## How do you size a solar array?

We often receive a question from our customers about how to size a solar array, taking into consideration electricity loads that are not yet shown on their electric bill.

For most retrofit solar projects, meaning projects for existing homes that have at least 12 months of electric usage history, the technical requirement for sizing a system is fairly simple. We simply look at your electricity usage, something that can be easily obtained from PNM’s website (or obtained whatever utility the customer uses), and we calculate the number of solar panels that would be needed to offset 70% to 100% of that usage on average. Obviously, there are other technical constraints such as shading, available roof space, or even budget considerations that might change what would normally be an “optimal” system.

## How is my solar array affected if I have an electric vehicle?

I recently had occasion to figure this out with my own existing solar array when my wife and I purchased a Nissan LEAF, an all-electric vehicle. In doing the research, I found some very helpful information that would allow us to determine the number of additional solar panels that would be needed to compensate for the electric vehicle, an electric load that would not be shown on our electric usage history.

**Calculate How Many Panels Are Needed To Accommodate Your Electric Vehicle**

A helpful website is http://www.fueleconomy.gov/feg/evsbs.shtml. At that site, you can locate most of the electric vehicles that are currently available on the market. Here is the information you need to gather:

- The operative number that you need is the number of kilowatt (kilowatt) hours per 100 miles, and we recommend using the “combined” number, unless you are certain you will not be using the vehicle on the highway at all. In that case, you could use the even lower number of “city” kilowatt hours to calculate your formula.
- The next number you will need to know is obviously the number of miles you will drive the car per year. You could break this down by month and simply add the kilowatt hours to each monthly total, but this is a somewhat inexact science. Since you are sizing the system, in most cases, for a grid-tied system and not one using batteries and other off-grid equipment, there is a margin of error. That is to say, if you do not get the calculation exactly correct, the utility will still happily sell you the kilowatt hours needed should you have a deficit of kilowatt hours produced by your solar array. On the flip side, it is probably not a good idea to over-size your system due to the unfavorable economics of a solar array that produces greater than 100% of the energy used over the course of a year. For more on that topic, please contact us to discuss the economics of solar array sizing.

**Example: **For the Nissan LEAF, the calculation for the additional number of panels needed to accommodate our electric vehicle would go as follows:

- We used 8,000 miles per year as a baseline calculation. This may be a little less than we will actually drive, but again, we do not want to oversize the system.
- The combined average kilowatt hours per 100 miles for the Nissan LEAF is 29. The first step that is needed is to break the 8,000 miles into a number of 100-mile units. That is fairly simple. Just eliminate the last two zeroes of the annual driving estimate. In our case that gives us 80 such 100-mile units.
**80 x 29 = 2,320** - How many additional solar panels do I need to produce 2,320 kilowatt hours over the course of a year? Any solar company will have sizing tools that will allow it to automatically make this calculation, but the basic calculation involves dividing that annual kilowatt-hour total by the number of days in the year to determine a daily total of AC kilowatt hours needed to be produced by the solar array.
**2,320 ÷ 365 = 6.36 kW hours per day** - Here is where things get a little bit technical, but again, a competent solar consultant can easily plug in the total kilowatt hours needed to correctly up-size your system to accommodate for the electric vehicle. But again, so you can see how the concept works, and more importantly, so we can get an answer to our question about the Nissan LEAF (as the example), let’s continue. 6.36 kilowatt hours per day is the number of kilowatt hours in alternating current. The solar panels produce electricity in direct current, which then must be inverted into alternative current. In that process, the power is reduced somewhat due to the inversion process. A good conservative factor to use is 75%. We refer to this in the industry as the “derate.” So, for our system, to back into the number of DC hours needed to be produced, we would divide the 6.36 kilowatt hours per day by a derate of 75%.
**6.36 ÷ 0.75 = 8.48** - 8.48 is the number of kilowatt hours needed to be produced per day in direct current. This number is then divided by the “average solar day” as that number is provided by the National Renewable Energy Laboratories (NREL) in Boulder, Colorado. For Albuquerque, New Mexico, where we live, that number is exactly 6.0. So the daily total of DC kilowatt hours needed, 8.48 kilowatt hours, is divided by the six-hour solar day, giving us the following:
**8.48 ÷ 6.0 = 1.41 kW**This step breaks the kilowatt*hours*into the additional kilowatts needed for the system size to accommodate the vehicle.[1]

## So, How Many Solar Panels Are Needed To Accommodate My Electric Vehicle?

At the end of the day, the energy that my vehicle needs during the year using the above calculations is accommodated by adding to my existing solar array an additional capacity of 1.41 kilowatts. Assuming a panel size of approximately 250 watts (a fairly average number for most solar panels in the modern market), then the calculation would be as follows for the additional number of panels: **1.41 kW = 1,410 watts 1,410 ÷ 250-watt panel = 5.64 additional panels** Since you cannot make a design that calls for a 0.64 panel, you would need a sixth panel to accommodate the necessary energy for the vehicle.

We need an additional six panels on top of whatever solar array we would otherwise need to accommodate the rest of our electricity usage in order to power the Nissan LEAF throughout the year.

[1] A kilowatt is akin to a snapshot in time, whereas a kilowatt hour is the amount of energy produced in an hour. If the system produced exactly one kilowatt in power over the course of an hour, that system will have produced a kilowatt hour at the end of that hour.