DIY Series Technology

Solar Install, The Numbers

We crunch through the numbers to see if a solar array is cost effective. The numbers surprise us.

In my previous post, I introduced the idea of installing a solar photo-voltaic system. In this second post, I will discuss the actual numbers and show how we are able to set up a solar system with installation at no additional cost to us.

This is our 2nd install of a solar photo-voltaic system. Our first house in the California desert was a natural for a solar system. We needed around 6kW to provide all of our daily needs. Our house was extra insulated to provide cooler temperatures against the desert heat. Additionally, our stove and heating were gas instead of electric. Federal and state tax credits paid for 45% of the cost of the system.

The cost of the system in 2011 was $26,000 for a 6kW system. After tax credits, and a down payment, our remaining costs were $14,000. A small inheritance paid this amount off. When we sold the home, the solar was an advantage in a slow housing market. We recovered all of our costs for the solar and made a nice profit off the house.

We now live in Huntsville, Alabama, where there are many more cloudy days. The California desert has an average of 6 hours daily of sunlight available for producing electricity. Here in Huntsville, the average is 4 hours of sunlight daily. Positioning the array in the correct direction (directly south) and angle (our latitude is 35 deg North) will maximize the sunlight we do get. See the map below for average daily hours of sunlight.

Average Hours of Direct Sun per Day

Our current house is completely electric and is constructed of standard 4 inch wall studs. It was built in 1989, with standard building materials. We added extra insulation in the attic and reworked the air ducts to seal leaks. We spent $2500 on sealing up the house. We also spent $7500 on new more efficient windows on the house.

The house is less leaky and warmer in the winter and cooler in the summer. We replaced the dish washer with a more efficient model. We switched the thermostat out for one that would reduce the use of electric resistance heat in our heat pump. Heating or cooling the house slower reduces the electricity use for our heat pump. When the heating and cooling system end in around 5-7 years, they will be replaced as well. Our improvements reduced our electricity needs to 14kW per day instead of nearly 25kW.

Now that our house is a bit more energy efficient, we can see about what we need for a solar system. Because we plan to generate all of our electricity needs for solar, we need a larger system. To save space in our backyard, we are using higher energy panels. Each panel will generate 320 watts per day. A total of 40 panels will give a total of 13kW average per day.

Here’s how we came up with our figures for covering our electric bill:

1. Take your electricity bill and get the average monthly amount of electricity consumed in a month. In our case that was 1300kW per month.

2. Divide this by 31 to get the amount of electricity used per day. 1300kW/31 = 41.9kW per day.

3. Multiply by the percentage of electricity coverage desired (0.0-1.0). 100% in our case, so multiply 41.9kW by 1.0. If 50% is desired, multiply by 0.5.

4. Get the number of average hours of direct sunlight per day. In our location that is 4 hours per day. (see the map above)

5. Divide the amount of electricity per day by the number of productive hours of sunlight. 41.9kW/4 = 10.8kW per hour.

6. Multiply the hourly figure by 1.25 to account for losses of the wiring and inverter. 10.8kW x 1.25 = 13.5kW needed for the solar system to generate per hour at our location in northern Alabama.

Now that we know how much power to generate, the next step is to determine the number and type of solar panels needed. We looked at various systems, including a few DIY systems and ultimately chose to have an experienced solar contractor install the system for us. We worked with the contractor and determined that 320w panels would be what we would need.

To calculate the number of panels needed was to take our needed hourly amount per hour (13.4kW per hour for an average of 4 hours per day), convert to watts per hour (13.4kW x 1000 = 13,400w per hour), then divide by the size of the panel (320w per hour). 13,400/320 = 40 panels.

Once we knew the size of our system, the contractor could give us an estimate. We decided to also mount our system on the top of a pergola, so we would have some additional shade in the back yard to replace the young oak tree where the solar pergola will be.

The cost of the system with the pergola will be $51,000 with $3000 included as the cost of the pergola. We will get a federal tax credit of 30% for the amount of $15,300. There is no state tax credit in Alabama. We will be borrowing $36,000 by refinancing our house at a lower rate and including it on our house note. Our house payment will go from $850 per month to $1050 per month for a total of 15 years. Our electricity bill averages $200 per month. So lower interest and the electricity generated by the solar will pay for itself.

In one month we will start building the system. We paid $1200 to have a tree removed from the site that we will be putting the solar on. This system is only possible for us as lower interest rates became available. Our home loan was at 3.5% and the new loan will be at 2.9% per year. We will be paying off our home over 15 years instead of 30 years. Additionally, we will make extra payments to get the house paid off in under 10 years. If we were unable to do this, the solar would not be feasible here in Alabama unless we had a much smaller system. Next, I will cover preparing the site for building.

The Solar Install Series:


About Anna Nyms

Anna Nyms loves to write about numbers and what can be done with them. She is an advocate for privacy and stronger online security. Her interests are encryption, privacy, hacking, and programming. Anna currently lives in Huntsville, Alabama with her husband and young daughter. She is a former US Air Force officer, spent nearly 20 years in the California desert working at Lockheed Martin ADP, and now works as a computer programmer.

1 comment on “Solar Install, The Numbers

  1. Pingback: Solar Install, Preparing the Site – Numbers Station

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: