How To Calculate Solar Energy Use And Determine The Size Of Your Solar System

Categories: Solar PV Panels.

Our sun is a natural nuclear reactor. Every second it releases a massive amount of energy, including light energy in photons. 

How do you calculate a solar energy system to use that free energy? 

How do you determine the size of your solar panels

How many solar panels will power a house?

calculate a solar energy system

Photons are the tiny packets of energy that enter into the atmosphere as daylight. Photons travel the 93 million miles distance from the sun to Earth at the speed of light taking approximately 8.5 minutes.

There is sufficient capacity in the photons arriving on Earth in a day to generate enough solar energy to satisfy the current global energy needs of the planet for a year. When these photons strike a solar panel, there is a photoelectric effect in which free electrons are produced. The freed electrons from the photovoltaic (PV) cell flow through a circuit as solar generated direct current electricity. Several solar cells linked in series form a photovoltaic solar panel. This collection of PV panels forms the solar energy generation system.

calculate a solar energy system -Working of a solar panel

Working of a solar panel

How Many Kilowatts Does A Solar Panel Produce?

To determine how many solar panels you needed for your household, first you need to know the home's energy load and set your energy goals. In other words you will calculate the solar energy system's inputs. 

The first way to achieve your energy goal is to conserve electric power by reducing appliance use and using efficient appliances. This factor considers reducing your carbon footprint.

The next goal most people strive for is to attain the investment cost of your solar panel system including setup costs. Also, you can get quotes. The initial expenditure outlay can be forecasted by the solar power price calculator to calculate a solar energy system outputs. 

There are a number of online solar energy calculators that direct the reader to get a quote from the sponsor after being shown the value of savings available. Solar calculators are often based on the location (PVWatts is US Based) and roof area with input of the current electricity costs.

Some solar calculators link into Google maps wherein you select the house roof and the software calculates everything for that location. The solar panel system's price is calculated by the power price calculator. 

To calculate the number of solar panels needed for your solar power system, we use the metrics below. 
These metrics help you calculate the kilowatts required from your solar panel setup. The metrics are:

  • The solar energy requirement of the household 
  • The roof’s usable surface area
  • The climate and peak sunlight in the specific area
  • The wattage and efficiency of the photovoltaic (PV) panels being used in the solar system
  • Energy storing capacity of the battery used
  • Regulators used

Storing Energy - Capacity Of The Battery Used

To calculate the solar electricity that needs to be generated, you need to know the amount of energy that can be stored in the solar PV system's rechargeable battery

The Solar panels selected should be sufficient to recharge the battery. The key is to match the recharging energy requirement to the solar panel and the charge controller DC output. 

calculate a solar energy system - A solar battery

Calculate A Solar Energy System Requirement

You need to match the kilowatt hours (amount of power generated over a period) of your solar PV system to the kilowatt hours that your appliances will use. To approximately estimate the energy requirement for a period of time, we need to multiply the hourly power consumption by the number of hours of intended use. An example for the same would be a 40W appliance in domestic use that will be used for 2 hours per day. The daily energy consumption for it would be 40*2 = 80 Wh.

Repeat that exercise for every appliance individually to calculate the daily energy requirement for your solar energy system. Add in any appliance that will use power while connected to the solar panel. The result will give you an indicative total energy requirement for the solar panel to produce for the day. You can work out the number of panels needed a few ways.

You calculate how many solar panels you may need by dividing the maximum hourly solar energy system output by the wattage output of one single panel. The number of solar panels of the size used in the calculation will theoretically meet that energy demand during a sunny day.

Another way is to multiply the household’s hourly energy requirement by the peak sunlight hours for the area. Then divide that answer by the wattage of the solar panel intended to be used.

And yet another way of calculating the energy requirement is to review your past electricity bills to determine your use in kilowatt-hours. You can find the kilowatt hours used from your most recent meter reading, less the reading from the earlier one.

Using The Kill A Watt Meter

As an example, the average US household uses 900 kWh power per month. For 30 days per month we get 30 kWh per day, and for an average day you might get between 6 and 10 hours of effective sunlight. The answer you get is between 3 kW and 5 kW per hour. So this monthly average wattage is what your solar panel system should be equipped to generate.

The calculations done this way will not be accurate. There are other factors affecting the generation of solar energy. The daily production might fluctuate greatly on a daily basis.

You could calculate a solar energy system power consumption of an electronic appliance using a low cost plug in device, called ‘Kill A Watt meter’. Once you know how much power an appliance is consuming per day, it will help you to determine your power consumption. Use a solar power price calculator.

You can use this device to count watts in kilowatt/hours (kW/h). This device is made up of a three-prong electrical plug-in, along with some buttons and a display monitor. Once you plug the monitor into the wall, you also plug any appliance into the monitor. The device tells you exactly how much is the electricity being used by the appliance, including situations when the appliance is just plugged in only without being turned on.

Here is a video to watch the working of kill a watt meter. 

Calculate The Roof’s Usable Area And Type Of Panel Required

The roof size available for setting up the solar panel would need to be ascertained in square feet. Orientation plays an important role, especially if the roof size is small or not of a usable geometric shape.

The size available needs to be divided by the size of the solar panel that is going to be used. If the number of panels is too few due to the size of the roof then one needs to look at the wattage output. Possibly each solar panel will be uniform in size so then you'll select the panel for its efficiency.

There are various kinds of solar panels available but the differentiating factors among them are the size, wattage, and efficiency of the panel.

Panel efficiency is the ability of the panel to convert sunlight into energy and is a key factor of the cell technology. Panel type will need to be taken into account as a  factor to calculate a solar energy system output.

Sizing Of The Solar Panel

The sizing of the panel also forms a critical part in sizing of the solar system.

That sizing in turn works along with the available roof area. If the area available for use on the roof is large, then can one can buy larger panels.

Larger solar panels will be cost efficient and might not need to be at the top efficiency band in generating power. In case of a limited roof size, or where the usable roof area is small or some section of the roof is partially shaded, you need to use higher efficiency panels, albeit in lesser numbers.

The panels are set close to each other across the roof in order to maximize the efficiency of racking. The power generation capacity of a Solar panel is given in Watts. The Photovoltaic (PV) solar panels are used in residential installations come in varying wattages from 150 watts to 370 watts per panel. You calculate the power supplied or calculate a solar energy system output from the panel to the battery using its Watts-peak and exposure.

Multiply the nominal Watts of the solar panel by the numbers of hours of sunshine it is exposed to. You might still need to judge or account for other factors like the best angle at which to face the solar panel.

calculate a solar energy system - installing commercial roof solar

The Climate Of The Area And Peak Sunlight Hours Calculation

The sunlight hours available to your location will directly impact the watts generated from the solar set up.

The more hours of sunlight available the less number of panels required to generate a given output.

Inversely, with lesser sunlight hours, more solar panels will be required. By dividing this average hourly wattage requirement by the number of sunlight hours daily at the location, you calculate the watts your panels need to produce every hour. Using this figure you can select the panels required. The sunlight hours and sun angles vary by season. This will also be a factor for calculating the number of available sunlight hours and thus the panels required.

Let’s look at a simple calculation to understand this concept better.

Let’s say the summer season sunlight at the location is available for 6 hours and for mid winter it is 4 hours. Then in mid winter a 10w panel will provide 10w * 4h = 40watt-hours of DC power into your battery. In summer the amount would be 10w*6= 60watt-hours. There would be a little energy lost due to resistance through the regulators, cables, and batteries chosen. In calculating the solar power system you would built some assumption of loss into the final calculation.

Regulators Used

The last metric to be considered is the Charge Controller or Voltage Regulator.

This device controls the charge put into your battery. The charge controller stops the battery from overcharging.

Also, at night the device prevents the flow of power from the battery back to the solar panel.

The higher the energy consumption or usage per hour from the household, the less hours the battery will run before it will need recharging from the solar generator.

calculate a solar energy system - solar charge controller

The Final Calculation Of The Size Of The Solar Power System Needed

Once the daily power requirement estimation has been done, we need to check those calculations for each season. For instance, say the figure applicable to daily power requirement we arrived at is 240w.

We divide the daily watts required by the number of sunshine hours for the season to get the panel size (for summer conditions).

  • 240 watt-hours/ 6 h = 40 Watt panel.

Similarly the calculation for winter daylight hours gives:

  • 240watt-hours / 4h= 60 Watt panel.

Depending on the brand of solar panel that you choose to buy, you will know the size of the solar panel you need. So if your chosen company makes 30 watt panels, then you need two panels connected in series. Note that based on the above calculations, the most satisfactory requirement for all year would be to use the higher number of watts and thus solar panels.

The above factors and metrics will help you in calculating the energy requirements for the solar power system that needs to be set up. There may be a difference in the estimated numbers from the actual watts output delivered. When the system is up and functioning you can adjust it.

calculate a solar energy system - how solar works

Future of the solar power system

As solar energy systems become cheaper and more efficient, more people will see some economic value with incorporating them into residential projects.

With the integration of the solar power monitoring systems with the Internet, it will be easy to monitor solar power output in real time.

More solar power systems will upgrade over time to integrate with smart technology and selling household solar PV power that is in excess to need. Solar power accounts for around 35 percent of all new electrical power generated in the U.S.

To enhance their PV sales some installation companies are offering innovative financing solutions to people to help set up their solar panels system. These solutions use the financial return of the power savings to pay for the PV solar system capital cost.

A full solar system with batteries is not only a cleaner source of renewable energy but a correctly sized solar system is a good investment too.



  1. Hey! This is my first visit to your blog! We are a group of volunteers and starting a new initiative in a community in the same niche. Your blog provided us useful information to work on. You have done a marvelous job!


    Dear sir
    i am looking a head to developed new work
    i would like to get help and complete information to began my work in solar energy
    thank you for your help
    Hassan Eldawaki


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