How to Calculate Solar Power System Size

solar system size

The idea of switching to solar energy is incredibly attractive for its environmental benefits and cost savings. But before you dive in as a homeowner or business owner, there are many factors to consider. Several elements will influence the installation of a solar system.


Important factors include your home or office building’s location, energy consumption, and desired energy offset, to name a few.


Needless to say, accurately calculating the best size of your solar system is crucial for efficient solar adoption. Why? Because it will determine how much energy the system will generate and how much you can use.


In this article, we explore the various considerations that influence the size of your solar panels system. The step-by-step guide breaks down how to calculate your solar panels system size.

Step 1: Understand Solar Power Constraints

When planning to install a solar power system, it is crucial to consider your budget and space constraints. You must ensure that you can achieve the most efficient and cost-effective solar system installation for your home or building.


Optimize available space by installing solar panels on rooftops, vertical walls, or even your lawn to maximize energy generation. Of course, the best setup depends on the characteristics of your property.


While determining your budget, also consider the scope for future expansion.

Step 2: Assess the Level of Sun Exposure

The availability of sunlight in your area will impact the output of your solar panels. Here are a few resources to estimate your home’s level of sun exposure:

Solar Resource Maps and Data from NREL

The geospatial data science wing of The National Renewable Energy Laboratory (NREL) has solar maps and images pertaining to the yearly and monthly average solar irradiance across the United States.


Irradiance data is vital to calculate the energy output (in kWh) of your solar system.


The formula is: E = A x r x H x PR




A is the total area of the solar panel,

r is the solar panel yield,

H is the average solar radiation,

and PR is the performance ratio (a constant).


You should also look at peak sun hours in your area to know when your solar panels will receive maximum sunlight.


PVWatts is a free online tool designed by NREL. It calculates the estimated energy production of grid-connected PV systems (solar systems) across the world based on historical irradiance data.


You must provide the size, tilt, and other parameters of your solar system to get an accurate estimate. This means you’d have to reverse engineer the tool to arrive at a desirable solar system size estimate.


That said, your solar system company can help you calculate the best solar system size for your home or building.

Step 3: Determine Your Energy Usage

Track at least a year’s worth of energy bills to determine your general electricity consumption in kWh. Get the average figure for each month, too: take the sum of 12 months’ usage and divide it by 12. Then divide the result by 30 to get your household or office’s average daily usage in kWh.


When you have these numbers, you can determine how much your solar system can offset your regular power consumption. Do you want to install a system that will take you off the grid entirely? Or are you alright with just covering around half of your consumption? Your budget and solar system size will interlink with your power coverage.


Now, you can calculate the kWh output of one solar panel and estimate how many panels you will need to offset your usage.


From the panel manufacturer or solar system installer, you can determine the area of a single panel. Correlate the same with your roof or wall to determine how many panels they can accommodate. Then, you can arrive at the maximum estimate of how much solar energy your hypothetical system can generate.


Don’t forget to consider the cost of installation with your budget.

Solar Power System Size Calculation Example

The wattage of one solar panel ranges from 250 to 400 W. Multiplying that with average peak sun hours will give you the daily Wh output of a single panel.


For instance, if a 300 W solar panel receives 6 hours of peak sunlight daily, its maximum theoretical output is 360 Wh of power every day. Dividing it by 1000 would give the kWh figure, which is 1.8 kWh/day.


Multiplying that by 30 gives you one solar panel’s monthly average kWh output (54 kWh). You can then correlate this with your monthly energy usage in kWh.


The average monthly energy consumption of households in the US is about 886 kWh. By these numbers, you will need 886/54 = 16.4 panels to fully offset your monthly power consumption.


Given that a typical residential solar panel is 17.5 square feet, you will need (16.4 x 17.5) =  290 sq. feet of solar panels. This translates to the solar system occupying a small portion of the typical 1700 sq. feet of the roof area of homes in the US.

Other Factors that Affect the Size of Your Solar Panels System

The above example is largely theoretical. Many factors will affect the size of your solar panels system and its productivity:

Roof Characteristics

A roof with a larger surface area and a sun-facing orientation will generally allow for more solar panels to be installed. On the other hand, a roof with shading from trees or other buildings may reduce the available space for panels.


As such, consider the orientation of your roof while trying to determine the size of your solar system.

Solar Power Battery Storage

If you want to use your solar system to provide power when the sun isn’t shining, you will need to include a battery storage system. This battery will ensure you won’t lose power in case of outages in your area.


Adding a battery to your solar system will affect its productivity and price.

System Losses

Energy losses can occur in any solar system due to wiring, inverter efficiency, module mismatch, and other technicalities. These losses can reduce the amount of energy your solar system produces and should be considered when calculating the size of your system.

Net Metering Program

Under net metering, any excess energy generated by a solar system is fed back into the utility grid, and the owner of the solar system is credited for it. As such, it can influence how much solar energy you will want to generate and may factor into the size of your solar system.


Check with your local utility company to understand the specifics of their net metering program and how you can take advantage of it.

2024 Update

With the advancements in solar technology, accurately calculating the size of your solar system has become easier and more precise. Modern solar panels are more efficient, meaning you can generate the same amount of power with fewer panels. Additionally, the availability of advanced solar calculators and tools allows homeowners to input their specific energy consumption data to get a tailored system size recommendation. Considering local solar incentives and net metering policies in 2024 can also impact the optimal system size, potentially reducing the overall cost and improving the return on investment.


At the end of the day, the best solar system size for your home or commercial building depends on your energy usage, offset preference, and roof structure. It’s important to work with a professional solar installer who can help you design and implement the optimal solar system for your needs. This way, you can ensure that it is appropriately sized to provide reliable and cost-effective renewable energy for your home or business.

Authored by Ryan Douglas

Authored by Ryan Douglas

NRG Clean Power's resident writer and solar enthusiast, Ryan Douglas covers all things related to the clean energy industry.