How Do I Calculate How Many Solar Panels I Need?
How to calculate how many solar panels you need
Discover your solar saving potential
Calculating how many solar panels you need is fairly easy if you have the relevant information. It all begins with the load you need solar to supply. If it’s a home, how many kWh does it use?
Use the following steps for solar panel sizing:




 What is the load? How many kWh does your home use?
 What is the irradiance in your location (Peak.sunhours)
 Use the load and irradiance to find theoretical solar system size
 Adjust solar size to account for system losses
 Divide adjusted solar system size by individual panel watts



Calculate the number of solar panels your home needs by estimating the previous year’s energy bill and using your location’s irradiance value in kWh/m2/year (Peaksunhours) to calculate the theoretical solar production needed. Adjust theoretical solar kWh by the loss factor of 1.44 and divide by the individual solar panel watts rating.
Video – How Do You Calculate How Many Solar Panels You Need?
How many solar panels are needed to power an average house?
The average house size in the US is 2500 square feet, which should logically equate to the average energy usage of 11000kWh per year, or 30kWh per day.
However, there are variations in house size from state to state and also in solar panel power output. This is because the sun’s energy, or irradiance, varies by geographic location – very important when considering the cost of a solar farm.
Table – Average house size in square feet – 10 US States compared
U.S. State 
Area (Square Feet) 
Alabama 
1800 
Colorado 
2126 
1694 

Kentucky 
1750 
Michigan 
2000 
New Mexico 
1838 
Pennsylvania 
1700 
Texas 
2031 
Wyoming 
2052 
Historical values for irradiance can be found for your location on sites like GlobalSolarAtlas.info. The average irradiance value for the USA is 5.322 kWh/m2/day, so I’ll use that value of peaksunhours for our calculation.
Using average US values, you can find the theoretical solar power output by dividing the daily kWh by the irradiance value in daily peaksunhours:
Solar power required = 30000 watthours/5.3 peaksunhours = 5660 watts
If 300 watt solar panels were used, the number of solar panels for the average US house would be:
5660 watts / 300 watts = 18.86 (19) solar panels
However, all solar PV systems have losses of about 23%. This can be taken into account my multiplying the solar power required by 1.4:
Adjusted solar output = 5660 x 1.4 = 7924 watts
Using 300 watt solar panels, the actual number of solar panels needed would be:
7924 watts / 300 watts = 26 solar panels
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How many solar panels do I need to power a 3000 square foot house?
We know the average sized home requires 7924 solar watts with the average US irradiance of 5.3 peak sun hours/day, which equates to 3.17 watts per square foot.
Multiplying this number by 3000 square foot, we get 9510 watts of solar required.
9510 watts / 300 watts = 32 solar panels of 300 watts rating each
How many solar panels do I need for a 3 bedroom house?
The average size of a US home with 3 bedrooms and 2 bathrooms is 1300 square feet. I previously worked out that solar requirements with average irradiance of 5.3 peak sun hours per day is 3.17 watts per square foot.
Cost of solar panels for 3 bedroom house:
1300 square foot x 3.17 = 4121 solar watts
If 300 watt solar panels are installed, the number of panels required is found by:
4121 watts / 300 watts = 14 solar panels
At just under $3/watt average installation cost, a 4000 watt solar solar panels system would cost $12000 excluding any incentives.
How many solar panels do I need to power a 4 bed house?
The average size of a US home with 4 bedrooms and 2 bathrooms is 1700 square feet. In the last section I worked out that solar output needed with average irradiance of 5.3 peak sun hours per day is 3.17 watts per square foot.
Solar calculation for 4 bed home:
1700 square foot x 3.17 = 5389 solar output in watts
If 300 watt solar panels are used, then the number of panels needed is found by:
5389 watts / 300 watts = 18 solar panels
How many solar panels does it take to run a house off grid?
As a general rule, you need the same amount of solar panels as for a gridtied system, but with extra panels depending on how much autonomy you require.
Let’s say you have a home that uses 30kWh per day and the irradiance in your location is the US average of 5.3 peak sun hours per day. A 5.7kW solar system would meet your needs in theory, ignoring PV system losses.
Calculating solar the right way
Most of your home’s energy usage, maybe 75%, is used in daytime. You need to make sure that your battery bank is fully charge during the day so that you have uninterrupted electrical supply through the night.
Also, what if you have several days of cloudy days with low solar output? In this case the energy storage battery bank would have to meet the shortfall in energy production.
I would add another 25% to the estimated solar system size required, in this case making the system size 7.5kW and a battery bank sized to supply the home for 48 hours with no solar generation.
The average US house (30kWh day) running with offgrid solar would need 25 solar panels each rated at 300 watts.
How many solar panels to power a house calculator
Is a 4kw solar system worth it?
With the average U.S. irradiance of 5.3kWh/m2/day (peaksunhours), a 4kW solar system generates about 21.2kWh of energy per day, or 7738kWh/year.
This is quite a lot less than the average energy consumption for a U.S. home is 11000kWh/year. A 4Kw solar system may or may not meet a home’s energy requirement, as calculating solar depends on the location and how much the home uses.
Whether such a system is worth it or not also depends on the electricity cost, which varies from state to state. The cost of the solar system must also be known, and this asset cost may be reduced by government or state solar incentives.
Example – Is a 4kW solar system worth it in Houston, Texas?
Calculate The Solar Payback For A 4kW Solar System In Houston, Texas
 City: Houston, Texas
 Solar system power output: 4kW
 Solar installation cost = $10960 – 26% federal tax credit = $8110
 Irradiance level at Houston, Tx = 1552 Peak Sun Hours
 Power generated by solar panel system = 1552 x 4kw = 6208kWhrs (kilowatthours)
 Houston home electricity cost = 10.98 cents/kWh
 Yearly savings = solar output x electricity cost = 6208 x 10.98c = $690
 Solar payback time in Houston, Tx= solar cost/annual savings = 8110/690 = 11.75 years
The payback period might well be less than this, down to 9 or 10 years. Professional solar installers will give you accurate estimates based on their historical data, and the irradiance level in your geographic location.
How much should I pay for a 4kW solar system?
The average cost of a solar power system installation in the U.S. is just under $3 per kW. There are variations according to state – see the table below:
Table 6 – Cost of a 4kW solar power system by US State
State 
Installation price Per Watt ($) 2021 
Cost of 4kW Solar System 
2.45 
$9800 

2.79 
$11160 

3.61 
$14440 

2.63 
$10520 

2.68 
$10720 


2.44 
$9760 
3.65 
$14600 

2.63 
$10520 

2.61 
$10440 

2.33 
$9320 

2.67 
$10680 

2.52 
$10080 

3.08 
$12320 

3.03 
$12120 

3.23 
$12920 

3.07 
$12280 

2.34 
$9360 

2.92 
$11800 

2.88 
$11520 

2.85 
$11400 

3.13 
$12520 

3.15 
$12600 

3.11 
$12440 

2.64 
$10560 

2.96 
$11840 

2.42 
$9680 

2.83 
$11440 

2.62 
$10480 

2.83 
$11440 

2.81 
$11240 

3.22 
$12880 

2.87 
$11480 

2.68 
$10720 

2.67 
$10680 

2.82 
$11280 

2.62 
$10480 

2.54 
$10160 

2.99 
$11960 

2.92 
$11680 

3.13 
$12520 

2.39 
$9560 

3.04 
$12160 

2.74 
$10960 

2.95 
$11800 

3.06 
$12240 

2.91 
$11640 

2.69 
$10760 

2.64 
$10560 

3.05 
$12200 

2.57 
$10280 
How much does a 8kW solar system cost?
You can expect to pay about $24000 for an 8kW solar power system, based on the average price in the U.S. Use the table above to get a more accurate estimate (multiply the results fo a 4kW system by 2).
How many solar panels do I need for 5kW?
The exact number of solar panels needed for a 5kW solar power system will depend on the irradiance in the system’s geographic location.
The calculation is:




 5000/300 = 17 solar panels 300 watt rating
 5000/200 = 25 solar panels 200 watt rating
 5000/100 = 50 solar panels 100 watt rating



How much should I pay for a 6.6 kW solar system?
Using the average U.S. installation cost for the USA, you would expect to pay about $19500 depending on your state.
How many panels does a 6.6 kW solar system have?
A 6.6kW solar system will have:




 6600/300 = 22 solar panels rated at 300 watts each
 6600/200 = 33 solar panels rated at 200 watts each
 6600/100 = 66 solar panels rated at 100 watts each



How much power can a 6.6 kW solar system produce?
With an average irradiance of 5.3 peak sun hours/day, a 6.6kW solar power system will generate:
6600 x 5.3 = 34.98kWh/day or 12767kWh/year
This output can vary up or down depending on the irradiance value state to state.
How much power does a 7kW solar system produce per day?
Using the U.S. average irradiance of 5.3 peak sun hours per day, a 7kW solar installation will produce:
7000 x 5.3 = 37.1kWh/day or 13541kWh/year
This output could be more or less depending on the peak sun hours value from state to state.
How many solar panels do I need for a 6kW inverter?
In general, the solar system size should be the same as the inverter size, so 6kW of solar output. This is because inverters are most efficient when they are fully loaded.
A 6kW solar system will need:




 6000/300 = 20 solar panels, each rated at 300 watts each
 6000/200 = 30 solar panels, each rated at 200 watts each
 6000/100 = 60 solar panels, each rated at 100 watts each



How much power is needed from a solar array for the average monthly electric bill – case study
How much electricity you need of course depends on the appliances you use in your home. Another important factor is how you use them. The key factors that determine a solar system size are:
 Kilowatt hours consumed – the amount of power required (from your electricity bill)
 Peak sun hours for your geographic location
 Solar panel wattage (power rating) – how many watts of power
 Solar power system type – gridtie or offgrid
 Type of solar panel
It goes without saying that how much energy you need from a solar energy system depends on your household energy needs i.e. how many kwh of electricity your home consumes each month.
The easiest way to determine this is to check last month’s utility bill. Once you know the amount of energy needed, you can move on to the solar calculator.
Should you use a solar installer or can you install residential solar panels yourself?
Although it’s entirely possible to save up to 50% of installation cost by installing yourself, for most people the best way is to rely on a professional solar installer. Such an installer will take into account:
 Solar system production ratio
 Amount of sunlight at you location – direct sunlight is important
 Your average energy bills for the past year
 Available roof space and amount of electricity it could generate
 Hours of sunlight per day
 Advise on efficient air conditioners to reduce the size of the system
 Advise on different types of solar panels
… and a host of other factors you never even though of! Above all, using a pro will ensure you’ll have enough electricity when you need it.
Are monocrystalline panels efficient solar panels?
Monocrystalline are more efficient than polycrystalline solar panels, but they are getting closer all the time. However, if you’re limited with square feet of roof space, mono is the way to go. Keep in mind that a typical 250 watt panel has an area of about 16 square feet, so a complete 5kW solar system for a typical home has considerable square footage.
Whichever type of panels you install, how much sunlight you get in your geographic location is vital for maximizing the output of any size kw system. Each kwh of energy has to be won from the sun, so higher efficiency panels will definitely help.