Using Solar to Achieve Net Zero: How to Size My Solar Panel System

Inspiring Awareness and Change

How does change happen? You raise the awareness and inspire the change. The single most important problem for our generation is the following: How do we provide clean and sustainable energy for our growing population while simultaneously lowering CO2 production?

Reducing the amount of energy consumed by the operation of buildings can help solve this problem. To accomplish this, we need to design and build net zero energy buildings. Net Zero Energy buildings create more energy than they consume, leaving a surplus of energy to be sent back into the grid.

Solar as a Solution

To create Net Zero Energy buildings, we need an energy production system for our buildings to offset the energy used during operation of the building. There are several energy sources available including solar, wind, geothermal, nuclear, biomass just to name a few. Currently, solar is emerging as the most widespread system in use. The price of solar panel systems has gone down significantly in recent years and the long-term benefits/savings are outweighing the upfront costs. Financial incentives from governments are also helping offset the costs.

Designing My Solar Panel System for Net Zero

This first step to designing your system is to calculate the amount of energy that your building is currently using or will use. This calculation will allow us to size the system appropriately in order to achieve Net Zero Energy.  

• Calculating Electricity Usage for Your Existing Building

If you have an existing building that you would like to add solar to, the simplest method to calculate your electricity usage is to use last year’s electricity bills. Calculate your average monthly use in kWh (Kilowatt Hours) by adding each month’s usage up and divide by 12 (12 Months in the year). Divide this figure by 30 to determine your daily kWh usage.

Sample electric bill showing total kWh’s used for a home.

• Calculating your projected Electricity Usage for Your New Building

To design a solar system for your building, the best way to do this is by a power analysis. Begin by creating a table with each appliance, equipment, and device that you use in your building. List out the device with its power in watts in one column and the hours of use per day in the next column. Create a third column that lists out the Watt Hours. Find the Watt Hours by multiplying the Watts each device uses by the time it is used each day. This can be an estimate, but it is best to round up. When using this method, we recommend consulting with an electrical engineer to be sure you have all your electricity loads accounted for. See the table below for an example power analysis.

Example Power Analysis

According to the latest data from the U.S Energy Information Administration, in 2016 the average American household used 897 kWh per month. Your energy usage may vary depending on the size of your building, location, amount/type of fixtures in the building, and the users daily routines. For our example, we will use a home with an energy usage of 1,000 kWh per month.

Find the Peak Sun Hours for Your Location

Next, use the map below to find the average peak sunlight hours for your area. A peak sun-hour is an hour during which the intensity of sunlight is 1,000 watts per square meter. This step is very important because not all areas of the world receive the same amount of sunlight. This diagram is simplified for the purposes of the article but can provide a rough estimate of the peak sunlight hours.

Map from Wholesale Solar.com

Use the map above to determine the average kWh per square meter per day for energy at your location.

Calculate your System Size in kW Based on Your Energy Usage and Location

After you have your monthly kWh and peak sunlight hours, use the equation below to calculate your total kW output. Take your monthly kWh energy requirement from above (we assumed a house that used 1,000 kWh per month) and divide by your peak sun-hours x 30. This will give you to total kW output you need for your system.

kWh per month / (avg sunlight per day * 30) = kW solar system

For example, if you’re using 1000 kWh per month and your location has an average 5.0 hours of sunlight per day, your equation would look like:

1000 / (5 x 30) = 6.66 kW

If you want your building to be completely Net Zero, your solar array will need to produce at least 6.66 kW of energy per month based on the calculation above. This is easily achievable with the current technology and products available on the market. The average solar PV system size in the U.S is around 6 (kW). Again, this is an average and can vary widely based on location and power usage in the building.

Calculate How Many Panels You Need

After we know our system output requirements, we can calculate how many panels we need to provide the required power. Today, you can buy solar panels that range from around 45 -315 Watts. Solar panels typically cost $3.05 per watt in 2019. This means that a 6-kW array would cost approximately $18,300 before any tax credits. If you are constricted with the area to locate your solar array, consider purchasing higher wattage panels so you can have fewer panels. To calculate how many panels you will need, first multiply your energy usage per month (in kW) x 1000 to get watts. For example, a 6.66 kW system is equal to a 6,660 W system. We are simply converting kilowatts to watts.

6.66kW x 1000 = 6600 watts

Then divide the kW output by your panel’s efficiency (let’s assume we are using 250-watt panels) to get the total number of solar panels for your system. The calculation would look like this:

6660 watts / 250 watts per panel = 26.64 solar panels

So, to install a 6.66 kW solar array, which will produce 1000 kWh per month in 5.5 hours of direct sunlight, will require (27), 250-watt solar panels.

Locating Your PV System

As mentioned above, you will want to measure the amount of area you have to install your system. Measure the size of each panel based on the manufacturer and multiply the number of panels you need. If you have minimal area to work with, consider higher wattage panels to reduce the number of panels required. Factors such as panel orientation, tilt and shade from surrounding obstructions, will all have an effect of the efficiency of your system.

Shaded panels can have a significant negative impact on the efficiency of a solar system. Some studies have shown that if only 5% of the solar panel is shaded, it is possible to lose up to 80% of energy production from the panel.

Congratulations, you now have a general idea of your solar panel system requirements and position. This information can serve as a great starting point for designing your solar system.

It is important to note that this article is intended to help owners get a rough estimate of their solar requirements and costs. This is not a replacement for having a licensed electrical engineer/contractor design a system for you. We always recommend working with a licensed engineer and certified installer to implement your solar panel system into your project.