How To Calculate Solar System Size

Solaris National | Oct 2021
Updated: October 30, 2021
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Correctly Sizing A Rooftop Solar System For Your Home

This article is to help estimate the correct size of your solar panel system to provide sufficient solar energy for your home. You should be able to figure out how many solar panels you will need to match or exceed the electricity you use and what time of day you use it.

Click Here To Use Our Solar Calculator To Estimate Your: System Wattage Size, Number of Solar Panels, and Roof Space Required.

Things to remember as you read:

  • If you have been thinking about going solar on your home, you will need to know how much generation capacity your solar system will need to provide.
  • The two primary sources of information that you will want to have at your fingertips to determine the size of your solar system are:
    1. Your monthly electric bill for a period of one year.
    2. An energy audit of your home that includes all appliances that consume electricity.
  • Don't pay so much attention to the number of panels, instead, look at the solar system’s overall power generating capacity.
  • Because the cost of solar panels has dropped so much in the last few years, it makes sense to get as much as you can reasonably fit on your roof.

How To Calculate Solar System Size with

When we talk about sizing a solar voltaic system, what we mean is” what is the maximum electricity output a particular solar system for your home can deliver.” It's important to remember that this is not about the total number of solar panels you can fit on your roof, but it is about the total capacity that all the panels combined can produce.

It's not unusual for a home to use 20kWh per day, That daily amount would be well served by a 5kW solar system for your home. If you have a solar power plant on your roof that is made up of 20 solar panels and they each have a capacity of 250W (20 x 250W = 5000W), Or if you had 25 panels of 200 watts each, either way, you would end up with 5000 Watts or 5 kilowatts. The size of your system measured in watts is the number that matters in the end.


  • Understanding How Much Electricity You Really Use
  • Estimating The Size Of Your Rooftop Solar System
  • Realistically How Big Should Your System Be
  • Grid-Paired/Grid-Connected Versus Off-Grid Systems
  • What's Involved In a Solar Installation
  • Working With Your Installer
  • Advantages Of A Whole-house Battery Backup System​

Understanding How Much Electricity You Really Use

It's not possible to estimate the correct size of your rooftop solar system until you have a good handle on how much electricity your home currently uses. Fortunately, the electric utility has given us the tools we need to accurately determine our electricity use. Looking at your past electric bills, you will be able to accurately determine how much electricity your home has consumed in the past month, the past quarter, and the past year.

Using these figures, you can easily determine how much electricity you use on average every day. If you have a smart meter installed, it's an even easier process. You can see how much electricity you use on a daily basis by checking your utility account online. Your consumption of power is always measured and invoiced using kilowatt-hours (kWh).

An average home in the United States uses 25 to 35 kWh per day. But of course, different homes will vary significantly in the use of their electricity. For instance, a single person living in a smaller home could use as little as an average of 8 kWh per day. On the other side of the coin, a household with 5 or 6 people with a pool, a dryer, and multiple electronic devices might use around 50 kWh per day.

Electricity Use Measured Daily And Seasonally

An important factor to consider is when you are using your electricity. If you have a home that is usually empty during the weekdays (because everyone is at work or school) you would expect that most of your power consumption happens at night time.

If that is the case, solar panels on your roof might not be used to their highest value, because they generate power during the day, when there is sunlight. If this is the case, you would probably want your solar panels to store the excess electricity by charging a battery bank or be exported to the grid to be used later at night.

Another thing to be aware of is that some days regularly consume more power than other days. For example, sometimes weekends are big power days because everybody is at home. But there are some houses that are empty during the weekends because those people go away on the weekend. You also need to consider that seasonally, some periods consume a lot more power than others. For instance, in the summer we use air conditioners and in the winter we use heaters.

All of these different factors need to be considered so you will have an accurate understanding of your power usage each day, on peak days, the times of the days, and seasonally. Once you have a good grasp of how and when you use the most electricity, you can begin to engineer a solar rooftop system that will work for your situation.

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After collecting the numbers from your utility bill and energy audit, you should know how much electricity you normally use and the times of day, and what times of the year you use it. Now we can answer the question, “What size solar panel system do I need on my house?

Since the most common solar panel setup is a grid-tied/grid-paired system, we will use that configuration for our example. The biggest benefit of a grid paired system is that you have your solar panels generating your daytime use electricity and a connection to the electrical grid for energy supply during the nighttime (or other low generation days) when solar panels don't generate enough electricity. During these times, your house will draw electricity from the power grid.

It is possible for you to have a solar system on your roof that will generate enough electricity for your use, both day and night. However, this would require a sufficient battery backup system as part of your solar panel array.

Check out our Solar Battery Bank Calculator and Calculate your Battery Size Here

How Much Actual Electricity Should You Expect Per Kw That Your Solar Panels Produce?

In the Solar industry, all things electrical are measured in watts or kilowatts. You will notice that we describe systems as 5 kW,10 kW, or 20 kW. A kilowatt is not the same as a kilowatt-hour. A kilowatt is how we describe the size of a photovoltaic system - and a kilowatt-hour is the measurement we use to describe the amount of electricity you are using at a particular time.

A good rule of thumb is approximately 1 kW of solar panels = 4 kWh of electricity generated per day. That means for each KW of your solar panel array, you should estimate that you will get about 4 kWh per day of electricity production. If you have a 5 KW solar system, you will produce around 20 kWh (kilowatt-hours) of electricity.

This describes the production on a good solar day which means you would have a lot of sunshine on your panels and it's not extremely hot (solar panels do not work at peak efficiency when they become hot).

The actual amount of power that is generated per kilowatt of solar panels depends on several factors: Things such as the location of your home, the season of the year, the amount of direct sunlight on your panels, shade, etc. Different geographic locations inside of the United States produce different amounts of electricity for the same kilowatts.

Since the cost of solar panels has dropped significantly in the last few years, it's probably a good idea to install a larger system than homeowners would have 10 years ago. That way you will ensure that you will have sufficient power even if you experience a string of heavy cloudy days.

Shift The Time Of Your Heavy Power Use

Because you're probably installing solar on your house to realize a decrease in the cost of electricity it makes sense to make the best use of the solar power at peak production times. Try to shift your electricity-intensive activities to those times when your solar panels are producing the most electricity.

Activities that you would normally do at night that are high power-consuming activities should be shifted to the daytime, if possible. These include things such as running the dryer, the pool pump, and pretty much anything that uses a lot of electricity. On the things such as your dishwasher or washing machine, using a timer or the machine’s delay function to put off the “start” can save you from using electricity from the utility.

Using Online Solar Calculators

There are available several useful online "solar calculators" that can help you estimate the best size of your solar system. Look at our solar panel system size calculator here. A good calculator like this will estimate your system size correctly to give you the answer.


Grid-Paired/Grid-Tied Or Grid-Connected

These two types of solar systems are the most common in the United States. Both of these systems have solar panels on the roof or are placed on an unshaded area of the ground, and they are connected to the utility power grid. With both of these systems, the solar panels convert daylight into electricity. The home uses this electricity and in cases where it is insufficient, it draws power from the electric grid. This power shortage most commonly happens at night times but can be an issue at other times when the panels can't produce enough power for your home. Times such as extended periods of cloudiness and other low sunlight time periods.

A grid-connected system can be configured with whole-house batteries or without batteries for storage.


The off-grid system is a completely autonomous house that does not draw any electricity from the electrical grid. With this setup, all of the power that the home uses comes from the solar array on the roof. It is possible to tie in other types of power generation like wind, or standby generators. Off-grid systems come with significant battery storage capacity. The batteries are used to capture and store excess electricity during the day which would be used during the night or during low solar productive days.

Standby generators are generators that sit alongside the home and instantly spring to life in the case of a power outage. These types of generators are usually diesel-powered or natural gas-powered. The downside to standby generators is that they use fuel which is usually difficult to obtain during a widespread blackout.

Off-grid systems are usually more expensive than grid-connected systems because of the cost of the storage battery capacity. While battery technology is getting better every year and their costs are declining, batteries are expensive and they have a shorter life than solar panels. Traditionally, off-grid systems have been the choice of people with remote properties where it is expensive to make a connection to the grid. These types of installations are becoming more common because of the power outages we have seen across the nation recently - especially in Texas.


Roof Area

One of the first things a solar engineer will look at is the amount of available space on your roof to put the solar array. Most single-family homes have enough roof area to install the panels that that family would require. The most frequent complication to the roof areas is that shade might fall on the roof. Shading from nearby trees or other buildings can significantly reduce the efficiency of a solar array. However, if your roof has insufficient space for the panels that you need, you might want to consider a “​ground mount”.

solar panel ground mount

A ​ground mount is an installation that uses an open area alongside the home to put the panels. The panels are put on racking which raises the panels off of the ground.

Roof Orientation

Because we live in North America, it is best to have your solar panels on a south-facing roof. This will provide them the most amount of sunlight during the day. Of course, not all homes were built with solar in mind and a south-facing roof might not exist. Your solar installer will be able to measure the orientation of your roof to see if there is sufficient sunlight to provide a short payback period.

There are situations where a mix of panels facing east and west can provide the most efficient configuration. These panels might have a lower amount of direct sunlight in the middle of the day but can produce electricity in the morning as well as the afternoon. if your power use is higher at those times, this kind of setup might make more sense.

The Solar Professionals

Calculating just how much sunlight you will need for your solar panels is not always a simple matter. Neither is it to figure out how much space is usable on your roof for panels. Most reputable solar companies will have an engineer who can do these calculations for you and provide you with a written estimate. Normally, these estimates include the cost of the system, any financing you need, and the payback period.

The payback period is that point in time when the savings from the solar panels is enough to pay for the solar installation. The payback period is normally five to 10 years. After that point in time, all of your electricity is free.

We hope that this article has been helpful for you and has helped you to find a solution to calculating your solar system size. If you're interested in getting solar panels for our roof try our short solar quiz here to see if your home is fit for solar panels.

This article is to help estimate the correct size of your solar panel system to provide sufficient solar energy for your home. You should be able to f...
October 20, 2021 | Solaris National
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