How Do I Calculate How Many Solar Panels I Need?

Updated on November 14, 2022

How do I work out how many solar panels I need?

How to calculate how many solar panels you need

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:

1. What is the load? How many kWh does your home use?
3. Use the load and irradiance to find theoretical solar system size
4. Adjust solar size to account for system losses
5. 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 (Peak-sun-hours) 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.

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
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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 peak-sun-hours for our calculation.

How many solar panels – how much solar do I need to power an average house in the USA?

Using average US values, you can find the theoretical solar power output by dividing the daily kWh by the irradiance value in daily peak-sun-hours:

Solar power required = 30000 watt-hours/5.3 peak-sun-hours = 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?

Solar panels, solar battery charger, batteries and an inverter used in off-grid solar systems

As a general rule, you need the same amount of solar panels as for a grid-tied 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 off-grid solar would need 25 solar panels each rated at 300 watts.

Is a 4kw solar system worth it?

With the average U.S. irradiance of 5.3kWh/m2/day (peak-sun-hours), 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

How many years to payback a 4Kw solar system in Houston?

What is the average solar payback period for a 4kW solar power 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 (kilowatt-hours)
• 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
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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:

1. Kilowatt hours consumed – the amount of power required (from your electricity bill)
2. Peak sun hours for your geographic location
3. Solar panel wattage (power rating) – how many watts of power
4. Solar power system type – grid-tie or off-grid
5. 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:

1. Solar system production ratio
2. Amount of sunlight at you location – direct sunlight is important
3. Your average energy bills for the past year
4. Available roof space and amount of electricity it could generate
5. Hours of sunlight per day
6. Advise on efficient air conditioners to reduce the size of the system
7. 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.