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Step 2: How Many Solar Panels Do I Need? – sizing your PV modules

In this tutorial, I will show you how to calculate the number of panels you need in your solar array. This is the second step in six-step tutorials on how to design a solar PV system. On the way, you'll learn factors that cause inefficiency in PV system and how to cater for them.
Step-By-Step Guides

Step by step guide on how to design a solar PV System

In this tutorial, I will show you how to calculate the number of panels you need in your solar array. This is the second step in six-step tutorials on how to design a solar PV system.

If you jumped into this article from a google search, I strongly advise you to visit the first step: Determining your Power Consumption Requirements. The information in that first article is very critical to understanding this second step. More so, if you’re a complete solar energy beginner, I recommend Fundamentals of Solar Energy, that article is written for you in mind.

Before I show you how to size your PV array, I will want you to understand factors that cause inefficiency in PV array and how to cater for them when sizing your array.

From there I will dive into solar panel quality issues, manufacturer ratings, Sun hour in Nigeria and panel installers issues. You’ll learn how all these have direct impact on the size of your array. After that, I will then show you how to calculate the number of panels you will require in your PV projects.

Factors that Cause Inefficiency in PV Module

Below are factors that can cause loss of energy from the panel:

1. Dirt or Rubbish – Paper, dead leaf, dust and all sorts of rubbish on your panels could prevent them from receiving adequate sunlight to produce their maximum power.

2. Heat – sunlight produces energy and heat at the same time. Heat on a solar panel means it produces less energy.

3. Stringing (wiring) –There’s always going to be loss of energy in all wired electrical connections. It’s a natural phenomenon.

4. Partial Shading – trees, tall houses may be casting shadow on panels

5. Module Mismatch – example, mixing mono and poly or mixing different brands of panel

6. Orientation mismatch – example, panels on different roof planes

Some other factors you can control include:

Bad Installers – Connectors are not cork up properly; switch gear, fuses are not well fixed, you are going to lose too much power. If you want installers to do your installation, do not base your selection only on price. You may invest in the best panel in the market, but a bad installer can ruin the whole effort.

Bad Solar Panel Quality – choose a well-respected brand and avoid oversized ratings.

Catering for Losses

Because we don’t know how much dirt, heat, wiring, partial shading, bad installers, etc., are going to affect our system but we know for sure that they are going to cause energy loss in the system.

How to Cater for Losses

To cater for those losses, we add 30% of our targeted output to the same targeted output (30% is international best practice. However, if you’re using unknown brands, especially, those from China, I strongly advise that you use 40% instead. In this tutorial, we are going to stick to 30% because at ASSDIA, we always go for the respected and cost-effective brands)

Original Target Output (Total Appliance Watt hour per day) = 5,120W

Target output (inclusive of 30% loss) = 30% X 5,120 + 5,120W

= 1,536+5120 = 6,656W

Therefore, 6,656W is the output we are expecting from the panels every sun hour.

The next thing we are going to do is to find Sun Hour in your area.

Sun Hour in Abuja

What is sun hour? My trainees often ask me what I mean by sun hour. Beginners always mistake daylight with sun hour. I made the same mistake. Check out other terrible mistakes I made when starting out, so you can avoid the same. Sun hour could be said to be that time in a day when the intensity of sunlight in your area is highest. This definition is okay for the beginners. But more advanced solar energy professional knows that it’s hour(s) in a day when sunlight produces 1,000 watts of PV power per square meter.

To figure out the sun hour in your location, you may need to ask google.

Google search on Sun Hour in Nigeria

Google search result shows that coastal state like Lagos, Rivers, Cross River, etc., receive average of 3.5 h while Northern states like Kano, Sokoto, Borno receive 9 h. Abuja receives 7.2. I know this based on personal research.

This information tells us that the sun will strike our panels for 7.2 solid hours per day.

To size the Capacity of our Panels:

Divide the Target Output (Inclusive of energy loss) By 7.2 Sun hour to get the total Capacity of the panels needed to operate the appliances.

Therefore, 910.55W is the capacity of our array. So, we need a panel that can output 910.55W. It’s likely that you won’t see a single panel that can output 910.55W. Therefore, you will need to combine two or more depending on the rating of the panels.

Manufacturer Ratings:

Panel Power Rating is the highest amount of power (watt-peak) a panel can produce under factory test conditions.

In solar PV world, panels can be tested under STC or PTC

STC – Standard Test Condition. This kind of test is conducted under artificial test condition. This means that there’s bound to be a wider difference between the outcome and the real-world testing.

PTC – PV Test Condition. This test condition is conducted under real mode test condition. PTC is closer to the real-world condition but it isn’t.

How to Interpret the PV Ratings:

Model = a coded number manufacturer uses to categorize their panels

Pm = Peak Max – highest power the panel can generate

Vm = Voltage Maximum power – max Voltage you will get when load is connected

Im = Current Maximum power – max Current you will get when load is connected

Isc = Current Short Circuit – current you will get when nothing is connected

Voc = Voltage Open Circuit – Voltage when nothing is connected

Size = Physical size of the panel measured in square meter.

Number of Panels Needed

To know the number of panels needed in this project:

Divide the capacity of our array by the rated output Watt-peak of the PV modules available to you. In this project, the rated watt-peak available to us is 260Pm (w).

Therefore,  910.55W
                    ________     = 3.50
                      260W

Increase any fractional part of result to the next highest full number and that will be the
number of PV modules required. So what we need is 4 number of 260W panel.

Result of the calculation is the minimum number of PV panels. If more PV modules are installed, the system will perform better and battery life will be improved. If fewer PV modules are used, the system may not work at all during cloudy periods and battery life will be shortened.


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