3 ways to calculate solar system size for your home



3 ways to calculate solar system size for your home


Solar system size is the first thing to look for when researching or buying a solar panel system for your home. That’s because it's the size of your solar system – not the solar panel efficiency or brand of equipment – that plays the biggest role in determining how much solar power you’ll generate, as well as how much the installation will cost you. 

So how do you calculate the right solar system size for your home? Read this article to find out.

Use the SolarReviews calculator to see recommended solar system size, costs, and savings

Key takeaways

  • The term “solar system size” describes a solar panel system’s capacity to produce electricity.‘Solar system size’ refers to installed power generation capacity, i.e. the maximum power output the system is designed to produce.
  • You can calculate solar system size by using PVWatts, the SolarReviews calculator (recommended), or by asking a local solar company.
  • Calculations must account for your usage, your roof’s characteristics, sunlight and weather where you live, and whether or not you’ll add solar battery storage.
  • Average system size varies from state to state, as does average power generation per kW of installed capacity.

On this page

    What is solar system size, and why is it important?

    The term ‘system size’ is used by the solar industry to describe how much power a solar panel system can generate. 

    When we say that a system is 8 kilowatts (kW) in size, we mean that it will produce a consistent 8 kW of solar power under Standard Test Conditions. [1 kilowatt (kW) = 1,000 watts (W)]. 

    System size also tells you how much solar power will be produced under real-world conditions at a specific location, such as the roof of a home. For example, an 8 kW system would generate an average of 40 kilowatt-hours (kWh) of power on my roof. On your roof, a 8 kW system could generate anywhere from 24 kWh to 60 kWh of average daily power output, depending on where you live, and your roof’s layout. 

    Most solar panel systems used in homes today are between 4 kW and 16 kW in size. To measure system size, you simply add up the wattages of all the solar panels in the system. If a system uses ten 400-watt solar panels, for instance, it has a system size of 4 kW. If 20 of those panels were used instead, the system would be 8 kW in size. 

    Solar panel installation costs are largely dictated by system size. For example, a standard solar panel system that is 8 kW in size costs about $24,000 on average. There are other factors that affect costs – choice of brands, your location, your roof’s layout – but system size is the single biggest one. 

    All said, solar system size will tell you:

    • How much it will cost you to go solar
    • The number of solar panels you’ll need
    • The amount of roof space required
    • How much solar power you will produce
    • The total value of incentives and rebates
    • Your annual utility bill savings

    What are Standard Test Conditions (STC)? All solar panels are assigned their power output rating – AKA size, wattage, or nameplate capacity – under lab testing performed under Standard Test Conditions. This involves shining exactly 1,000 watts per square meter of sunlight on the solar panels, and operating them at a consistent temperature of 77°F (25°C).

    1. PVWatts

    PVWatts is a free online tool developed by the National Renewable Energy Laboratory (NREL), a national lab of the Department of Energy. PVWatts draws on historical solar irradiance and weather records to perform photovoltaic (PV) analysis on buildings around the world. 

    pvwatts calculator

    Based on your location and the building’s characteristics, PVWatts provides a highly accurate breakdown of system output down to the hour. 

    It’s a powerful tool, and its calculations are used to develop advanced solar estimates and analyses by solar companies, as well as consumer-facing tools such as the SolarReviews calculator

    PVWatts’ biggest flaw is that it doesn’t actually calculate solar system size for you - you’ll have to use the tool to do it yourself. This takes a bit of reverse engineering, requiring you to take your total annual energy usage and divide it by the amount of solar energy 1 kW of panels can make.

    PVWatts pros and cons
    Pros Cons
    Government system powered by millions of data points Difficult to use for the average consumer
    Free to use Calculating system size requires some reverse engineering
    Works globally Lacks system price information
    Trusted by the solar industry Lacks financial analysis

    2. SolarReviews calculator

    The SolarReviews calculator is designed for easy use by homeowners. 

    It generates a ballpark solar estimate by using a vast database of utility rates and electricity usage. 

    It tells you all the important things you need to know to go solar - at a relatively high level of accuracy - based on just your address and your monthly power bill.

    SolarReviews offers several major advantages over PVWatts, including:

    • Ease of use: The calculator quickly tells you the system size recommended for your roof 
    • Provides cost estimate: Shows you the estimated cost to buy recommended solar system size with either cash or a loan
    • Option to see live prices: The SolarReviews calculator allows you to see live prices from pre-screened solar installers in your area

    No other solar calculator available to consumers online offers this combination of accuracy, functionality, and ease of use. To try the SolarReviews calculator, just enter your address into the box below.

    Use the SolarReviews calculator to see recommended solar system size, costs, and savings
    SolarReviews calculator pros and cons 
    Pros Cons
    Easy to use Only available in the U.S.
    Can calculate system size with just your average utility bill amount Only works for homes (i.e. it doesn't cover commercial buildings)
    Recommends system size for 100% bill offset (maximum savings)  
    Calculations based on data from PVWatts and NREL-audited Genability  
    Uses database with information on tens of thousands of utility rate plans  
    Provides cost estimate and pricing information for recommended system size  
    Provides financial modeling (cashflow, payback period, etc) for recommended system size  
    Shows all incentives and rebates available to you  
    Trusted by tens of thousands of consumers each year  

    3. Solar company

    The third way to calculate system size is to ask a solar installation company to do it for you. 

    rooftop solar installers

    Solar companies can combine tools like PVWatts with an in-person assessment of your roof to develop the most accurate projection of solar panel production. A reputable solar company would be happy to calculate the right solar system for your energy needs and provide a free quote for you to consider. 

    To make sure you only talk to solar companies with a strong track record, use SolarReviews to find the best solar companies in your area. 

    If you’re a savvy consumer who likes to be prepared when talking to vendors, here are a couple of easy things you can do: 

    Pros and cons of using a solar company
    Pros Cons
    Most accurate calculations Require at least one year of electricity bills 
    Can adjust calculations based on your requirements Many people prefer to get an estimate before talking to a solar company
    Knowledge of local rules and regulations  
    Includes financial modeling for system size  
    Can provide you a quote for a solar installation  

    What factors determine the right solar system size for you?

    There are five key factors that are used to calculate solar system size: 

    • Usage
    • Available sunlight
    • Solar insolation and weather
    • Roof characteristics
    • System losses
    • Battery storage


    Your home’s energy usage is the most important factor in determining what size solar system you need. 

    Many homeowners aim for a solar system size that allows for 100% electricity usage offset. In simple terms, this means your solar panels will be able to cover all of your electricity needs, thus slashing your utility bills and maximizing your savings

    Of course, the higher your energy usage, the larger the solar system size required to achieve, or at least get close to, 100% offset. 

    As such, it’s important to know what your energy usage is when calculating the right solar system size for your home. Here’s how to do that:

    • SolarReviews calculator - tell it your average monthly power spend: The SolarReviews calculator only needs your monthly electric bill – even an estimate will do. It then uses data on local utility rates and energy usage patterns to estimate your annual usage, and estimate the system size required to achieve 100% offset.
    • Solar company – give them a year’s worth of utility bills: The solar company will use your electric bills to establish your historical energy usage and then propose a system that matches that.
    • PVWatts – calculate usage yourself: Unfortunately, PVWatts doesn’t calculate electricity usage. You’ll need to check your utility bills and total up a year’s worth of electricity usage in kilowatt-hours, and then perform the appropriate calculations to come up with the system size you need.

    Available sunlight

    Sunlight availability can vary greatly from place to place. Take, for instance, Tucson, Arizona, and Seattle, Washington. Comparing how much sunlight each city receives – measured in peak sunlight hours (PSH) – we see that sunny Tucson gets almost 8 PSH, roughly twice the 4 PSH of sun received by Seattle.

    That means that, all things equal, a solar system in Seattle will need to be twice the size of one in Tucson to achieve the same solar power output!

    Any method you choose to calculate solar system size must account for sunlight availability in your specific location. Luckily PVWatts, SolarReviews, and your local solar company all rely on data from the National Solar Radiation Database, which has years of detailed solar radiation and meteorological data from every part of the country.

    Roof characteristics

    The orientation (‘azimuth’) and pitch (‘tilt’) of your roof will determine the direction and angle of your solar panels. Both factors, especially direction, can have a considerable impact on solar energy production.

    A solar array facing east or west will produce approximately 15% less output than one facing south. If you can only install on an east or west-facing roof, you’ll have to increase the size of the solar system to account for that. 

    System losses

    You will not have access to the full DC output of your solar panels; that’s because all solar power systems experience system losses.

    There are typically wiring losses of about 2% during the transmission of solar power from the solar panels to the solar inverter. There’s an additional ‘conversion loss’ of 3% when the solar inverter changes the power from DC to AC electricity. That means that if you install a typical grid-tied solar power system, only about 95% of the solar power generated will be available for use in your home.

    Battery storage

    The addition of battery storage to a solar power system can also have a considerable impact on solar system size. Exactly how much it will affect the solar system size you need depends on the following factors:

    Amount of backup power required 

    Do you just want to have a few hours of emergency lights? Or do you want enough solar-plus-storage to power your entire home for 24 to 48 hours if there’s an extended grid outage?

    The former won’t affect system size too much, but the latter option will require you to have a system size that’s large enough to cover your daily energy consumption and fully recharge your batteries, even on days with low solar production (i.e. a cloudy day in winter). 

    Battery efficiency 

    There is a small energy loss every time electricity enters or leaves a battery.

    DC-coupled battery systems like the LG Chem Resu are the most efficient. They have an average round-trip efficiency of roughly 95%, which means only 5% of power is lost during each charge and discharge.

    AC-coupled batteries, such as the Tesla Powerwall, see greater losses – with round-trip efficiencies of around 90%, they lose 10% of power when power moves in and out. 

    You will need to compensate for these losses by oversizing your solar panel system accordingly.

    Average system size and power output by location

    Home solar panel systems are between 6 kW and 12 kW in size in most states. 

    System size differences between states are explained by factors such as energy usage patterns and local weather conditions. 

    In places like California, for example, household energy usage is relatively low, while sunlight availability is excellent. Homeowners here can make do with smaller-sized solar panel systems, and the average system is 7.33 kW in size.   

    In Texas and Florida, meanwhile, homes tend to be bigger, and households consume more energy. As a result, homeowners here tend to install much larger systems, between 11 kW and 12 kW on average.

    Average solar system size (power capacity) in selected states
    State Average system size  Avg. daily power generation* Annual power generation
    California 7.33 kW 31 kWh 11,326 kWh
    Texas 10.32 kW 42.3 kWh 15,441 kWh
    Florida 10.75 kW 43.6 kWh 15,939 kWh

    *Daily kWh generation figures can see large variations between seasons, and even from day to day

    The system sizes and output figures are just system averages for these states, that is California, Texas, and Florida

    Your system size and power generation could be very different, even if you live in one of the states shown above. This could be due to regional weather variations, or the unique characteristics of your roof. 

    Don’t worry if your requirements are different from what’s typical in your state or city – the three calculation methods we’ve described above will account for your unique situation, and there’s a good chance that solar panels will still be worth it for you.

    Space requirements based on solar system size

    Lots of homeowners looking to install solar panels are curious about the physical size of the solar panel systems. 

    This is important information, as it lets you check if your roof is big enough for a solar panel installation. 

    Here’s how many solar panels and how much square footage of roof space is required for average solar panel installations in CA, TX, and FL.

    Space required for an average solar system
    State System size  Panels required Space required
    California 7.33 kW 19 392 sq ft
    Texas 10.32 kW 26 558 sq ft
    Florida 10.75 kW 27 577 sq ft

    Assuming the use of 400 W solar panels.

    There are a couple of important things to note here:

    • Solar panel efficiency affects your space requirements – more efficient panels use less space to produce the same amount of power.
    • You’ll need additional space for access and setback, i.e. how big a pathway you need to keep open to the panels, and how far they must be from the peak of your roof. These rules are established for firefighter access and general safety. Your AHJ (Authority Having Jurisdiction) will be able to tell you what the rules are in your area.
    Find out how much you can save annually by switching to solar

     - Author of Solar Reviews

    Zeeshan Hyder

    SolarReviews Blog Author

    Zeeshan is passionate about promoting renewable energy and tackling climate change. He developed these interests while studying at beautiful Middlebury College, Vermont, which has a strong focus on sustainability. He has previously worked in the humanitarian sector — for Doctors Without Borders — and in communications and journalism.

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