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How Many Solar Panels Do I Need?

Mpfana Manu is a mechatronic engineer who's working in green energy electronics products.

how-many-solar-panels-do-i-need

I wish there was one correct answer to this question. It would have been very easy and convenient for everyone but the reality is there are many correct answers. How many solar panels you need for a house is determined by several considerations. In this article, we'll learn about those considerations and how exactly you can calculate the number of solar panels you need for any house.

To calculate the number of solar panels you need, you need to go through the following.

  • Scoping what you want to achieve
  • Fleshing out the technical specifications
  • Doing a power analysis
  • Deduce the size and number of solar panels you need

This is the work every solar system technician goes through before everything else. It's the paperwork that starts first. If you do a good job with the paperwork, the hands-on work will be more clear and obvious so put diligence in the paperwork, even if it's the boring part of installing a solar power system.

Scoping what you want to achieve

You want solar panels for a reason.

What is that reason?
It is advisable to start with something simple as your scope.
Reasons for wanting solar panels can range from something like...

  • I want to power my holiday house so that I don't rely on the grid supply at all.
  • I want lighting for my poultry farm that can't be supplied by the grid
  • I want to make money by producing surplus electricity that I can feed into the grid.
  • I just wanna learn how to design a solar power system

As you can see, the reasons can be anything.

You know your reason. Write it down and try to be as descriptive as possible. Write why you want solar power.

When what you want is clear, you can then start a more in-depth approach into estimating what you need.

Fleshing out the technical specifications.

I'll use an example to explain how to draw out the required specifications. Say I want to power my holiday house so that I don't rely on the grid at all.

From this example, I can then judge the type of solar power system that best suits what I want. The system that would be best in this example would be a stand-alone solar power system. If you don't know the different types of solar power systems to choose from, I've wrote another article explaining all of them. Read the types of solar power systems here.

After choosing the type of system that is best for your wants, you'll have to choose an operating voltage for your system. The operating voltage will affect the number of solar panels you'll need because it'll will determine the level of inefficiencies in your system.

A solar system with high inefficiencies will require more electrical power, therefore more solar panels, than a system with low inefficiencies to support the same load.

There are two options for the operating voltage.

High voltage

The grid supply already uses a high voltage when it supplies electrical power. So if you're going to install a solar system that has to be attached to the grid (grid-fallback system, grid-tied system), you're inevitably going to supply solar power at high voltage too.(anything greater than 48V)

Using a high voltage reduces the inefficiency caused by electrical resistances in the wiring of your system. High voltage is associated with low AC current. Less electrical power is lost in the wiring when low current flows through it.

However, a solar system requires an inverter to run at high voltage. The inverter changes the DC current generated by the solar panels to AC current, and it also has some resistance that adds more to the inefficiency of the system.

Low voltage

This option is applicable for stand-alone systems only. Modifying the grid wiring (for grid-related solar systems) to operate at low voltage is usually not worth it and is not cost-effective.

Operating at low voltage(less than 48V) is associated with high DC currents. No inverter is required but more electrical power will be lost in the wiring.

Operating at low voltage is best when the connecting wires are very short and the appliances being supplied are low-voltage appliances (say a solar system for an RV).

Low-voltage appliances can include televisions, radios, refrigerators, computers and light bulbs. Electronics manufacturers started making low-voltage appliances ages ago to accommodate low-voltage power supplies.

how-many-solar-panels-do-i-need

For the example that I took, which is to power my holiday house without the grid at all, the operating voltage that I'll choose for the stand-alone system is a low voltage of 24V.

I chose a low voltage because all the appliances I have in my house are low-voltage appliances. I cook using propane gas so my kitchen won't demand a lot of electrical power. And also, my solar panels will be on the roof which is closer to all my appliances so the wiring will be relatively short.

In whatever case of yours, please exclude the cooking and heating economies from the appliances being powered by your solar system. It is not a smart move to use solar electrical power to cook or heat anything.

Those appliances require so much of power that the cost and size of the solar system will get out of control. Find other means to cook or heat. Use gas or solar heating systems instead, using solar electrical power is not worth it.

Doing a power analysis

If you like math, this part will be a little more interesting. You're going to calculate the exact amount of electrical energy that you'll need from your solar panels.

First, you have to list all the appliances that you want to be powered by your solar system. And then list the corresponding wattage for each electrical appliance.

There are two reliable ways to get the wattage of any electrical appliance.

  • Some electrical appliances have their wattage written on them. Usually at the back of an appliance or in the manual of an appliance. The wattage may be written in terms of current and voltage. For example, if a cellphone charger is written to be using 1.5amps at a voltage of 5volts. You can calculate the wattage by multiplying the current with the voltage. Power(watts) = Current(amps) × Voltage(volts)
    Power(watts) = 1.5amps × 5volts = 7.5watts
  • You could also get the wattage of any electrical appliance using a watt-meter. A watt-meter is a device designed to measure the wattage of any electrical appliance that can be plugged to an AC supply. The watt-meter is plugged into an AC supply socket, then the appliance is plugged into the watt-meter. An LCD display on the watt-meter will show the exact wattage used by the appliance.

After noting down the wattages of all the appliances, you calculate the total electrical energy that is required from your solar system. Let me show you the calculations using the example I used earlier.

The stand-alone solar system of my holiday house is going to operate at 24V, and I've determined all the wattages of my appliances. I'll then calculate the total energy required by finding the product of the wattage and the hours of use as shown by the table below.

Electrical applianceOperating voltageWattageHours of use per dayEnergy in watt-hours

TV

24V

60W

5

300Wh

Radio

24V

30W

7

210Wh

Computer

24V

40W

4

160Wh

Bedroom lights

24V

10W

6

60Wh

Kitchen lights

24V

10W

5

50Wh

Dining room lights

24V

10W

5

50Wh

Charging cellphones

24V

9W

4

36Wh

Total electrical power required per day

886Wh

According to this power analysis, my solar panels have to produce 0.866kWh of electricity every hour. But there are also inefficiencies of the system that have to be included.

Inefficiencies depend on the wiring used, the inverter, the batteries, the controllers and the type of solar panels used. For the sake of this example, I'll use an inefficiency of 10% for the whole system.

So the new total power will be calculated as follows.

10% of 886Wh is 88.6Wh
Then add that 88.6Wh to 886Wh to get 974.6Wh
So 0.975kWh is the new total power required per day.

Deducing the size and number of solar panels you need

The hands-on work starts here and everything can get really complicated. I'll keep it simple for the sake of understanding the whole picture.

Lets assume that your site for installing the solar panels is good(with no light obstacles) and the average solar irradiance at that site is 1kW per square meter. Solar panels are rated with a standard solar irradiance of 1kW per square meter.

The holiday house will need a maximum of 974.6W from the panels running at 24V, so there are multiple configurations of the number and size of the solar panels that you'll need. Here are some of the possible configurations

One 1000W, 24V solar panel.
Two 500W, 12V solar panels connected in series.
Two 500W, 24V solar panels connected in parallel.
Five 200W, 24V solar panels connected in parallel.

Whatever configuration you choose, you just have to make sure that the total wattage produced is equal or greater than the wattage you calculated in the power analysis. But it is always safer to keep it greater.

Conclusion

So that is how you get the number of solar panels you need. You'll need to be familiar with basic electricity principles to even attempt this.

Please note that I've oversimplified everything in this article, a real solar system project is a lot more work. I didn't include how other components like batteries and controllers fit in but much of what determines how many solar panels you need is laid out.

You just need a systematic way of determining the demand of your appliances and then match it with the supply of enough solar panels.


Comments

Soul on November 02, 2020:

Very clear and simple tutorial, Thanks

Patrick Kamau from Nairobi, Kenya on January 26, 2020:

This article is very informative, thanks for sharing this guide on how many solars one might need.