solar during the performance period, but that this time the solar went online at the same time the performance period started.
To do this in Metrix or Option C
- You only need one meter, as we are not tracking Solar to Grid kWh.
- Do a regression of base year Bill kWh
- Enter a baseline modification for Solar Produced. Make sure the solar kWh are negative in the baseline modification.
- For post retrofit bills, enter in Bill kWh. Remember, Bill kWh = Grid kWh – Solar to Grid kWh.
In this example, we are given 2 days of usage in a month. In day 1, the solar produces more than the building uses. In day 2, the building uses more than the solar produces. Remember, since we are assuming that there is no solar installed, then we can assume there is no solar rate, and therefore this are no different costs for buying and selling kWh. Therefore, we do not need to break it up into Grid and Solar to Grid categories in our calculations.
|Day||Fit line of Bill kWh||Solar Produced Modification
(Fit line – Modification)
|Solar Produced kWh||Total Building Usage
|Net kWh Supplied to Building
|ESP Savings NOT Including Solar
Given the numbers in this table, we want our savings results to match the last column, or 25 kWh.
Savings = Baseline kWh – Actual kWh
Savings = (Fit line of Bill kWh – Solar Produced kWh) – Bill kWh
Savings = 185 kWh – 120 kWh – 40 kWh = 25 kWh
How to Handle Demand
You would treat demand exactly as you treat energy. So:
Savings = (Fit line of Bill kW – Solar Produced kW) – Bill kW