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
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
- If possible, add a variable for solar insolation or average sunlight percentage.
- Do a regression of the base year Bill kWh (hopefully with this solar variable).
- 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.
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|
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|
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 $.
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.
|Solar Produced kWh||Fit line of Net kWh
(Includes Solar Produced)
|Building Usage kWh||Solar Produced kWh||Net kWh Supplied to Building
|NOT Including Solar
Given the numbers in this table, we want our savings results to match the last column or 145 kWh.
Savings = Baseline kWh – Actual kWh
Savings = Fit line of Bill kWh – Bill kWh
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
Savings = Fit line of Bill kW – Bill kW