Calculating what size solar system do I need…. on my own


How much solar power do I really need?

Most solar companies have salesman that can size your solar need. But some of you may still want to do this exercise on your own – which is awesome. My name is Yoge and I am a first time blogger. In this post, I will show you exactly how to determine how much solar power you need for your home.

Usually, some cold calling solar company sends a salesman to your door. That person first asks you for your electricity bill, curiously (or pretends to be) inquires about your daily energy consumption habit, then finally tries throwing some size requirement at you.

There is nothing wrong with that; as long as the person who is sizing knows what he is doing. But all solar companies (and their salesmen) are not created equal. So, how do you test them for their work? By educating yourself first….am I right?

If we were still living in the year 2010, I would not tell you what I am about to tell you now next…….

There is one important difference about getting solar in – 2016 vs. 2010. The reason is…

Few years ago, how much solar you installed on your rooftop was determined by:

  • How much $$ you were willing to spend?
  • How big roof space you had?, and..
  • The maximum size you were allowed to install by your electricity company.

But wait! what about my need? Ironically, that didn’t matter so much.

Of course, only few installed solar back then only to match their need. Most of them produced more solar power than what they self-consumed. Homeowners would then sell the rest of it to the power company at a rate much higher (2-3x) than what you were buying for (that’s an income – from the power company). Which is why most homeowners installed maximum size system that was allowed – as long as they had the roof space and could afford.

Things are quite different today.  Although the incentive for exported electricity still exists, it’s a fraction compared to 2010 rates. Some states don’t even offer any financial reward for the export anymore. 


Why am I telling you this? And what does all this mean to you?

It means one thing – there is no point spending money on solar panels that you don’t need. And just because you don’t need it doesn’t always mean you won’t get it. Be aware of push sales from the solar companies.

Now, let’s get back to the main agenda of this article, which is, (in case you forgot)..

How to calculate your solar power requirement today?

Today, this is a two-step process.

  1. First, we need to find out how much energy you consume each day that can be replaced by solar.
  2. Then, we find the size of solar system that is able to supply that energy for you. Not more, not less.

So, let’s start with something really basic – your electricity bill.

On that electricity bill, there are mainly two types of information. First, there is information about how much you owe to the energy retailer. This is in dollars.

Second, the invoice shows what your total energy consumption was for that billing period. This is in kWh.

Tips: Don’t look at dollars. Look instead for the consumption in kWh for sizing.

An average Aussie household uses about 17 kilowatt-hours (kWh) of energy per day. In your case, that number may be different.

How do you calculate daily usage from the utility bill?

This is really simple.

On your periodic electricity bill, monthly or quarterly, there is a bar graph that shows your total consumption for each month for the past 12 months. Or, for each quarter for the past four quarters. You can add all those values up to twelve months and divide by 365 days, which gives you your daily consumption averaged for last year.

Worst case scenario: If you don’t have those bar charts on your invoices, or you haven’t been saving the power bills, don’t worry. Simply call your provider and ask for your historical  usage data over the phone.

Tips: While calculating daily usage, use the past twelve months consumption data to account for seasonal variations.

If you were like an average Aussie family, the daily usage you just calculated from your bill is likely close to 17 kWH.

Next, let’s discuss how much of that daily usage can be offset using solar electricity… (that’s because solar panels don’t work all the time)

Understanding intra-day energy consumption profile

If you are like any, you consume energy twenty-four hours a day. More in some hours than others. How much you consume in each hour is dictated by things such as your own power habits, family size and more.

A typical energy consumption of a working family may peak twice in a day: once in the morning and once in the evening. For entire 24-hour, the power consumption profile may look somewhat like what I have shown in the diagram below.


Note: This hourly consumption data is not included in your electricity bill.

Once you have solar, the only grid consumption you are going to offset is what you use during the day. And that’s roughly the area under the sun (yellow) in the above diagram.

Because you don’t have your hourly usage information, the easiest way to estimate your consumption under the sun (day time usage) is by estimation. Unfortunately, this is also the only way, but for smaller residential systems, a wisely estimated value is as good as any.

Tips: Guesstimating your daytime usage is crucial because that’s going to decide how big of a solar system you will ultimately need. 

Let’s say your average daily consumption matched the national average of 17kWh. And if you guesstimated 60% usage is in the day, that’s about 10kWh energy you use during the day.

Next, we will take this 10kWh and explore how big of a solar panel is needed to produce that amount of energy in a day so it can offset your daytime grid import and save money on your electricity bill.

But before we do that, let’s summarize three major points we’ve covered so far.

  • Solar sizing in 2016 is about self-consumption, not export. 
  • Your daily average consumption of electricity can be easily calculated from your utility bill.
  • Estimating your day time energy usage is crucial because that determines how many solar panels you’ll need.


Now that we know the amount of daytime usage we want to replace with solar (which is 10kWh for our example), let’s dive into the second and final piece of the puzzle.

How do we know how many solar panels is required to produce 10kWh in a day?

Size of a solar power system is measured in Kilowatts. For example, John recently installed 4.5 kilowatts solar power system for his need. Each panel is rated for its power output and today, a typical panel outputs about 250 Watts. So, if John needs 4.5 kilowatts (or 4500 watts) and each panel is 250 watts, that’s 18 panels on his rooftop. 

A standard panel can output maximum between 250 watts to more than 300 watts per panel at solar peak hours.

Scientific studies and years of historical meteorological data have made the calculation of sizing a solar power system really simple for you and me.

For major cities across Australia, what we already know is how much 1 Kilowatt solar panel system produces on average in a day. Once you have that information for the nearest major city from where you live, you can easily calculate how many Kilowatt of solar panel system is needed to generate 10 kWh.

According to the guidelines published by Clean Energy Council, the peak body for clean energy industry in Australia, below are the ten major cities in Australia and their respective production estimate for 1 kilowatt system.

City Average Daily production (from 1 kW solar system)
Adelaide 4.2 kWh
Alice Springs 5.0 kWh
Brisbane 4.2 kWh
Cairns 4.2 kWh
Canberra 4.3 kWh
Darwin 4.4 kWh
Hobart 3.5 kWh
Melbourne 3.6 kWh
Perth 4.4 kWh
Sydney 3.9 kWh


As you can see, some locations are far better for solar than others. In Alice springs, a 2 Kilowatt system would produce 10 kWh in a day, while in Hobart, you would need almost one additional kilowatts of solar panels to produce the same.

Across ten major cities in Australia, here are different solar system sizes you would need to produce the same amount of energy – which is 10 kWh per day.

City Solar Size (for 10kWh daily)
Adelaide 2.4 Kilowatts
Alice Springs 2.0 Kilowatts
Brisbane 2.4 Kilowatts
Cairns 2.4 Kilowatts
Canberra 2.3 Kilowatts
Darwin 2.3 Kilowatts
Hobart 2.9 Kilowatts
Melbourne 2.8 Kilowatts
Perth 2.3 Kilowatts
Sydney 2.6 Kilowatts

There you have it.

If you lived in Sydney, 2.6 kW system would produce 10kWh to meet your daily need. Likewise, if you lived in Perth, a slightly smaller size, 2.3 kW, would do.

Note: This system size doesn’t weigh in shading on the roof. If you have moderate to heavy shading where the panels will be installed, you should request for shading analysis with every solar company you deal with.

Note: This system size also doesn’t weigh in battery storage. If you are likely to add batteries to your solar system in the future (when the batteries become affordable), you should consult about that with your solar company. What you want is enough panels on your roof to supply power to your house during the day and charge the batteries at the same time.

Wrapping up!

The basic concept of sizing your solar power need is really simple. However, if you have a special case such as shading or batteries need, the result from the exercise above will only give you a minimum size requirement. 

With solar, what is also important is that you have enough power demand in the afternoon to make your investment meaningful.

Imagine if you installed solar panels and no body is home in the afternoon. You have just wasted thousands of dollars on a system that you don’t need.

Once you find out your daily usage from the utility bill, think carefully and estimate wisely on how much of that daily usage can be replaced by solar.

I hope you enjoyed reading this article. If you have any questions or comments, please don’t hesitate to leave comments below.

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