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How many solar panels do i need?

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There are a few elements to consider when determining the size of a solar panel system. Every property is different, and the reasons people install solar varies. 


Below we will consider some important elements when choosing a system size, including: 

  • How much electricity does your property use?
  • When you use electricity in your home?
  • Where can you install solar panels on your property?
  • What is your primary aim with solar?
  • What is your budget?

Before we launch in, it’s important to understand how the size of a solar system is measured.

When we talk about the size of a solar system, we’re referring to its solar PV ‘Peak Output’ which we measure in kiloWatts. A Watt being the basic measure of electrical power and the kilo means we’re talking in 1000s. i.e 1 kiloWatts (kW) = 1000 Watts. 

A system is made up of multiple panels, each with an individual ‘Peak Output’ measured in Watts. By adding more panels we increase the size of the overall system. For example, a system made up of 20, 250W solar panels would be 20 x 250W = 5000W ‘Peak Output’ or 5kW ‘Peak Output’ sometimes referred to as 5kWp.

But there is one more thing to consider, and that is a solar panel’s rating. 

A solar panel’s rating is tested and determined during manufacture. During this test, a panel is exposed to 1000W/m2 of light and the resulting output in Watts is its rated output. All reputable solar panel manufacturers have a +3% rating system, meaning the Wattage of the panel will be the rated output indicated as a minimum and up to 3% more. e.g a panel with a rated output of 320W could actually be 329W.  

Now to give you an idea of how this works under local conditions, at midday in January, you would expect to receive 1002W/m2 in the Nelson Lakes area. In July that figure reduces to 391W/m2. This is why solar panels perform better in the summer: more light = more electricity. 

Let’s look at what a what 1 kiloWatt of power actually represents in your home:

  • Operating two desktop computers across a standard work day.
  • Running a 60-Watt light bulb from sun-up to sun-down.
  • Charging your phone for 2 hours/day over the course of a month.
  • Running the microwave for 2 minutes every day for a month.


OK, so now let’s look at how much electricity your property uses

The easiest way to find out is to look at your most recent electricity bill. Most suppliers have an app or an online portal as well, where you can find out your energy usage over time and look at how many kWh you have used. 

Let’s look at an example:

If you use 7000 kWh per year, which is similar to the average New Zealander.

Divide your annual kWh by 365. That is the daily usage. This comes out as 19kWh’s per day.

In Nelson Lakes District, if the system is facing north they you are likely to produce 1400kWh for every 1kWp installed. So 7000 kWh divided by 1400 kWh, equals 5kWp

So you would need a 5kWp solar system to generate the same amount of electricity that you use.


Next step is to look at when you use electricity in your home.

You use your solar energy in one of two ways depending on whether, at any moment in time, you are:

1) consuming all your solar electricity in your home (using the same or more than you generate).
2) exporting your solar electricity out to the grid (generating more than your house can use).

To get an idea of how solar power is used in a property with solar, lets have a look at this graph:

Solar Usage

The red line is the electricity use over 24 hours.
The grey line is the typical output of a system.

The closer you can match your generation to your usage the better your return will be.

You will save around $0.35 for every kWh of solar electricity you don’t need to purchase from the grid.
Likewise if your home has surplus electricity to your demand you will export to the grid, the current rate you will be paid by your energy retailer is around $0.09 for every kWh. 

Installing solar will likely make you think about your energy use, simple actions like turning the washing machine on during sunshine hours will help you get the best out of your system. There are also lots of smart technologies available to help match your solar generation with your usage. 

Another option is to install batteries to store the spare solar electricity you generate during the day, to be used in your home in the evening.


Next we need to look at how many solar panels can fit on your property. 

Ideally, we’re looking for a north-facing roof or elevation, free from shading and at a pitch of between 10 and 40 degrees. 

A standard panel measures 1.6m x 1m. According to our calculations above, most houses will require between 10 and 30 panels to cover their usage. Each property is different however, using specialist software we can model a property to design how many solar panels would fit and how the system would perform. 


What is your primary reason for installing solar panels? 

It could be to reduce your energy bill, to reduce your carbon footprint or perhaps you want to get a better return on your savings than putting it in the bank. Or all of the above and more!

The size and design of your solar system can be matched to ensure it performs against your values and motives. We always recommend that you receive a detailed proposal from a trusted solar expert, demonstrating the financial and environmental benefits of installing solar panels so you can make an informed decision. 


Finally, what is your budget? 

A typical solar panel system can range from $8k to $28k depending on your home, needs and your reasons for going solar. Spending more doesn’t necessarily result in a better performing system or bigger reduction in energy bills. Make sure you do your research and feel comfortable with your system proposal before making the investment. 


So how many solar panels you need really does depend on your personal circumstances. The simple answer is to get good advice from a qualified solar expert who can design a system size with you, to suit your needs.


If you are interested in installing solar panels then fill out the form below and one of our team will get in touch with you to see how we can help.

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