How Many Solar Panels Do I Need?

How Many Solar Panels Do I Need?

When we talk about solar power, the first thing that comes to mind are the solar PV panels, the central component of your solar power system. When determining to build a solar energy system, the first question pops up is how many solar panels do I need?

Well, it depends on five key factors:

  1. Amount of energy you use on an average day
  2. Peak sun hours in your city
  3. Roof space available to install solar panels
  4. Capability of your energy storage system
  5. Your budget

Amount of energy you use on an average day

View your past utility bills

To determine the amount of energy your home uses in a typical month, week, or day, look on your bill for "kilowatt hours (kWh) used" or similar. Usually, this is the figure for 30 days’ usage, so just divide it by 30 to get a rough estimate of average daily energy consumption.

Check your meter readings

Alternatively, you can take a look at your electricity meter readings. Take the most recent figure and subtract the previous one. Then divide the result by the number of days between the two readings, and there you have an average daily consumption figure. If you can, we suggest doing this several times, between multiple pairs of readings, to get a more accurate average.

Use formulas to calculate manually

For a more thorough look at your usage, take the following steps:

Firstly, find out the wattage of each of your devices.

Most electronic devices have a label which details their wattage, typically found on the bottom or back; alternatively, you can see this information in the user manual.

If you can't find the wattage information, here are three ways to figure it out:

1) Use this formula to calculate it: watts = amps x volts

2) Use a wattage measuring tool, watt meter or multimeter

3) Contact the manufacturer with your device's model number

4) Search your device online

Next,calculate watt-hours per day:

After figuring out the wattage of your electronic device, multiply it by the number of hours you use it in a day to get the watt-hours. For example, a 150-watt television is used for three hours per day:

Wattage of TV (watts) x hours TV is used per day= watt-hours (Wh) per day for TV

150 wattsx 3 hours= 450 Wh/day

At last, how many watts are in a kilowatt?

You'll notice that the electricity on your bill is measured in kilowatt-hours (kWh), not watt-hours. And one kilowatt equals 1000 watts. Take our TV that uses 450 Wh per day:

watt-hours (Wh) / 1000 = kilowatt-hours (kWh)

450 Wh/ 1000= 0.45 kWh

So, using a 150-watt television for three hours a day will consume around 0.45 kilowatt-hours of energy per day.

When calculating your overall household energy consumption, we recommend making a list of all the appliances and devices you plan on running and then using the above formula.

Use Renogy Super Solar Calculator

Figuring out energy usage can be tricky to get right. We wanted to make this process quick and painless for you, so we developed the

 Renogy Super Solar Panel Calculator

Renogy Super Solar Calculator is an intelligent calculator system that will help you estimate how much energy you need to power your RV, home, cabin, or boat with a super easy-to-use interface.

Just select the appliance, tell us how long you use it on an average day, enter the wattage, voltage, and current, and we’ll do all the math. Can't find the device you're using? Just enter its details to create a new record. Once you’ve told us about all your appliances, you’ll get your daily energy consumption result.

 

Solar powered calculators can tell you the minimum and recommended system size, as well as the recommended battery output. Additionally, this solar panel calculator automatically creates a custom wiring diagram based on your power needs.

Peak sun hours in your city

The second factor which determines how much you can get out of your solar panel system is the amount of sunlight your solar panel system can absorb where you live.

“Peak sun hours” (also known as PSH) is not the same as “hours of sunlight”. Hours of sunlight refers to the total number of hours from sunrise to sunset, while peak sun hours is the intensity of sunlight in a specific area. It’s defined as an hour of sunlight that reaches an average of 1000 watts per square meter (around 10.5 square feet).

Why should we use peak sun hours to calculate power generation instead of hours of sunlight? The rated power wattage of every solar panel you purchase is tested under conditions of solar irradiance of 1000W/㎡. So, the solar panels can perform at their maximum effectiveness and generate maximum electricity when the sun's intensity is highest.

Average peak sun hours by state

In the U.S., peak sun hours vary by state. Many states have the same sunlight hours, but some have five or six peak sun hours per day on average, while some only have three. The number of peak sun hours will increase the closer an area is to the equator and, more generally, during the summer months.

Here are the average peak sun hours per day of each state:

*Data comes from  TurbineGenerator

How will peak sun hours impact my solar panels?

When you know the peak solar hours of your city, you can quickly calculate how large your solar panel system needs to be. For example:

Your home uses 10 kWh per day.Peak sun hours in your city are 5 hours.

10 kWh per day÷5 peak sun hours per day= 2 kW

You need to install a 2 kW solar panel system, at least, to satisfy your daily energy consumption.

 

However, peak sun hours occur on sunny days; what about rainy or snowy days? If you’re planning to live completely off-grid, your power storage system, usually a battery bank, should be larger than your daily energy consumption to back up your power needs when the sun isn’t shining. This means more solar panels need to be installed to generate enough power to keep your battery system charged.

Roof space available to install solar panels

Before sizing your roof space, you need to evaluate the condition of your roof to determine whether it is suitable for solar panels.

Roofing structure and materials

A comprehensive roof structure analysis is crucial before a solar panel system installation. You need to know whether your roof is flat, sloping, or curved. Different roofing structures require different mounting methods and different types of solar panels. Flat roofs support the installation of rigid and flexible solar panels; but for curved roofs, flexible solar panels are the best option. You also need to examine the roof structure to see whether it can withstand the overall weight of the solar panels and the mounting systems.

The age of the roof

The majority of solar panels on the market have an expected lifespan of around 25 years, so it’s best if your roof can bear the weight of all the solar panels installed on it for that long. So, if you have any roof repair plans, we recommend finishing them first. Once the solar energy system has been installed, you will need to remove it in order to carry out any work on your roof, then re-install it; that will cost you more money.

Calculating the weight your roof needs to bear

Say your house consumes 5 kWh of energy per day, peak sun hours where you live are

five, and you intend to install

 Renogy rigid 320-watt solar panels(which weigh roughly 40 lbs per panel). First, figure out what size your system needs to be:

5 kWh/ 5 peak sun hours= 1 kW

Then calculate how many panels you’ll need:

1 kWx 1000/ 320W= 3.125

Obviously, you’ll need to round that up to the next whole number, which gives you a total of four panels required. So, then you multiply that by the weight of each panel:

4 x 40 lbs= 160 lbs

Don’t forget, finally, to add the weight of the mounting system, including wires, brackets, and connectors. It’s true: Analyzing your roof condition is not an easy job; for peace of mind, we recommend asking a certified expert to do a comprehensive assessment of the condition of your roof.

Usable roof space

It’s crucial to check the actual

usable space available on your roof for solar panels.

Air conditioners, chimney stacks, or windows all take up space. Plus, if part of your roof is shaded by trees or other buildings at all times, we don’t recommend installing solar panels there – not only will they not absorb enough direct sunlight to generate sufficient electricity, but panels which are shaded for long periods of time may suffer from the effects of ‘hot spots’, which can result in irreversible damage.

So, remember to subtract the shaded or otherwise unavailable space from your total roof space. Say you have 1000 square feet available, and you want to install Renogy rigid 320-watt panels:

Usable roof size:

1000 square feet

Panel size:

65.6”x 39.4”= 2584.62 square inches

Convert that to square feet:

2584.64 square inches/ 144= around 18 square feet

Number of solar panels you can install:

 

1000 / 18= around 55

It’s crucial to consider the length and width of the entire solar array system. We strongly recommend that you make an accurate drawing/plan of your rooftop layout, with the solar array, to ensure you don’t exceed the length and width available on your roof.

Capability of your energy storage system

In most solar power systems, energy generated by the panels is stored in a battery bank, which supplies your appliances through a power inverter.

There are multiple types of batteries on the market, including lithium-iron phosphate (LiFePO4) batteries, lead-acid batteries (Flooded, GEL, AGM), and ternary lithium batteries.

Your electricity consumption each day will fluctuate: It will never be as precise as your calculations. Additionally, all batteries have a healthy range of charging and discharging. Many people believe that they can discharge their lifepo4battery to zero capacity every time. In fact, battery professionals recommend against this, as it quickly reduces the battery's lifespan. Therefore, it's best to reserve some back up power for the LFP battery to meet your unexpected needs and to maintain a healthy battery lifespan.

Say your house consumes 10 kWh of energy per day, and you intend to build a 12-volt lithiumiron phosphate battery system with 12.8 V 200 Ah batteries:

Calculate the electricity available in a 12.8 V 200 Ah battery:

12.8 Vx 200 Ahx / 1000 = 2.56 kWh

Calculate the number of batteries you need:

10 kWh/ 2.56 kWh= 3.9 (round up to 4)

Next, you need to calculate how much energy the solar panels need to generate to fully charge this battery bank system:

4 x 200 Ah x 12.8 Vbattery bank system/ 1000= 10.24 kWh

This means that if you want to consume 10 kWh of electricity per day, you’ll need to install a solar panel system that can generate 10.24 kWh per day to fully charge your energy storage system and power your appliances. So, you need to use 10.24 kWh to calculate how many solar panels you need to power a house.

For more information about various types of solar panel batteries, check out related articles below:

 

Your budget

You buy solar panels on your budget, which means the amount you invest in building your solar power system will influence the number, quality, and type of solar panels you intend to purchase.

Monocrystalline vs. polycrystalline solar panels

The light blue polycrystalline panels are the cheaper option of the two. Dark grey – almost black – monocrystalline panels come in at a higher price as they are more compact but also more efficient than other panels with the same wattage.

Extended knowledge:

What's the difference between monocrystalline and polycrystalline panels?

Rigid vs. flexible solar panels

Rigid panels are the most common solar panels on the market. They are suitable for flat roofs. Made of multi-layered sheet laminate structures, they are durable enough to last for decades. They can be installed with tilt mount brackets to optimize angle for better sunlight absorption.

If you need to mount your solar panels on an uneven surface such as a curved roof, flexible panels are your best option. They weigh less than rigid solar panels and can be installed directly on the roof of your RV with glue, without drilling any holes; however, they cost nearly twice as much as rigid panels.

Extended knowledge:

 What are the pros and cons of rigid and flexible solar panels?

So, if you are planning an installation with limited flat roof space, we recommend you purchase monocrystalline panels because they are more energy- and space-efficient. If money is the driving factor, you may go with the less expensive polycrystalline panels. If the roof is not flat, we recommend flexible solar panels.

So, how many solar panels do I need?

Follow the guidelines above and you’ll get an answer that will help you determine how many solar panels should be installed to meet your electricity needs, whether it’s at home or out on the open road!

For your home we do recommend asking a professional expert to assess your roof architecture, angle to the sun, usable space, and other factors, to see how best you can install sufficient solar panels on your roof to reach your daily energy production goals. Then contact certified solar installers for installation.

For off-grid RV living, our

 Renogy Super Solar Calculator

is the best tool to help you determine what size system will best fit your needs, including the number of solar panels, batteries, charge controller, and inverter. A custom wiring diagram can be created automatically based on your power needs.

Where to buy solar panels?

Going solar is a great way to become energy independent, save money on monthly utility bills, and ensure reliable access to energy when you need it most, like an unexpected power blackout caused by a meteorological disaster. Check out the best solar panel kits on Renogy!

If you want more knowledge about solar panel, you might be interested in our other articles:

Solar Panels 101: A Beginner's Guide