Whether you want to help our planet or just save some money, the solar panel calculator might be just the tool you want to use. It's created to help you find the perfect solar panel size for your house depending on how much of your electric bill you'd like to offset.

If you're willing to make such an investment, it may be a good idea to compare the cost of going solar versus solar savings. Read on to learn more about it!

Why are solar panels for home use a way to go?

Undoubtedly, renewable energy has a number of benefits affecting several areas of our lives. Many countries are turning towards hydroelectric power or wind turbines depending on which best suits their natural environment. Why should you follow the trend? Let's see:

  • Switching to solar panels can indirectly improve your health. The more people go solar, the less traditional power plants you will need in your country. If these are coal or natural gas plants, this means your air quality should increase. As Neil Armstrong said, "One small step for man, one giant leap for mankind".
  • Stable prices. The vast majority of solar savings comes from the fact that the fuel is free. Oil prices skyrocketing? Well, that's none of your business if you have your own solar panel power output (and it's not too cloudy). This applies also to the Tesla charging cost if you own one.
  • Solar panels for home use can also offer reliability. Not only is it rare for them to break, but they can also save you if there's a power shortage in your area. A battery might be a good idea so that you have some saved energy in case the weather or season isn't favorable.

What solar panel size should I choose? Calculate your solar panel needs

Solar panel dimensions are critical if your roof is small or of an unusual shape. Why? These factors affect the usable area, so whatever you sacrifice in size, you'll need to make up for in efficiency. It may seem confusing, so let's go through the whole decision-making process step by step:

  1. Begin by calculating your solar panel needs, the solar array output. This is when our solar panel calculator steps in. Alternatively, you can just use the formula:

solar array output = electricity consumption / (365 * solar hours in a day),

where the electricity consumption is yearly and expressed in kWh (our energy converter can help if your electric meter uses other units). Solar hours in a day depends strongly on your location.

  1. You need to account for efficiency and how much you want to depend on solar power. In other words, how much of your electricity bill you'd like to offset. The equation is:

solar array size = solar array output * (bill offset / efficiency),

where both bill offset and efficiency are percentages.

  1. The tricky part begins now. You need to estimate your roof area. Exclude the bits you cannot place anything on or are normally in the shade since they won't generate any power. This will give you an idea of the maximum solar panel dimensions.

  2. There's no size fits all solution here, and you'll have to research your local options regarding solar panels. You've calculated your solar panel needs, so it's time to check where you can get photovoltaic cells that are the closest to the ideal.

  3. To see if any of the panels available will fit your roof, you will first need to compute the number of solar panels needed:

required panels = solar array size * 1000 / (solar hours per day * panel output in watts).

Typically, the output is 300 watts, but this may vary so make sure to double-check!

  1. The last step is determining the area the potential panels would occupy. The following equation will help you:

area occupied = required panels * panel width * panel length,

where both width and length are in meters.

If the area occupied is smaller than your roof area, the system should fit just right!

How many solar panels do I need?

You can find the number of solar panels you need from the equation:

number of panels = system size / single panel size,

where system and single panel sizes are their wattages, not actual dimensions. The system size determines the power you expect from the solar panels.

The number of solar panels you need depends on the following factors:

  • Your solar panel needs;
  • Your usable roof area;
  • Solar panel dimensions;
  • Photovoltaic cell efficiency.

So, for example, if you have a small roof, it might be a good idea to invest in fewer highly efficient panels. Bear in mind that as long as the total power output fulfils your needs, it doesn't matter how many solar panels you have.

Cost of going solar vs. solar savings - an example

Photovoltaic cells are often advertised as an investment that saves you money in the long run. Although, as we've mentioned, each case is different, we can check it with an example.

Let's consider a nice house somewhere near Boston, Massachusetts. The average residential power use is 627 kWh per month, priced at 14.91¢/kWh. Rounding it up, we pay $94 for electricity monthly, and $1,128 yearly.

Now, the house has a gable roof, and one side of it is usually in the shade, so a solar panel power output there would be close to zero. It's better to exclude this bit completely. If the total roof area was 1750 ft2, halving it means that we have approximately 875 ft2 (81.3 m2) of usable area.

Inputting the data into the solar panel calculator shows us that to offset 100% of electricity bills, we need a solar array producing 7.36 kW, assuming 70% efficiency. The average installation cost for an 8 kW system is $25,680. Dividing this by yearly electricity cost, we see that the solar panels for home use would return the investment after nearly 23 years.

However, this is a bad scenario, as solar panels are more efficient when used closer to the equator. Bear in mind that often there are incentives that help offset the installation price. Moreover, we didn't account for inflation that causes electricity costs to increase. Although the final decision is yours, hopefully, our solar panel calculator can help you make an informed choice.


How to calculate solar panel output?

To find the solar panel output, use the following solar power formula:

output = solar panel kilowatts × efficiency × solar hours per day.

The output will be given in kWh, and, in practice, it will depend on how sunny it is since the number of solar hours per day is just an average.

How is the efficiency of solar panels calculated?

The solar panel efficiency is usually calculated from the formula:

efficiency = (Pmax / area) / 1000 x 100%,


  • efficiency - Expressed as a percentage;
  • Pmax - Solar panel peak power in Watts;
  • area - Length multiplied by width of the solar panel, in meters squared; and
  • 1000 - Standard Test Condition irradiance in Watts/m2.

How to calculate the solar panels needs for camping?

To determine your solar panel needs for camping:

  1. Calculate the consumption of all appliances you're going to use. To do that, multiply the power consumption by the hours you intend on using each item.
  2. Look up the solar hours in the place you're going to.
  3. Multiply the solar panel kilowatts by the number of solar hours and its efficiency to find the output.
  4. If the output is greater than or equal to, you're good to go. If not, you will need a larger panel.

How many solar panels to charge a Tesla?

Let's consider an upgraded Tesla Model S with a battery capacity of 100 kWh. If you used half of its capacity daily, then you'd need a solar array of approximately 14.99 kW, which translates to 13 solar panels to offset the costs entirely. This is assuming 4 solar hours a day, which is the yearly average for the US, and 300 W panels.

Madhu Raman and Michael Darcy
Electricity consumption
per year
I know my solar hrs per day
Solar hours per day
hrs. per day
Bill offset percentage
Efficiency of solar panel
Solar array size estimate
Estimate required roof area
Your roof area
No. of panels needed
Required area
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