# Case 1: Solar was installed before the ESP. Solar PV kWh are used at the building and the remainder goes to the grid. Utility Buys and Sells kWh at the Same Rate.

I am assuming the point of M&V in this case is that you want track energy cost savings.  That means, you want to track the reduction in net kWh sold by the utility.

Remember, Bill kWh is the net kWh provided by the utility, and that is what we are trying to reduce.

Bill kWh = kWh contributed by grid to building – kWh contributed to grid by solar PV system

or

Bill kWh = Grid kWh – Solar to Grid kWh

We are not tracking building energy usage, Building kWh.  We are tracking Bill kWh, which is what the utility is charging for.

We will do a regression on pre-retrofit Bill kWh:

Baseline kWh = Fit line of Bill kWh

You could try adding a variable for solar insolation, or average sunlight percentage per day.  If you add this variable, then you might get a better fit.  By having a variable, you would also be able to adjust baseline usage based on how available sunlight is.  If a particular month is cloudy one year and not the other, the baseline would capture this difference.  The Actual bill will capture it as the actual conditions led to the actual usage.  Adding a variable is the better way to do this.

For Actual data, again, we want to just track the net amount that the grid provides, as the solar was in the pre-retrofit bills too.  So:

Actual kWh = Bill kWh

kWh Savings = Baseline kWh – Actual kWh

kWh Savings = Fit line kWh of Bill kWh – Bill kWh

### To do this in Metrix or Option C

1. If possible, add a variable for solar insolation or average sunlight percentage.
2. Do a regression of the base year Bill kWh (hopefully with this solar variable).
3. For post retrofit bills, enter the Bill kWh.

So basically, we are using the same set of data for pre- and post-retrofit, which is Bill kWh.  This is how much energy the grid provides to the building minus what the solar system provides to the grid.

### Reporting Complications

In your reports you will want to break out the kWh for the two types of meters.  The Grid kWh meters can be combined at the Area, Site or Project levels.  But it makes no sense to combine the Grid kWh and the Solar to Grid kWh.  These are two different, but related beasts.  If your ESP saves 200 kWh during the hours when the building feeds the grid, then all of the savings will go to the grid, like the example below.

 Grid kWh Solar to Grid kWh Baseline 1000 kWh Baseline 100 kWh Actual 800 kWh Actual 300 kWh Difference 200 kWh Saved Difference 200 kWh Increase

If your ESP saves 200 kWh at night from parking lot lights, then none of the savings will go to the grid, like the example below.

 Grid kWh Solar to Grid kWh Baseline 1000 kWh Baseline 100 kWh Actual 800 kWh Actual 100 kWh Difference 200 kWh Saved Difference 0 kWh Increase

Nearly all projects will be somewhere between these two extremes.

It doesn’t make sense to add or subtract these kWh values at all.

You are just going to have two different kWh values.  If you are reducing energy usage from your ESP, then you see a reduction in Grid kWh and an increase in Solar to Grid kWh.  Report both, but don’t add them together.

Really, and I know this is controversial, but performance contracting it is all about dollars, not kWh, and therefore, what matters is the cost reports.

It may make sense to combine \$ at the area, site or project level for the two types of meters, but even this can be confusing.  I think the best way to report \$ would be to report the \$ associated with the Grid kWh meters separately from the \$ associated with the Solar to Grid kWh meters, and then add a page that combines the two.

When you combine \$, you are going to have to treat the \$ associated with Solar to Grid kWh as negatives.  Think about it.  \$ associated with Grid kWh is what you pay, and it is a positive number.  \$ associated with Solar to Grid kWh is what they pay you, and therefore it should be a negative number.  Overall, you are tracking what you pay the utility.  If you combine Baselines, then it is Grid \$ – Solar to Grid \$.  If you combine Actuals, it is Grid \$ – Solar to Grid \$.  If you combine Baseline – Actual, then it is Grid \$ – Solar to Grid \$.

### 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.

 Baseline Post-Retrofit Savings Day Building Usage kWh Solar Produced kWh Fit line of Net kWh (Includes Solar Produced) kWh Building Usage kWh Solar Produced kWh Net kWh Supplied to Building (Bill kWh) NOT Including Solar (kWh) Day 1 195 100 95 80 100 -20 115 Day 2 110 20 90 80 20 60 30 Total 305 120 185 160 120 40 145

Given the numbers in this table, we want our savings results to match the last column or 145 kWh.

Savings = Baseline kWh – Actual kWh

or

Savings = Fit line of Bill kWh – Bill kWh

or

Savings = 185 kWh – 40 kWh = 145 kWh

### How to Handle Demand

You would treat demand exactly as you treat energy.  So:

Savings = Baseline kW – Actual kW

or

Savings = Fit line of Bill kW – Bill kW