Case 8: Solar was installed by the client during the performance period of the ESP. Solar PV kWh are used at the building and the remainder goes to the grid. Utility Buys and Sells kWh at Different Rates

Remember, the adjusted baseline represents how much energy would have been used if there had been no ESP.  If the ESP had never been implemented, the client would have still had a reduction in energy due to the solar project.  Therefore, the adjusted baseline would have been reduced by the kWh produced by the solar project.  To achieve this reduction, a baseline modification will have to be created to handle the reduction in energy usage due to the solar project.

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, rate.

Baseline kWh is calculated using the regression model of base year bills, before the solar was installed, which is what the building was drawing from the grid.

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.

So, 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 did not have to fool with the different costs for buying and selling kWh, this ends up identical to Case 4.

This is also exactly like Case 7, which precedes this case.

To do this in Metrix or Option C

  1. You only need one meter, as we are not tracking Solar to Grid kWh.
  2. Do a regression of base year Bill kWh
  3. Enter a baseline modification for Solar Produced. Make sure the solar kWh are negative in the baseline modification.
  4. For post retrofit bills, enter in Grid kWh supplied by utility to the building.

Example Calculations

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.

Baseline Actual Savings
Day Fit line of Bill kWh Solar Produced Modification


Baseline kWh

(Fit line – Modification)

Solar Produced kWh Total Building Usage

(Bldg kWh)

Net kWh Supplied to Building

(Bill kWh)

ESP Savings NOT Including Solar


Day 1 95 -100 -5 100 80 -20 15
Day 2 90 -20 70 20 80 60 10
Total 185 -120 65 120 160 40 25


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