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How Many Solar Panels Do I Need?

“I’ve always been interested in solar.  We live in a two-story that’s about 2500 square feet.  How many solar panels do I need?”

We get this question a lot, and the short answer is:

The size of your home has very little to do with how many solar panels you need. 

There are a lot of factors that contribute to how much energy your family consumes, including:

  • How many people live in your home and how often are they there?
  • Do you have children living in the home (“I’m not air conditioning the outdoors!”)?
  • What are your current day and nighttime thermostat settings?
  • Do you and/or your spouse work from home?
  • How energy efficient is your home and how new/old are your major appliances?
  • How old is your HVAC system and what is the SEER rating of your air conditioning condenser unit(s)?

When we look at a house’s solar potential, we start with gathering your last 12 months energy consumption, focusing on the number of kilowatt-hours (kWh) your home  consumed each month, not how much you spent on electricity (although that number is important).  It’s a good idea to have this information handy before reaching out to solar contractors, since it’s the first question 99% of us are going to ask.

Haven’t been in the home for 12 months?  That’s okay.  Just give us what you have.  We’ve collected so much usage data over the last 10 years that we can easily model the missing months with confidence and accuracy.

Many utilities give access to detailed consumption data that can easily be exported.  You should be able to find this information when you log in to your online utility portal.  If you’re old school, we can go through your bills and gather the data that was as well.

Once we understand consumption patterns, we can begin reverse-engineering how many solar panels you need to eliminate your electric bill.

This is where our Design Process shines.

The Design Process

Great data leads to great design.  A Professional Site Assessment focuses on four critical areas:

  • A detailed set of photos of the home, roof, electrical equipment location(s) and any potential shade obstructions
  • Map and measure each roof plane, including the slope and directional azimuth
  • Document the current condition of the roof, checking for damage and potential areas that need repair
  • Document the existing electrical equipment at the point of interconnection (code changes every 3 years)


When calculating how much solar a home needs, we’re also considering factors like your utility rebate guidelines,  and taking into account any potential changes to your future usage patterns.  Are you considering an add-on to your home?  How about a new pool or hot tub…or maybe you’re going to be empty nesters fairly soon.  These are all things that will come to light during the initial conversation with your Account Manager well before your project lands on a designer’s desk.

Solar panels work for over 25 years.  Not taking these factors into consideration could mean over- or under-sizing your new solar system.

Now that we’ve settled on your goals and have a clear target to shoot for, we start laying panels on the roof inside some specialized software, working towards your offset target.  We focus our efforts on South facing roof planes first, then West facing and East facing roof planes, in that order.  If you have preferences, we’ll certainly take those into consideration, with a watchful eye on the overall aesthetics of the array(s).

Not every company has the same design philosophy.  At Longhorn Solar, ours is based around our Mission Statement, which focuses on providing the best value to our customers.  How that translates to design is simple.  We always utilize equipment that provides the best balance of quality and price.  We don’t come out of the gate with higher priced high wattage modules unless that is what the project calls for, and sometimes it does.

More often than not, we can achieve a customers goals with lower wattage modules, which cost less while providing excellent quality.  They’re also manufactured by some of the largest and most reputable companies in the solar industry.

How Do We Estimate Solar Production?

We’ll have a pretty good idea of how many solar panels it’s going to take to achieve your offset goals.  To verify everything though, we use some industry software to check our numbers, and verify our design.  One of the most common tools is readily available to the public.

PV Watts Solar energy Calculator from NREL

The National Renewable Energy Labs (NREL) is a national laboratory of the U.S. Department of Energy, and they maintain (via the Alliance for Sustainable Energy, LLC) a tool called The PVWatts Calculator.  It is the predominant software used in the solar industry for estimating solar production, and is the recognized tool used by most utilities in Texas.

PVWatts was developed NREL, allowing homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations at specific locations based on the project zip code.  It knows, based on 25 years of historical data, what the average sun hours are for where you live and then models how much energy the proposed array will produce.

Once we verify our estimates and finalize the design, it’s on to Proposal Creation, where we bring it all together so we can come show you what your new solar system is going to look like, how much it will cost, and what the environmental and financial benefits are, specific to your home.

Steve Petrik has served in the solar industry for over 10 years.  He holds a NABCEP Technical Sales Professional certification (#TS-102415-012889), and sits on the board of the San Antonio Solar Alliance.  For questions, comments or feedback, you can reach him via email at steve@longhornsolar.coT