How Many Solar Panels Do I Need?

How many solar panels do I need is one of the most frequently asked questions by solar enthusiast.

We suppose that you have already made your home or office more energy efficient and that you know the different types of solar panels available in the market .

The right choice of the solar panel type is very important.

It predetermines the number of solar panels, because every solar panel type shows different efficiency in converting solar energy to DC power.

Having said that, let’s go straight to the point.

First, you should calculate your daily energy target.

You have to decide what percentage of your annual electricity bill you want to offset to your grid tied system.

Let’s say your annual energy usage is 7,000 kWh. You want to offset 40% of it to a grid-tied system.

This means that energy target is:

7,000 kWh x 0.40 = 2,800 kWh

Since there are 365 days in a year, your daily energy target is:

2,800 / 365 = 7.7 kWh

If you know your daily energy target and the average annual PSH (Perfect Sun Hours) value for your area, you can calculate the amount of peak power you need to install on your roof:

Installed ‘peak’ solar power = Daily energy target/(SLF*PSH)

SLF is the System Losses Factor, a.k.a system efficiency ,which takes into account system losses or system inefficiency. For a grid-tied system system efficiency is assessed usually between 70% and 80%. This means that we lose (20-30)% of the energy in the system and our panels must have higher installed peak power so as to compensate for those loses.For an off-grid system the system efficiency is somewhere between (50-65)%.

If your daily energy target is in Wh, then you obtain the peak solar power in Wp (watts-peak). If daily energy target is in kWh, you obtain the peak solar power in kWp (kilowatts-peak).

PSH is abbreviated from ‘Perfect Sun Hours’ and refers to the number of hours per day during which the solar irradiance equals 1,000 W/m2. PSH are measured in kW/m2/day and it can be found by using solar maps.

By NREL (National Renewable Energy Laboratory of the United States Department) (http://www.nrel.gov/gis/solar.html) [Public domain], via Wikimedia Commons

If your daily energy target offset is 7.7 kWh, the area you live has an average annual PSH = 4.5 hours, and you assume system efficiency = 75% 0r SLF= 0.75, then the needed total peak installed power is:



7.7 kWh / (4.5 hours x 0.75) = 2.28 kWp or

This is the installed ‘peak’ solar power needed to generate the required energy target.

At this stage it is important to assess how much area you need to install the solar array. Based on your energy needs, you can determine whether the area of your roof would be enough to fit all the panels (for example: RENOGY 200 WATT ) needed. Here we don’t talk about a specific panel model but rather about solar panel type – monocrystalline, polycrystalline or thin-film.

The area required for installing the solar array, so that your PV system would meet the energy offset target, depends on:
Peak power installed on the roof (in kWp or Wp)
The kind of modules you use (monocrystalline, polycrystalline, thin-film).

To estimate the area you need to install the required peak power, you should use the following table:How to estimate the area you need to install the required solar peak power

The required roof area is calculated by the formula:

Total area needed =

= Installed solar power in kWp x Area needed for 1 kWp

a) If your target peak power is 2.28 kWp, and you decide to use monocrystalline solar modules, the area you need is:

2.28 kWp x 7 m2 = 15.96 m2 or 2.28 kWp x 75 ft2 = 171 ft2

b) In case you prefer to use polycrystalline solar modules, the area you need is:

2.28 kWp x 8 m2 = 18.24 m2 or 2.28 kWp x 86 ft2 = 196.08 ft2

c) Should you decide to buy thin-film solar modules, the area you need is:

2.28 kWp x 15 m2 = 33.87 m2 or 2.28 kWp x 161 ft2 = 367.08 ft2

Finally, to find out how many solar panels you need, you should divide the total installed power by the rated power of a single panel you are going to buy, and round the result up to the nearest integer.

For example, if you have chosen to buy panels of 160 Wp rated power each, the number of panels required is:

2,280 Wp / 160 = 14.25, which should be rounded up to 15.

Please, have in mind that such a number is reasonably exact for budgeting purposes only.

The reason is that solar panel output power changes with temperature and solar (sun) energy deviation. Such power output deviation forms the ‘operating window’ of a solar panel or solar array.

Furthermore depending on the solar power system you use, the next to the solar panel component which can be either a charge controller or an inverter, has its own operating input window as well.

To provide an efficient energy delivery to the loads in your home or office over time, the operating window of the solar array formed by these solar panels should be always within the operating input window of either charge controller or the inverter (depending on the solar system type) and match the capacity of the battery bank (if any needed).

You can estimate the number of solar panels needed for your solar power system by using our free solar panel calculator .

You can find even more money saving solar information and more precise calculators helping to build or buy an affordable yet efficient solar power system in our informational Solar packages. Click Here to learn more about our Solar Packages Now!