How to Calculate Your Solar Panel System Size

One of the most common questions we get from our customers pertains to how to calculate their solar panel system size to meet their energy needs. Matching your solar panel system design/configuration to match your energy usage will produce maximum cost-efficiency. 

Let’s explore how to calculate your power needs, how to estimate power output from solar panels, and how to reconcile those to find the right size system for your home or business.

Close up of man technician in work gloves installing stand-alone photovoltaic solar panel system under beautiful blue sky with clouds. Concept of alternative energy and power sustainable resources.

Bear in mind that there’s a little bit of math involved – but we’ll provide some online tools to make the calculation work as easy and accurate as possible.

Figuring your energy usage

The first thing you need to do is determine how much power you typically use in your residence or commercial property. Here’s how to do that:

  • Get a copy of your last 12 energy bills dating back a year. Your energy use is presented as kilowatt-hours (kWh). This is the key metric to pay attention to.
  • To identify your individual kWh use, your monthly power bill will have a section titled “energy usage history” or a similar phrase. Your energy use will be presented in graph form using kWh. If Florida Power & Light Company is your provider, use this guide on how to read your energy bill
  • Calculate your average monthly power consumption. This can be done by adding all 12 months’ kWh together and dividing by 12. This is important because power usage tends to rise in winter and summer relative to spring and fall (due to air conditioning and heating).
  • Calculate your daily energy usage. Divide the monthly kWh consumption by 30 to determine your energy needs on a day-to-day basis.

Calculating peak sunlight

Knowing how much peak sunlight you get in your area – which varies widely by latitude and climate – is important. Fortunately, in the Sunshine State, we get a lot.

Use a free online peak sunlight calculator for convenience.

Calculating solar panel system size

Now that you’ve got your daily kWh power usage and your peak sunlight hours, plug those numbers into the following equation to determine your ideal solar panel system size:

Daily kWh ÷ average sun hours) x 1.15 efficiency factor = DC solar system size

Using a PVWatts® Calculator to determine your system’s power output

The PVWatts® Calculator is a free, handy online tool developed by the US National Renewable Energy Laboratory that you can use to calculate your system’s estimated power output in kilowatt-hours on an annual and month-by-month basis.

You can adjust the estimates based on the features of your property and system, including:

Here’s a great tutorial to learn how to use and navigate the platform:

Ideally, your system’s estimated power output will meet or exceed your kWh energy needs.

Factors that affect solar panel power output

Aside from the most obvious element – available sunlight — the following factors affect solar power output:

  • Available roof space. The roof is the ideal location because it is the most cost-effective, usually the most sun-exposed, and the easiest rack to install. But if you don’t have enough roof space to match your energy needs, alternative mounting options may be viable.
  • South-facing roof space. Roof space facing southward captures the maximum amount of sunlight throughout the day. If you don’t have south-facing roof space, you will need extra solar panels to make up the difference.
  • Roof angles.
  • The type and quality of solar panel (standard vs premium efficiency vs. thin film)
  • Climate. Solar panels perform optimally in colder weather compared to warmer weather.
  • Shading. Trees or other obstructions may cast shadows on the solar panels at different times of day and hinder their efficiency.
  • Solar panel efficiency

Let Compass Solar help you size your solar panel system

If all these considerations and calculations seem overwhelming, that’s understandable. For best results, rely on expert help to design your ideal solar panel system.

Compass Solar is here to walk our Northwest Florida neighbors through the design and installation process. Contact us to get started. Get the ball rolling with a free solar quote.

We’re the only locally owned and operated company equipping our neighbors with high-quality solar batteries and generators. To get an idea of the quality of our work and products, take a look at testimonials from previous Compass Solar customers