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How To Figure Solar System Size

Factors That Affect Solar Panel Output Efficiency

How to Size your Solar Power System

All things being equal, the design of the solar panel determines how efficient it is. This, in turn, specifies how well each square foot can convert sunlight into energy.

For example, gridlines on most panels reduce the active surface area. Therefore, they are not as efficient as those without them. In addition, grid panels are susceptible to peeling, further reducing efficiency.

Quality plays a role, too. For example, if the construction is subpar and introduces corrosion and cracking, it can also impact the ability of the panel to absorb sunlight.

But as a rule of thumb, expect residential solar panels to give you between 150 370 watts. This translates to an average solar panels wattage per square foot of 15 watts.

Calculate The Cost Of Solar Panels

One standard solar panel produces about 1.24 kWh per day, and costs around 65 to 75 cents per watt. Solar panels from well-known manufacturers run as high as one to two dollars per watt.

Based on your results from the solar panel calculator above, you can multiply your recommended wattage by 65 or 75 cents to get a ballpark cost for all of your solar panels.

How Many Solar Panels To Power A House

When estimating how many solar panels your home could need, its important to consider the size of your home. An average homeowner would need about 28 to 34 solar panels to fully offset their electricity usage. The chart below is an estimate of the number of panels you could potentially need based on the size of your home.

*Data from the Residential Energy Consumption Survey.

How many solar panels do I need for a specifically sized solar system?

To determine the number of panels you need to achieve a given solar system size, divide it by the wattage of each panel .

For example, if youre aiming for a 4 kW system, youll divide 4 kW by 320 watts to get 12.5. Round up the answer to 13, which is the number of panels you need.

How many solar panels do I need for common appliances?

You can also determine the number of panels you need on a per appliance basis. This approach is helpful if you want to add panels because of increased usage or when buying a new appliance.

To calculate this, divide the average annual wattage of the appliance by the panel wattage. For example, a 600 kWh refrigerator would need two solar panels

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Solar Panel Output Per Square Metre

The most popular domestic solar panel system is 4 kW. This has 16 panels, with each one:

  • around 1.6 square metres in size
  • rated to produce roughly 265 watts of power

To work out the output per square metre, use this formula:

Number of panels x Capacity of solar panel system

Capacity ÷ Total size of system


  • 16 panels of 265 W each:
  • 16 x 265 = a capacity of 4,240 kW
  • Total size of the system
  • 4,240 ÷ 6 = 165 W per m2

Solar Panel Output Per Day

Off Grid Solar System Calculator Excel

Work out how much electricitymeasured in kilowatt hours your panels would produce each day by using this formula:

Size of one solar panel x 1,000

That figure x Efficiency of one solar panel

That figure x Number of sun hours in your area each day

Divide by 1,000

Read more on efficiency below.

To estimate the number of sun hours in your area, use this calculator.


  • Divide by 1,000:
  • 1,440 ÷ 1,000 = 1.44 kWh per day

*The number of sun hours varies greatly throughout the year , and will be much lower during winter months in particular.

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Choose The Right Solar Panels

If you have a small or odd-shaped roof, solar panel size is an important consideration when deciding on the size of a solar system. Take these factors into account:

  • With a large usable roof area, you can buy more larger panels to get to your target energy output.
  • If your usable roof area is limited or partially shaded, using fewer smaller high efficiency panels will ultimately be the most cost-effective, long-term solution. You can add more panels later on to accommodate increased energy needs.

Calculate Size Of Solar Panel

January 8, 2016

Calculate Size of Solar Panel, No of Solar Panel and Size of Inverter for following Electrical Load

Electrical Load Detail:

  • 1 Nos of 100W Computer use for 8 Hours/Day
  • 2 Nos of 60W Fan use for 8 Hours/Day
  • 1 Nos of 100W CFL Light use for 8 Hours/Day

Solar System Detail:

  • Solar System Voltage = 48V DC
  • Loose Wiring Connection Factor = 20%
  • Daily Sunshine Hour in Summer = 6 Hours/Day
  • Daily Sunshine Hour in Winter = 4.5 Hours/Day
  • Daily Sunshine Hour in Monsoon = 4 Hours/Day

Inverter Detail:

  • Future Load Expansion Factor = 10%
  • Inverter Efficiency = 80%

Step-1: Calculate Electrical Usages per Day

  • Power Consumption for Computer = No x Watt x Use Hours/Day
  • Power Consumption for Computer = 1x100x8 =800 Watt Hr/Day
  • Power Consumption for Fan = No x Watt x Use Hours/Day
  • Power Consumption for Fan = 2x60x8 = 960 Watt Hr/Day
  • Power Consumption for CFL Light = No x Watt x Use Hours/Day
  • Power Consumption for CFL Light = 1x100x8 = 800 Watt Hr/Day
  • Total Electrical Load = 800+960+800 =2560Watt Hr/Day

Step-2: Calculate Solar Panel Size

  • Average Sunshine Hours = Daily Sunshine Hour in Summer+ Winter+ Monsoon /3
  • Average Sunshine Hours = 6+4.5+4 / 3 =8 Hours
  • Total Electrical Load =2560 Watt Hr/Day
  • Required Size of Solar Panel = X Correction Factor
  • Required Size of Solar Panel = x 1.2 = 635.6 Watt
  • Required Size of Solar Panel = 635.6 Watt

Step-3: Calculate No of Solar Panel / Array of Solar Panel

If we Use 250 Watt, 24V Solar Panel in Series-Parallel Type Connection

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What About A Battery

A storage battery will capture the unused solar power generated during the day, for use at night and on low-sunlight days. Installations that include batteries are increasingly popular. See our case study of the first Australian home to install a Tesla PowerWall battery.

But for most homes, we think a battery doesn’t make economic sense yet. Batteries are still relatively expensive and the payback time will often be longer than the warranty period of the battery. However, it’s likely that battery technology and prices, together with future changes in how the electricity market works, will make batteries a good option for most homes within the next few years.

And remember: for most grid-connected systems, having a battery doesn’t necessarily protect you in the event of a blackout. You may still lose all power to your home, despite having solar panels producing power and a charged battery ready and waiting. This is because grid-connected systems have what’s known as “anti-islanding protection”. During a blackout, the grid and any engineers working on the lines must be protected from “islands” of electricity generation pumping power unexpectedly into the lines.

For most solar PV systems, the simplest way to provide anti-islanding protection is to shut down entirely. So, when it senses a grid blackout, your solar PV system shuts down and you have no household power at all.

How Many Solar Panels Will You Need To Offset Your Electricity Usage

How To Size Your Solar System

First I want to address most peoples motivation for going solar: I want to put on solar panels and get my electricity bill to zero!

Getting a zero dollar bill isnt as simple as installing enough solar panels to generate whatever your average daily usage is. As I explained previously, when you use electricity is almost as important as the amount of electricity you use. This is because a solar system only generates its maximum at midday on a good day.

Solar power, regardless of system size, wont do much to a massive electricity bill if its all caused by overnight air-conditioner usage!

Getting zero bills with only solar panels and no battery storage is possible. Until I added an electric car to my home, I regularly got credits on my electricity bills.

Yup my retailer was paying me for the privilege of being connected to the electricity network!

But for those of you with sky-high energy costs and not enough room for a huge solar power system, its important to focus on the amount solar energy can reduce your bills by, not whether or not it can simply get your bills to $0.

Or, to put it another way if solar power could take your $500 quarterly bill down to $50, would you be annoyed you didnt get your fabled $0 bill, or would you be jumping for joy your solar system saved you $450 in 3 months?

Now lets get into some hard numbers on system sizes with the help of my nifty solar calculator.

Example for a typical home in Sydney

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What Affects Solar Panel Output Efficiency

Heres where solar panel quality makes a difference. Not all solar panels are alike. Photovoltaic solar panels come in wattages ranging from about 150 watts to 370 watts per panel, depending on the panel size and efficiency , and on the cell technology.

For example, solar cells with no grid lines on the front absorb more sunlight than conventional cells and do not suffer from issues such as delamination . The construction of our cells makes them stronger and more resistant to cracking or corrosion. And a microinverter on each panel can optimize power conversion at the source, in contrast to one large inverter mounted on the side of the house.

Because of these wide variations in quality and efficiency, its difficult to generalize about which solar panels are right for you or how many youll need for your home. The main takeaway is that the more efficient the panels are, the more wattage they can produce, and the fewer you will need on your roof to get the same energy output. Conventional solar panels usually produce about 250 watts per panel, with varying levels of efficiency. In contrast, SunPower panels are known to be the most efficient solar panels on the market.*

To figure out how many solar panels you need, divide your homes hourly wattage requirement by the solar panels wattage to calculate the total number of panels you need.

So that average U.S. home in Dallas, Texas, would need about 25 conventional solar panels or 17 SunPower panels.

Average System Size And Power Output By Location

We know that real-world output for a given system size is affected by several factors and is constantly fluctuating. But dont worry – its possible to account for weather conditions in your area based on decades of historical data, which allows us to calculate average solar power output for a given solar system size at any specific location.

Average solar system size in selected states


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

Do note that the figures above are just averages. The amount of power generation will vary, especially if weather conditions where you live are different from the state average, or if your roof has a sub-optimal orientation or angle.

Dont worry though, you can still come up with accurate system size and production figures for your specific home – just use one of the three calculations methods we discussed earlier.

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Solar System Calculations & Final Cost

Once you have your solar system calculations figured out, you can start to understand what your cost to go solar is going to be. This largely depends on the cost per watt for solar in your state, as well as the solar incentives youre eligible for. Our financial experts will help you understand all your options and help you get the most savings possible!

Contact MOXIE today to get a free, no-obligation solar system sizing and turnkey cost estimate.

Solar Panel Installation Costs

Solar System Size Calculation

After using the solar panel calculator and purchasing everything you need for your system, it may be possible to install your home solar panel kits on your own. To do so, youll not only need to be handy, but youll need to have experience with electricity and the ability to safely and securely climb onto your roof.

The majority of people will need to hire professionals, which will add to their solar panel installation costs. In general, solar installers will charge somewhere between $0.75 and $1.25 per watt for their labor. This cost is another reason why its helpful to use a solar cost calculator to know how many watts you plan to install.

Factoring in the installation costs will also help you to determine your payback period, which is usually around eight years for most systems in the United States.

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How Do I Check That My Solar Panels Are Working Efficiently

Your solar panels are connected to a control panel called an in-home display. This is a wireless device you can use to monitor whether your system is generating as much electricity as it should be.

If youre concerned that your solar panels are underperforming, contact the installer or manufacturer. They can send out a professional technician to investigate.

We dont recommend ever tampering with the solar panels, as this can damage the system and invalidate your warranty.

Fast Payback On Additional Kws

The cost of increasing the size of the system to 5kW is only $657 per kW. If we assume that you receive a feed-in tariff of 11.1c for every kWh you export back to the grid, it will only take 4 years to pay back the extra 2kW this is assuming that all of the additional solar power output is exported back to the grid. If you self-consume the power, it will pay back much faster. You can expect the panels to continue generating solar power for another 20 years.

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The Cost Of Solar Panels Per Square Foot

It can be difficult to determine how much solar panels will cost you per square foot. This is because there are several factors such as size, type, and quality, that will affect the total cost of the panels. However, it is possible for a tough calculation to be given for the cost per square foot. The prices below include the following:

  • Panels

This will cover an estimated 225 square feet. Looking at a mid-range system once again, the average cost for this would be around £5,000 in total. This works out to £22,12 per square foot.

  • 4kw System

This would cover an estimated 301 square feet. Taking another kid-range system, it would cost around £7,000 to install. As a result of this, the cost per square foot would be £23.26.

Calculating Your System Size

How to size a solar panel system, solar power calculation formula, how many solar panels do i need

Knowing your average consumption and the full sun hours factor in your area, we can calculate the right system size for you to maximise self-consumption.

Take your average monthly consumption and divide it by the average number of days per month . This will give you your daily energy consumption.

Now multipy the average daily consumption figure by 30%. The result is the amount of energy that can be supplied for self-consumption by a solar system.

Now take this figure and divide it by the full sun hour factor in your area.

For example

Say a building is using 4000 kWh per month on average.

  • 4000 kWh / 30 days = 133.3 kWh per day
  • 133.3 kWh * 0.3 = 40 kWh per day
  • 40 kWh / 4.7 = 8.5 kWp
  • Thus the most economically viable size will be a 8.5 kiloWatt peak, grid-tied solar system.

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    How Much Solar Power Will You Need

    To determine your homes average energy requirements, look at past utility bills. You can calculate how many solar panels you need by multiplying your households hourly energy requirement by the peak sunlight hours for your area and dividing that by a panels wattage. Use a low-wattage and high-wattage example to establish a range . Note that the size of your roof and how much sunlight your roof gets are factors as well.

    If you work with an experienced solar installer, they will handle all these calculations for you. If youre searching for a calculator to figure out how many solar panels do I need?, look no further. You can use SunPower Design Studio to estimate your own system size, monthly savings, and the actual appearance of a solar array on your own roof. This interactive tool provides a solar estimate in just a few seconds and can be done on your own or on a call with a SunPower 786-7693.

    How Much Do Solar Panels Weigh

    If youre planning on installing a rooftop solar system, understanding the weight of your solar panels is another key factor to consider. Knowing a solar panels weight is the best way to be certain that your roof can support a full installation.

    While panel weights vary from brand to brand, most panels weigh about 40 pounds.

    SunPower panels are the lightest of all major brands*, with some of our panels weighing as little as 33 pounds. For comparison, at the top end of the range some conventional panels weigh as much as 50 pounds.

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    How Many Batteries Are Needed To Power A House

    How many batteries would you need to power the average home?

    If your home consumes 30kWh every 24 hours and you want to have 24 hours autonomy, then you can easily work out how many batteries you would need.

    There are two choices for home energy storage batteries lead-acid deep-cycle or lithium iron phosphate. I prefer LiFeP04 but as lead-acid are still the mnost common, Ill use them in an example.

    Deep-cycle batteries can be discharge up to 80% of their capacity, but 50% is recommended in order to maximize battery life.

    This means that we need to double the amount of energy required from the batteries, simply beacuse half of it isnt normally available. 80% Depth of Discharge is OK from time to time.

    The calculation looks like this:

    Energy for 24 hours = 30kWh

    30kWh in battery amp-hours = 2500Ah

    Total battery capacity needed for 24 hours = 2500Ah x 2 = 5000Ah

    25 batteries rated at 200Ah will be required to power this home for 24 hours.

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